Module 1: Classification of Materials, Lattice Imperfection,

Deformation & Strain Hardening

Engineering materials:

Almost every substance known to man has found its way into the engineering workshop at

some time or other. The most convenient way to study the properties and uses of engineering

materials is to classify them into „families‟ as shown in figure below:

1. Metals

1.1 Ferrous metals

These are metals and alloys containing a high proportion of the element iron.

They are the strongest materials available and are used for applications where high

strength is required at relatively low cost and where weight is not of primary

importance.

As an example of ferrous metals such as: bridge building, the structure of large

buildings, railway lines, locomotives and rolling stock and the bodies and highly

stressed engine parts of road vehicles.

The ferrous metals themselves can also be classified into "families', and these are shown in

figure below.

1.2 Non – ferrous metals

These materials refer to the remaining metals known to mankind.

The pure metals are rarely used as structural materials as they lack mechanical

strength.

They are used where their special properties such as corrosion resistance, electrical

conductivity and thermal conductivity are required. Copper and aluminium are used

as electrical conductors and, together with sheet zinc and sheet lead, are use as roofing

materials.

They are mainly used with other metals to improve their strength.

Some widely used non-ferrous metals and alloys are classified as shown in figure below.

2. Non – metallic materials

2.1 Non – metallic (synthetic materials)

These are non – metallic materials that do not exist in nature, although they are manufactured

from natural substances such as oil, coal and clay. Some typical examples are classified as

shown in figure below.

They combine good corrosion resistance with ease of manufacture by moulding to

shape and relatively low cost.

Synthetic adhesives are also being used for the joining of metallic components even in

highly stressed applications.

2.2 Non – metallic (Natural materials)

Such materials are so diverse that only a few can be listed here to give a basic introduction to

some typical applications.

Wood: This is naturally occurring fibrous composite material used for the

manufacture of casting patterns.

Rubber: This is used for hydraulic and compressed air hoses and oil seals. Naturally

occurring latex is too soft for most engineering uses but it is used widely for vehicle

tyres when it is compounded with carbon black.

Glass: This is a hardwearing, abrasion-resistant material with excellent weathering

properties. It is used for electrical insulators, laboratory equipment, optical

components in measuring instrument set, and in the form of fibres, is used to reinforce

plastics. It is made by melting together the naturally occurring materials: silica (sand),

limestone (calcium carbonate) and soda (sodium carbonate).

Emery: This is a widely used abrasive and is a naturally occurring aluminum oxide.

Nowadays it is produced synthetically to maintain uniform quality and performance.

Ceramic: These are produced by baking naturally occurring clays at high temperatures

after moulding to shape. They are used for high – voltage insulators and high –

temperature – resistant cutting tool tips.

Diamonds: These can be used for cutting tools for operation at high speeds for metal

finishing where surface finish is greater importance. For example, internal combustion

engine pistons and bearings. They are also used for dressing grinding wheels.

Oils: Used as bearing lubricants, cutting fluids and fuels.

Silicon: This is used as an alloying element and also for the manufacture of

semiconductor devices. These and other natural, non-metallic materials can be

classified as shown in figure 7. Limestone (calcium carbonate) and soda (sodium

carbonate).

Composite materials (composites)

These are materials made up from, or composed of, a combination of different materials to

take overall advantage of their different properties. In man-made composites, the advantages

of deliberately combining materials in order to obtain improved or modified properties was

Understood by ancient civilizations. An example of this was the reinforcement of air-dried

bricks by mixing the clay with straw. This helped to reduce cracking caused by shrinkage

stresses as the clay dried out. In more recent times, horse hair was used to reinforce the

plaster used on the walls and ceiling of buildings. Again this was to reduce the onset of

drying cracks. Nowadays, especially with the growth of the plastics industry and the

Development of high-strength fibers, vast range combinations of materials are available for

use in composites. For example, carbon fiber reinforced frames for tennis rackets and shafts

for golf clubs have revolutionized these sports.

Lattice Imperfections:

The fact that real materials are not perfect crystals is critical to materials engineering. If

materials were perfect crystals then their properties would be dictated by their composition

and crystal structure alone, and would be very restricted in their values and their variety. It is

the possibility of making imperfectly crystalline materials that permits materials scientists to

tailor material properties into the diverse combinations that modern engineering devices

require. As we shall see repeatedly in the body of this course, the most important features of

the microstructure of an engineering material are the crystalline defects that are manipulated

to control its behaviour. In this Chapter we shall identify and describe the various defects that

are found in crystalline solids. We shall hint at their consequences to indicate why they are

worth studying, but shall defer a detailed discussion of their behavioural role until we discuss

particular classes of engineering properties. It is useful to classify crystal lattice defects by

their dimension. The 0- dimensional defects affect isolated sites in the crystal structure, and

are hence called point defects. An example is a solute or impurity atom, which alters the

crystal pattern at a single point. The 1-dimensional defects are called dislocations. They are

lines along which the crystal pattern is broken. The 2-dimensional defects are surfaces, such

as the external surface and the grain boundaries along which distinct crystallites are joined

together. The 3-dimensional defects change the crystal pattern over a finite volume. They

include precipitates, which are small volumes of different crystal structure, and also include

large voids or inclusions of second-phase particles.

3.INTRODUCTION

The term defect ,or imperfection , is generally used to describe any deviation from an orderly

array of lattice points. When deviation from the periodic arrangement of the lattice is

localized to the vicinity of only a few atoms it is called a point defect or point imperfection

.However , if the defect extends through the microscopic regions of crystal , it is called a

lattice imperfection.

Crystal imperfections are broadly classified into 4 major classes:-

1)Point defects (0 dimensional)

2)Line defects (1 dimensional)

3) Planar or surface Defects (2 dimensional)

4)Volume defects 3D

2. POINT DEFECTS

All the atoms in a solid possess vibrational energy and at all temperatures above absolute

zero, there will be a finite number of atoms that have sufficient energy to break the bonds

which hold them in their equilibrium position .Once the atoms are free from their lattice sites

,they give rise to point defects. Also due to presence of impurity atoms point defects are

likely to come in crystals.

Point defects are classified as:

a)Vacancies b)Interstitial c)Impurity atoms d)Frenkel imperfction d)Schottky imperfection

a) VACANCIES

The simplest of point defects is a vacancy ,or vacant lattice site ,one normally occupied from

which an atom is missing, All crystalline atoms contain and it is not possible to create a

material that is free of these defects . The necessity of the existence of vacancies increases the

entropy of crystal. The equilibrium number of vacancies Nv for a given quantity of material

depends on and increases with temperature according to Nv=Nexp(-Qv/kT) .In this

expression , N is the total number of atomic sites ,Qv is the energy required for formation of

vacancy ,T is absolute temperature in kelvin k is Boltzman‟s constant.

Production of vacancies

There are 5 main methods of introduction of vacancy into a metal in excess of equilibrium

concentration.

1.Rapid quenching from high temperature can be effective in retaining the concentration of

vacancies which exist in thermal equilibrium at that temperatures.

2.Excess of lattice vacancies are formed when inter metallic compounds are formed. Eg. Ni-

Al ,Co-Al ,Fe-Al ,etc.

3.Bombardment of metal with high energy nuclear particles can produce both vacancies and

interstitials.

4)Vacancies are also formed during plastic deformation at jogs produced on dislocated lines

by intersecting dislocations.

5)Oxidation of some metals eg. Zn , Cd ,Mg ,Cu ,Ni is accompanied by injection of vacancies

into metal lattice.

Effects of vacancy :

The introduction of vacancies into a metal produces change in both its physical and

mechanical properties. Vacancy reduces yield strength and tensile strength of material due to

that material becomes weak.

By successive jumps of atoms it is possible for vacancy to move in lattice structure and

therefore play an important part in diffusion of atoms through the lattice. The atoms

surrounding the vacancies tend to be closer together, distorting the lattice planes .

b) Interstitials :

Interstitialcy occurs in a pure metal as a result of bombardment which are high energy

nuclear particles. Interstitialcy also caused by process of formation of solid ,liquid gaseous

state.

An atom that is trapped inside the crystal at a point intermediate between normal lattice

positions is called an interstitial atom, or Interstitialcy . The interstitial defect occurs as

a result of bombardment with high energy nuclear particles ( radiation damage) , but it does

not occur frequently as a result of thermal activation.

The presence of an impurity atom at a lattice position or at an interstitial position in a local

disturbance of the periodicity of the lattice, the same as for vacancies and interstitials . It is

important to realize that no material is completely pure.

Effects of Interstitialcy

Interstitialcy does not affect mechanical behavior of material but affects properties like

electrical conductivity and magnetic flux density in case of conductor and transformer core

material ie increase in Interstitialcy increases electrical conductivity and magnetic flux

density.

c) Impurity atoms :

An impurity represents the existence of secondary atom in the regular crystal lattice. It is

essentially an undesirable property presence responsibility for degradation of behavior

and contamination of composition. Following are various types of impurity :

1)Interstitial impurity :

When the presence of the impurity causes a substantial modification of properties it is called

interstitial impurity.

2)Substitutional impurities

A foreign atom substitutes or replaces parent atom in the regular crystal or if presence of

secondary atom is desirable it is called as substitutional impurity.

For interstitial solid solutions, impurity atoms fill the voids or interstices amount the host

atoms. For metallic materials that have relatively high atomic packaging factors, this

interstitial positions are relatively small. Consequently , the atomic diameter of an interstitial

impurity must be substantially smaller than that of the host atom. Normally, the maximum

allowable concentration of interstitial impurity atom is low (less than 10%).

Effects of impurity atoms :

Presence of O2, H2, N2 and C is highly significant and they are primary interstitial

impurity.These impurities reduce electrical conductivity, chemical resitivity and mechanical

formability.

d) Frenkel imperfection:

Here an atom is displaced from its regular site to an interstitial site,the defect is called as

frenknel imperfication.

e) Schottky imperfections

It is closely related to vacancies and is formed when an atom or an ion is removed from a

regular lattice site and replaced in an average position on the surface of a crystal.

In other words , vacancies are created by movement of atoms from position inside the crystal

to position on the surface of crystal.Then this defect is called schottky imperfections.

3 Line Defects: One Dimensional

Introduction:

A part of a line of atoms will be missing from its regular site and this missing row of atoms is

called dislocation. The dislocation is centered along a line and hence the line defect is called

as dislocation. Movement of dislocation is necessary for plastic deformation. The dislocation

is a boundary between the slipped and unslipped region and lies in slip plane. The structure

and behavior of dislocations affect many of the properties of engineering materials.

There are two basic types of dislocations.

(a) Edge dislocations. (b) Screw dislocations.

Edge dislocation:

It can be considered as a defect due to an insertion of an extra half plane of the atoms. If this

extra half plane of atom is above as shown in figure, the line defect is called as positive

dislocation and is denoted by „_|_ „ . If this extra plane is below, the defect is called negative

edge dislocation and is denoted by the symbol ' T „ The lattice above the line is in

compression, whereas below the line it is in tension and reverse is the case of negative

dislocation. The more inter-atomic spacing below the extra half plane for positive dislocation

and above extra half plane of atoms for negative dislocation favors segregation of interstitial

atoms and impurities.

Screw dislocation:

It is formed by a shear stress that is applied to produce distortion as shown in figure. The

upper front region of the crystal is shifted one atomic distance to the right relative to the

bottom portion. The atomic distortion associated with the screw dislocation is also linear and

along a dislocation line. The screw dislocation derives its name from the spiral or helical path

that is traced around the dislocation line by the atomic planes of atoms. Sometimes the

symbol ….... is used to designate a screw dislocation.

BURGER VECTOR :

The magnitude and direction of displacement are defined by a vector called the Berger vector

which characterizes a dislocation line.

Consider the perfect crystal shown in figure (a) starting point from 'P', if we go up by 'x'

number

of steps (x=2), then 'y' steps to the right (y=4),then x steps down and finally 'y' steps to the

left, we end up at the starting point. Here Berger circuit is perfect. If we do the same

operation on the edge dislocation, the starting point 'P' and we end up at „Q‟, we need an

extra step to return to point 'P' or close the Berger circuit. It is shown in figure.

The magnitude and direction of this step defines the Berger Vector (B.V.). Therefore,

→

B.V.=QP=b

The Berger vector is perpendicular to the edge dislocation line. Following are few important

points about edge dislocation by studying figure.

1) Relation between dislocation line and 'b' (B.V.) → (_|_er) perpendicular

2) Slip direction → parallel to B.V.

3) Direction of dislocation line movement relative to 'b' (B.V.) → parallel

4) Process by which dislocation may leave slip plane → Dislocation climb

Consider the hatched area 'AEFD' in figure (a) on the plane 'ABCD'. Let the top part of the

crystal over the hatched area be displaced by one inter-atomic distance to the left with respect

to bottom part as shown in figure (b).

Consider figure, in which Berger circuit starts from point 'v' it moves up by 2 atomic distance

then it moves towards right from point 'q' by 3 atomic distance (y=3) and reaches to point 'r'.

From 'r' it moves 4 atomic distance down (x=4) and reaches to point 's' then it moves 3

atomic distance (y=3) towards the left and reaches to point 'p'. Now it moves 2 atomic

distance towards up and reaches to point 'u' which is the end point of Berger circuit. Here,

starting point (v) and end point (u) are not same and to reach from end point (u) to starting

point (v) and distance (1 atomic) to close circuit.

For screw dislocation, produced at boundary 'EF' between displaced and undisplaced parts of

the slip plane. Let ‟t‟ vector be defined such that it points from right to left. The Berger

vector is determined by step needed to close circuit. The Berger vector is pulled 'b' has same

direction as the 't' vector. The Berger vector is parallel to screw dislocation line. Screw

dislocation line are symbolically represented by or depending on whether Berger vector and

the 't' are parallel or anti-parallel. These 2 cases referred to as positive and negative screw

dislocation.

Following are few important points of screw dislocation by studying figure.

1) Relation between dislocation line and 'b' (B.V.) → (||) parallel

2) Slip direction → parallel to B.V.

3) Direction of dislocation line movement relative to 'b' (B.V.) → (_|_) perpendicular

4) Process by which dislocation may leave slip plane → cross-slip

Distinguish between Edge Dislocation and Screw Dislocation:

EDGE DISLOCATION SCREW DISLOCATION

1. If one of the planes is not continuous from top

to bottom or bottom to top and ends part way

within crystal, it is called as edge dislocation.

1. If plane of atoms follows dislocation in a

helical or screw path then it is called as screw

dislocation

2. Edge dislocation are of two types positive

(_|_) and negative (T) dislocation.

2. clockwise and anticlockwise dislocation.

3. Relation between dislocation line and Berger

vector (b) is perpendicular to each other

3. Relation between dislocation line and

Berger vector (b) is parallel to each other.

4. Direction of dislocation line movement

relative to Berger vector is parallel

4. Direction of dislocation line movement

relative to Berger vector is perpendicular.

5. Edge dislocation can be removed from slip

plane by dislocation climb mechanism

5. Screw dislocation can be removed from slip

plane by cross-slip mechanism

4. SURFACE DEFECTS (TWO DIMENSIONAL)

Surface imperfections are 2 dimensional in the mathematical sense. They refer to regions of

distortion that lie about a surface having a thickness of a few atomic diameters. The external

surface of a crystal is an imperfection in itself, as the atomic bonds do not extend beyond the

surface.

Types of surface defects are

Grain Boundaries Tilt Boundaries Twin Boundaries

If new crystals are formed during solidification process it is called as recrystallization. These

new crystals are randomly oriented and grow by addition of adjacent regions. These crystals

impinge on each other. When 2 crystals impinge in this manner, the atoms that are caught in

between the two are being pulled by each of the 2 crystals to join its own configuration. They

can join neither crystal due to the opposing forces and so take up a compromise position. The

thickness of this region is only a few atomic diameters, because the opposing forces from

neighboring crystals are felt by the intervening atoms only at such short distances.

The boundary region is called as a crystal boundary or a grain boundary. Angle between 2

grains are more the10-15˚, so the grain boundaries are known as high angle boundaries.

b) Tilt Boundary

When angle between 2 crystals is less than 10˚, the distortion in the boundary is not so drastic

as to be compared with a non-crystalline material they are also called low angle boundaries.

Figure above shows neighboring crystalline regions are tilted with respect to each other by

only a small angle.

The tilt boundary can be described as set of parallel, equally spaced edge dislocation of the

same sign located one above the other. In figure there is a set of positive angle edge

dislocation which are equally spaced by height „h‟. the angle of tilt „Ɵ‟ is related to the

bergers vector „b‟ of the

edge dislocation,

Tan Ɵ = b/h

Where, h = vertical spacing between two neighboring edge dislocation. b = berger vector of

edge dislocation.

c) TWIN BOUNDARY:

Some metals like zinc, tin and iron deforms by process known as „Twinning‟. In deformation

by slip, all atoms in one block move the same distance, but in deformation by twinning atoms

in each successive plan within a block will move different distance and each half of the

crystal becomes a mirror image of the other half along a twinning plane.

Twins may form during solidification, deformation or during deformation and annealing of

metals.

VOLUME DEFECTS:

Volume defects such as cracks or stacking faults may arise when there is only small

dissimilarity between the stacking sequence of close packed planes in FCC and HCP metals.

Hexagonal close packed structure is produced by stacking sequence of the type AB

AB AB……in which any one atom from the second plane occupies any one interstitial site of

first plane. Third plane is stacked similar to first and fourth similar to second and so on. For

FCC structure by stacking sequence ABC ABC ABC…..first two planes are stacked in a

similar way to HCP and third plane (C) is stacked in different manner. The stacking fault

may change the above sequence to ABC ABC AB ABC ABC for FCC structure. The

stacking fault in this case is

due to the missing of “C” plane of atoms after 8th plane. This defect can be regarded as a

very

thin region of HCP stacking in a basically FCC structure as indicated below:

ABC ABC/FCC AB/HCP ABC ABC…/FCC

Stacking faults may occur during crystal growth or may result from the separation of two

partial dislocations.

* Dislocation pile-up:-

Dislocation frequently piles up on slip plane at barrier such grain boundaries, second phases

or sessile dislocations. The leading dislocations in the pile-up are acted on not only by in the

applied shear stress but also by the interaction force with the other dislocations in the pile-up.

This leads to a high concentration of stress on the leading dislocation in the pile-up. When

many dislocations are contained in the pile-up, the stress in the dislocation of the head of a

pile up can approach theoretical shear stress of the crystal. This high stress either can initiate

yielding on the other side of the barrier or in other instances; it can nucleate a crack at the

barrier. Dislocation piled up against the barrier produce a back stress acting to oppose the

motion of additional dislocations along the slip plane in the slip direction.

The dislocation in the pile up will be tightly packed together near the head of the array and

more widely spaced towards the source. The number of dislocation that can occupy a distance

L along the slip plane between the source and the obstacle is

n = kπτsL/Gb

where τs is the average resolved shear stress in the slip plane and k is a factor close to unity.

For an edge dislocation k =1-v, while for a screw dislocation k =1.

More the grain boundaries i.e finer the grain size, dislocation generation rate is faster. Fig.

shows dislocations pile up. In which dislocation are moving along slip plane and collected at

grain boundary.

When number of dislocation collected at grain boundary, material becomes weak and when

there is application of force in plastic region, it will fail at the grain boundary. The strength of

material is also reduced because of number of dislocations.

*Dislocation climb:-

An edge dislocation can glide only in the slip plane containing the dislocation line and its

burgers vector. However, under certain conditions an edge dislocation can move out of the

slip plane onto a parallel plane directly above or below the slip plane. This is the process of

dislocation climb.

Dislocation climb occurs by the diffusion of vacancies or interstitials to or away from the site

of the dislocations since climb is diffusion controlled, it is thermally activated and occurs

more readily at elevated temperature. In positive climb atom are removed from the extra half

plane of the atom at a positive edge dislocation so that this extra half plane moves up one

atom spacing. In negative climb a row of atoms is added below the extra half plane so that the

dislocation line moves down one atom spacing. The usual mechanism for positive climb is by

a vacancy diffusing to the dislocation and the extra atom moving into the vacant lattice site.

It is also possible, but less energetically favorable for the atom to break loose from the extra

half plane and become interstitial atom. To produce negative climb, atom must be added to

the extra half plane of the atoms. This can occur by atoms from the surrounding lattice

joining the extra half plane which creates a flux of vacancy away from the dislocation, or less

probably, by interstitial atoms diffusing to the dislocation.

The existence of compressive stress in the slip direction causes a force in the upward

direction of the climb. Similarly, a tensile stress normal to the extra half plane causes a force

in the direction of the negative climb. The superposition of stress on the high temperature

needed for diffusion in an increased rate of climb.

*Dislocation Source:

The low yield strength of pure crystal leads to the conclusion that dislocation must exist

incompletely annealed crystal and in crystals carefully solidify from the melt. There is no

general relationship between dislocation, density and temperature completely annealed

material will contain a density of about 104 to 106 mm-2, while heavily cold worked metal

will have a density of 108 to 1010 mm-2.

All metal with the exception of tiny whiskers, initially contain an appreciable number of

dislocation produce as a result of the growth of the crystal from the melt or vapor phase.

Gradients of temperature and composition may produce misalignment between neighboring

dendrite arms growing from the same nucleus. This result in dislocation arrange in networks

or in grain boundaries.

Irregularities at grain boundaries are responsible for emitting dislocation. It is thought that

emission from grain boundaries is an important source of dislocation in yearly stages of the

plastic deformation. In single crystal small surface steps which serve as stress raisers can act

as dislocation sources. The dislocation density in single crystal is appreciably higher at the

surface then at the interior because of this effect. However it is not significant in the

polycrystalline material because the majority of the grain are not in contact with the surface.

Another way that dislocation can form is by the aggregation and collapse of the vacancies to

form a disk or prismatic loop.

INTRODUCTION OF DEFORMATION:

The change in dimensions of forms of matter under the action of applied forces is called as

„deformations‟. It is caused either by mechanical action of external forces or by

physical

processes. To form various metallic shapes the deformation of metal is necessary. The

deformed or mechanically worked metals are much superior to cast metals from which they

are produced.

When metals or alloys are stressed (i.e. subject to load), they get deformed. Deformation is

caused by

1) Mechanical action of external forces.

2)Various physical and chemical processes.

Types of Deformation:

Deformations are of two types.

(1) Elastic: Deformations is completely temporary and after removing applied load it regain

its original shape.

(2) Plastic: Deformation is permanent even after removal of applied load is called as plastic

deformation.

*ELASTIC DEFORMATION:

If a metal is loaded within certain limits, a temporary deformation of the crystals takes

place through displacement of atoms. In elastic deformation, if load is applied on material,

atoms in the material are displaced from their original position. After gradually removal of

load, the atoms returns to their original stable position and crystal recover its original shape.

Fig. 5.2.2 shows various steps in the elastic deformation during application of load and after

removal of load.

Elastic deformation occurs with comparatively smaller deforming load. Under tensile

loading, metal piece becomes slightly longer as a result of a slight elongation of the unit cell

in the direction of tensile load. This elongation in the metal piece disappears as soon as the

deforming load is removed.

strain.

Within elastic limits, material obeys „Hook‟s Law‟ i.e. stress is proportional to

Elongation, e=PL/AE Where, P= Applied Force.

L= Length of Specimen.

A= Cross sectional Area of the Specimen.

E= Young‟s Modulus.

=Stress/Strain

Within elastic range, the strain is result of a slight elongation of the atoms in the direction of

the tensile load, or a slight contraction in the direction of compressive load.

*PLASTIC DEFORMATION:

If load is applied gradually on metal, within elastic limit, stress is proportional to strain.

When metal piece cross elastic limit, it enters into plastic limit. The specimen gets plastically

deformed in the plastic region. If load is removed in the plastic region, the metal piece does

not regain its original shape and a permanent (plastic)strain always remains in the metal.

Plastic deformation stages in the metal are shown in Fig.5.3.2

Plastic deformation is function of

(1) Applied Stresses: As stress increased, there is more plastic deformation.

(2) Temperature: If temperature is increased, at less load, there is more plastic deformation.

(3) Strain Rate: If strain rate is increased there is more plastic deformation.

Application of Plastic Deformation:

Permanent deformation is many times carried out in working and shaping processes such as

- Bending.

- Rolling, Stamping, Forging, Extrusion.

- Stamping of Automobile parts, Rolling of Boiler Plates, Rails and I-beams, etc.

ELASTIC DEFORMATION OF SINGLE CRYSTAL:

Grains are the smallest metallic units from the viewpoint of mechanical behavior, in order to

understand the behavior of metals, it is necessary to learn the way these grains or crystals,

react when subjected to mechanical loading. There is very careful control required to produce

single crystals of some metals that are several inches long.

The behavior within single crystals of metals is a function of

(1) The lattice type.

(2) The interatomic forces.

(3) The spacing between planes of atoms.

(4) The density of atoms on various planes.

In elastic deformation, there is no permanent displacement of atoms with respect to each

other. The displacement occurs by very slight warping of the lattice unit, without changing

their relative positions or losing their neighboring atoms.

Various loads can be applied on single crystal as shown in fig. 5.5.2. In fig.

5.5.2(b), when there is application of tensile load, single crystal‟s length increases as well as

there is decrease in diameter. When there is application of compressive load, there is decrease

in length and increase in diameter, as shown in fig. 5.2.2(c). But, after removal of these loads

material regain its original shape and size. Fig. 5.2.2(d) shows shearing load on single crystal.

*MECHANISMS OF PLASTIC DEFORMATION:

Plastic deformation in the crystal can occur by

1. Slip mechanism.

2. Twin mechanism.

Slip is more common mechanism. slip and twinning occurs by pure shearing process.

If a single crystal of a metal is stressed in tension beyond its elastic limit,it elongates

slightly,a step appears on the surface indicating relative displacement of one part of the

crystal with respect to the rest,and elongation stops .Increasing the load will cause movement

on another parallel plane ,resulting in another step. It is as if neighbouring thin sections of the

crystals had slip past one another like slidding cards on deck. Each successive elongation

requires a higher stress and results in the appearance of another step, which is actually the

intersection of a slip, plane with the surface of the crystal. Progressive increase of the load

eventually causes the material to fracture.

A more important factor in determining slip movement is the direction of shear on the slip

plane .slip occurs in the direction in which the atoms are most closely packed ,since this

requires least amount of energy.

Close-packed rows of atoms could slide past each other particularly easily without

coming apart. refer to figure, the atoms in a row in (a) are closer together and the rows are

farther apart vertically than they are in (b),so that less force is required for a given horizontal

displacement, as suggested by the slope of the black bars between the atoms. In addition less

displacement is required to move the atoms into unstable position from

which they will be pulled forward into stable ones ,when these stable positions are closer

together as in the figure(a).

Twinning

Twinning is the process in which the atoms in a part of the crystal subjected to stress

rearrange themselves so that the orientation of the part changes in such a way that the

distorted part becomes the mirror image of the other part. The plane across which the two

parts mirror images is called twinning plane or composition plane.

Every plane of atoms shifts in the same direction by an amount proportional to its distance

from the twinning plane.

The direct contribution to plastic deformation by twinning is negligible. However twinning

creates favourable conditions for slip and total deformation increases due to indirect

contribution by twinning. Twinning occurs by plastic deformation or by thermal treatments.

Twinning which occur by plastic deformation is called deformation twinning and is

common in HCP structures this also occurs in BCC structures at low temperature. This types

of twinning particularly occurs under fast or shock loading conditions.

Like slip Twinning also occurs along certain crystallographic planes and directions. These

planes and directions are called as twin planes and twin directions.

CRITICAL RESOLVED SHEAR STRESS IN SINGLE CRYSTAL

The slip in Single crystal depends on the magnitude of the shearing stress produced by:

External loads

Geometry of the crystal structure

Orientation of the active slip planes with respect to the shearing stresses.

Critical resolved shear stress

Slip begins with the shearing stress on the slip plane in the slip direction reaches a threshold

value called the critical resolved shear stress. The value is really the single crystal equivalent

of the yield stress of an ordinary stress strain curve.

Value of critical resolved shear stress depends on,

composition

Temperature

Different tensile loads are required to produce slip in single crystals of different orientation

can be rationalized by a critical resolved shear stress. This was first recognized by Schmidt

.To calculate the critical resolved shear stress from a single crystal as shown in fig. 1.28

tested in tension. It is necessary to know from x-ray diffraction , the orientation with respect

to the tensile axis of the plane on which slip first appears and the slip direction.

Consider,

A˳=cross sectional area of cylindrical single crystal.

ϕ =Angle between normal to the slip

λ = Angle between slip direction with tensile axis.

A˳=Transverse cross section all area of crystal

=A˳\cosϕ

Fѕ=Resolved force in the slip direction as per fig.

=Fcosλ

It is important to realize that in general the three directions [the tensile axis, the slip plane

normal

,the slip direction] are not coplanar, so that

ϕ +λ ≠ 90°

Indeed the minimum value of ϕ +λ = 90°.This is attained only when the three directions are

coplanar.

The slip plane area „Aѕ ‟ is greater than „A˳ ‟.It is shown in figure

As per above fig. Cosϕ =A˳/Aѕ

Therefore Aѕ=A˳/cosϕ …….(1.2)

Thus, critical resolved shear stress (τRSS) acting on slip plane and in the slip direction is

τRSS =force in the slip direction/ Transverse cross sectional area

τRSS = Fcosλ/Aѕ ……(1.3)

Put values of „Aѕ‟ as per equation ( 1.2) in the equation ( 1.3)

τRSS = Fcosλ/(A˳/cosϕ )

τRSS = F/A˳ . cosϕ . cosλ

OR

τRSS = σ/m

Where, σ = F/A˳

m=1/. cosϕ . cosλ

=Orientation of the slip system with respect to the tensile axis.

above equation gives shear stress resolved on the slip plane in the slip direction.

This shear stress is maximum when ϕ =λ =45° .

So that

τR=F/2 A˳

If the tension axis normal to the slip plane (λ=90°) if it is parallel to slip plane ( ϕ =90°), the

resolved shear stress is zero .slip will not occurs for these extreme orientations since there is

now

shear stress on slip plane .crystals close to these orientations tends to fracture rather than slip.

6. INTRODUCTION OF STRAIN HARDENING:

Strain hardening or work hardening is a phenomenon which results in an increase in hardness

and strength of metal when subjected to plastic deformation at temperature lower than the

recrystallization range (cold working). This increase in stress required to cause slip because

of previous plastic deformation or the increase in the strength of material due to

mechanical working is known as strain hardening or work hardening. Strain hardening

happens due to cold working process. Effect of strain hardening has to be relieved before

application of any manufacturing processes. If it is not relieved then there is chance of failure

of material.

Strain hardening is employed to pure metals as well as alloys which increases hardness and

strength of metal and reduces ductility and plasticity of metal.

EFFECT OF STRAIN HARDENING ON BEHAVIOUR OF MATERIALS:

If stress-strain is plotted for a mild steel.When load is increased gradually upto elastic region

it follows Hook‟s law i.e. stress is directly proportional to strain and follows path „OE‟. If

material is loaded gradually it enters in the plastic region by following path „EA‟ and at the

end of the plastic region (at point „D‟) material will break.

If load is gradually reduced when metal enters in the plastic region i.e. from point „A‟ instead

of following path „AEO‟ it follows path „AB‟ because strain „OB‟ will be induced in the

metal which cannot be removed. Now, if metal is reloaded from point ‟B‟, it will follow path

„BCD‟ and at point „D‟ it will break. There is an increase in ultimate tensile strength of

material because of removing load and reloading it till the fracture. The rise in stress during

plastic deformation is called work or strain hardening.

*THEORY OF WORK HARDENING OR STRAIN HARDENING:

All the theories of work hardening i.e. Cottrell , Taylor etc. believe that work hardening is

due to increased resistance to dislocations, when subjected to cold working. Some

dislocations become struck inside the crystal and act as source of internal stress which

opposes the motion of other dislocations motion i.e. work hardening occurs due to the

restriction to the motion of dislocations.

*STAGES OF WORK HARDENING:

The three regions of work hardening are distinguished as follows:

STAGE-I [Easy Glide Region]:

This stage follows immediately after the yield point. Dislocations are able to move over

relatively large distance without encountering barriers. Length of this region depends on

_ Orientation

_ Purity

_ Size of Crystals

STAGE-II [Linear Hardening Region]

This region shows a rapid increase in work hardening rate, the slope of which is

approximately independent of applied stress, temperature or alloy content. In this region, slip

occurs on both primary and secondary slip systems. As a result several new lattice

irregularities may be formed which will include,

_ Forest dislocations

_ Lomer-Cottrell barriers

_ Jog produced either by moving dislocations cutting through forest dislocations. Following

theories explain the stage-II hardening

(i) The Pile-up Theory (ii) The Forest Theory (iii) The Jog Theory

The Pile-up Theory

According to this theory, some dislocations given out by Frank-Reed source are stopped at

barriers e.g. Lomer-Cottrell barriers in lattice and other dislocations pile up behind. As

deformation proceeds, the number of barriers increases until each source becomes completely

surrounded by barriers. As per this theory, the hardening is principally due to long range

internal stresses from piled up interacting with guide dislocations.

STAGE-III [Parabolic Hardening Region]

It is a region of decreasing rate of strain hardening. At sufficiently high stress value or

temperature in region III, the dislocations held up in stage-II are able to move by a process

that had been suppressed at lower stresses and temperatures. The screw dislocations which

are held up in stage-II, cross-slip and possibly return to the primary slip by double cross-slip.

By this mechanism, dislocations can by-pass the obstacles in the glide plane and do not have

to interact strongly with them. For this region, stage-III exhibits a low rate of work hardening.

THREE STAGE ANNEALING

Cold working is defined as the working of metal and alloys below their recrystallization

temperature. During cold working , strain hardening is observed and also the grains become

distorted. Cold working increases hardness , yield strength , internal stresses and electrical

resistance as well as decreases ductility and corrosion resistance . after cold working, metal

becomes hard and brittle, hence it is not possible to work them without cracking beyond a

certain limit if cold working is to be continued, the metal must be made soft and ductile by

suitable heat treatment . So further working can be continued by relieving effect of strain

hardening. The heat treatment which is used to relive effect of strain hardening is called an

annealing.

Annealing

Annealing consist of heating the cold worked metals to some temperature above

recrystallization. Hold there for definite time for completely uniform structure and cool to

room temperature very slowly usually in furnace itself only by switching off the electric

supply.

The property changes and structure changes which occurs due to heating a cold worked

material is shown in fig. Annealing process consists of three stages such as recovery,

recrystallization and grain growth.

1) Recovery:

No change occurs in microstructure in this stage . Grains remain distorted hence mechanical

properties such as hardness, tensile strength and ductility do not change appreciably and

assumed to remain same that of cold worked metal. Due to heating, some imperfections like

vacancies, Interstitialcy existing in slip, may get eliminated. Rearrangement of dislocations

such as low angle grain boundaries due to recovery is shown in fig.

2) Recrystallization:

Old distorted grains are replaced by new equiaxed stress free, strain free grains by process of

nucleation and growth. Microstructure at the end of recrystallization process is much similar

to the original structure i.e. similar to the structure prior to cold working. Due to change in

microstructure, all mechanical properties are changed, and become almost similar to that

original material. Degree of cold work is defined as the percent reduction in cross sectional

area during extrusion, wire drawing, forging , rolling or similar working operation.

Recrystallization does not occur unless the degree of cold work is sufficient and

temperature is sufficiently high. The temperature at which a metal or alloy with a normal

degree of cold work completely [i.e. 95% or above] recrystallizes in a reasonable period

(usually 1 hour) is called Recrystallization temperature of metal.

Recrystallization temperature is high in the metal, if they are having higher melting point .

Recrystallization temperature of iron is 450C, steel 727C, aluminium is 150C etc.

3) Grain Growth:

If a specimen is left at the at high temperature beyond the time needed for complete

recrystallization, the grain begin to grow size .This occurs because diffusion occurs across the

grain boundaries and larger grains have less grain boundary surface area per unit of volume.

Therefore, the larger grains lose atoms atom and grow at the expense of smaller grains.

Larger grains will reduce the strength and toughness of material. This type of grain growth is

not desirable because it decrease the useful properties . However, it is useful in single crystal.

FACTORS AFFECTING RECRYSTALLIZATION TEMPRATURE

Recrystallization temperature is function of following factors:

1) Melting point :Higher the melting point of metal, higher is recrystallization temperature

2) Degree of cold work : higher the degree of cold work, lower is recrystallization

temperature

3) Grain size : finer the original grain size, lower is recrystallization temperature

4) Purity of metal : Presence of impurities increases recrystallization temperature.

5) Heating time: Longer the heating time, lower is the recrystallization temperature.

*Difference between Hot and Cold Working :

Module 2 : Failure Mechanism

1. Introduction to fracture

Simple fracture is the separation of a body into two or more pieces in response to an imposed

stress that is static (i.e. constant or slowly changing with time ) and at temperature that are

low relative to the melting temperature of the material. The applied stress may be tensile,

compressive, shear or torsional; the present discussion will be confined to fractures that result

from uniaxial tensile loads. For engineering materials, two fracture modes are possible:

ductile and brittle. Classificationis basedon the ability of material to experience plastic

deformation.

Any fracture process involved two steps –crack formation and propagation-In response to an

imposed stress. The mode of fracture is highly dependent on the mechanisms of crack

propagation.

2. Ductile Fracture

Ductile fracture is characterized by extensive plastic deformation in the vicinity of an

advancing crack. Furthermore, the process proceeds relatively slowly as the crack length is

extended. Such a crack is often said to be stable, That is it resists any further extension unless

there is an increase in the applied stress.

For ductile fracture, the presence of plastic deformation gives warning that fracture is

imminent, allowing preventive measures to be taken, more strain energy is required to induce

ductile fracture in as much as ductile material are generally tougher.

When a ductile material has a gradually increasing tensile stress, it behaves elastically up to a

limiting stress and then plastic deformation occurs. As stress is increased, the cross-sectional

area of the material is reduced and a necked region is produced. With a ductile material, there

is a considerable amount of plastic deformation before failure occurs in the necked region as

a result of excessive yielding as shown in figure.

Steps in ductile Fracture

Stages in ductile fracture are well explained by considering tensile test. When a ductile

material loaded gradually by tensile load in tensile test, steps in ductile fractures are as

follows:

01. Necking

02. Small cavities formation

03. Formation of crack

04. Cup and cone fracture

First after necking begins, small cavities or micro voids form in the interior of the cross

section as indicated in figure.

As deformation continues, these micro voids enlarge, come together, and coalesce to form

elliptical cracks, which have its long axis perpendicular to the stress direction. The crack

continues to grow in direction parallel to its major axis by the micro voids coalescence

process. Finally fracture ensues by the rapid propagation of a crack around the outer

perimeter of the neck by shear deformation at an angle of about 45º with the tensile axis –

this is the angle at which the shear stress is maximum. Sometimes a fracture having the

characteristic surface contour is termed a cup and cone fracture because one of the melting

surfaces is in the form of cup, the other like a cone. In this type of fractured specimen, the

central interior region of the surface has an irregular and fibrous appearance, which is

indicative of plastic deformation.

Brittle Fracture

Brittle fracture takes place without any appreciable deformation, and by rapid crack

propagation. The direction of crack motion is very nearly perpendicular to the direction of the

applied tensile stress and yields a relatively flat fracture surface.

When gradual tensile load is applied on material in tensile test, at the end of elastic limit,

without any prior indication material breaks. This type of fracture is called as brittle fracture.

Fig. (a) Shows that, there is no change in diameter of material after fraction and fig. (b)

shows that, material breaks at the end of elastic region. There is no plastic region in the brittle

material.

Steps to lead Brittle fracture:

When stress is applied, the bonds between grains in the material are elastically strained. At

some critical stress within elastic limits bond between grain break without any prior

indication, remember the material is brittle and there is no plastic deformation.

Factors Affecting Fracture

A factor affecting the fracture of a material includes:

(i) Stress concentration

(ii) The speed with which the load is applied

(iii) The temperature

(iv) Thermal shock loading

(1) Stress concentration: [ Notch sensitivity]:

A stress concentration is a location in an object where stress is concentrated. An object is

strongest when force is evenly distributed over its area, so a reduction in area, e.g., caused by

a crack, results in a localized increase in stress. A material can fail, via a propagation crack,

when a concentrated stress exceeds the material’s theoretical cohesive strength. The real

fracture strength of a material is always lower than the theoretical value because most

material contains small cracks or contaminants that concentrate stress. Failure cracks always

start at stress raises, so removing such defects increases the fatigue strength.

Notch sensitivity is defined as ‘A reduction in properties due to the presence of

stress Concentration.’ Any kind of irregularities produces stress concentration in material

such as

1. Crack

2. A grain Boundary

3. An internal corner of engineering part.

If you want to break a small piece of material one way is to make a small notch in the surface

of the material and then apply a force. The pressure of the notch or any sudden change in

section of a piece of material can significantly change the stress at which failure occurs. The

notch or sudden change in section produced in the metal is called as stress concentration.

They disturb the normal stress distribution and produce local concentration of stress. The

effect of various factors on increase in stress is as follows,

An increase in stress

Increases depth of notch

Reduces radius of tip

Increases change in section

A crack in a brittle material will quite have a pointed tip and hence a small radius. Such a

crack thus produces a large in stress at its tip, one way of arresting this progress of such a

crack is to drill a hole at the end of the crack to increase its radius and so to reduce the stress

concentration.

In brittle material, fracture will occur if the stress concentration exceeds. e.g. decrease in the

strength of material. It is shown in fig.

Notch sensitivity is defined as the reduction of nominal strength, static or impact, by presence

of stress concentration and is usually expressed as the ratio of the notched to the unnotched

strength. It is often determined by conducting tensile tests using both smooth and notched

specimen of the same material. Notched bar impact tests are often used to evaluate notch

sensitivity but they do not give ratio of notched to unnotched strength.

In ductile material, strains permit adjustments to be localized with a reduction of the stress

concentration. Notch sensitivity cannot have any relation unless we have specific knowledge

of the behaviour obtained experimentally of different classes of materials. It is shown in fig.

Eg. Good ductility in tension does not mean low notch sensitivity. There is no correlation

between yield or tensile strength and fatigue notch sensitivity.

2] The speed with which load is applied:

A sudden application of load and impact loading may lead to fracture where the same stress is

applied more slowly, it would not break. With avery high rate of application of stress there

may be insufficient time for plastic deformation of a material to occur and so what, under

normal conditions a material behaves ductile through it were brittle.

The Charpy and Izod test give a measure of the behavior of notched sample of material where

subject to a sudden impact load. The results are expressed in terms of the energy needed to

break a standard size test piece. The smaller the energy needed, the easier it is for failure. low

energy associated with material is termed brittle. Ductile materials need higher energy

for fracture.

3] Thermal Shocks:

Pouring hot water into a cold glass can cause it to crack. This is the cause of thermal shock

loading. The layer of glass in contact with hot water is trying to expand but is restrained by

the colder outer layers of the glass. These layers are not heating up quickly because of poor

thermal conductivity of glass. Result is the stressing up of stresses which can be sufficiently

high to cause failure of brittle glass.

Ductile Brittle Transition in Steel:

The temperature of a material can affect its behaviour when subject to stress. Many material

which are ductile at high temperature are brittle at low temperature e.g. steel may behave as a

ductile material above say 0°C but below that temperature, it becomes brittle.

The notched bar impact test can be used to determine whether or not a material experience a

ductile to brittle transition as a temperature decreased. In such transition at a higher

temperature the impact energy is relatively large since the fracture is ductile. As the

temperature is lowered, the impact energy drops over narrow temperature range as a fracture

becomes more brittle.

The transition can also be observed from the fracture surface, which appear fibrous or dull for

totally ductile fracture, and granular and shiny for totally brittle fracture . Over the ductile to

brittle transition features of both type will exist. While for pure materials the transition may

occur very suddenly at a particular temperature, for many materials the transition occur over

a range of temperature. This causes difficulties when trying to define a single transition

temperature and no specific criterion has been established . If a material experience a ductile

to brittle transition , the temperature at which it occurs can be affected by a variables mention

earlier , namely the strain rate , the size and shape of the specimen and relative dimensions of

the notch.

Fig shows how the impact tests result might vary with temperature. Properties of steel

changes with temperature. It behave as a ductile above 0°C temperature. If temperature is

reduced below 0°C, steel shows brittle in nature.

The ductile brittle transition is exhibited in BCC metals , such as low carbon steel , which

becomes brittle at low temperature at very high strain rates. FCC metals , however generally

remain ductile at low temperatures.

In metals plastic deformation at room temperature occurs by dislocation motions, The stress

required to move a dislocation depends on the atomic bonding, crystal structure and obstacle

such as a solute atoms, grain boundaries, precipitate particle and other dislocations. If the

stress requires moving the dislocation is too high, the metal will fail instead by the

propagation of cracks and the failure will be brittle. Thus either plastic flow (ductile failure)

or crack propagation (brittle failure) will occur, depending on which process requires

smaller applied stress.

In FCC metals, the flow stress i.e the force required to move dislocation , is not strongly

temperature dependent . Therefore dislocation movement remain high even at low

temperature and the material remain relatively ductile.

Plastic flow depends upon the movement of dislocation and this occurs in some finite times.

As a temperature decreases, the movement of dislocation becomes more difficult and this

increases the possibility of internal stress exceeding yield stress at some instant. The mode of

fracture changes at some temperature or in certain temperature range. This temperature

or temperature range is called Ductile-Brittle transition temperature or range. At Ductile

transition temperature .the stress to propagation a crack = σf is equal to yield stress σy i.e.

σf = σy

At transition temperature ,σf = σy Temperature below transition ,σf<σy Temperature above

transition ,σf>σy

1) Factors Affecting on Ductile-Brittle Transition Temperature : Following factors affect

Ductile-Brittle transition temperature.

- when grain size of material increases .

- when alloying elements are added in the material.

- when impurities in the material increases.

- when percentage of carbon in steel increases.

Fig. shows Ductile-Brittle transition for iron and steel having 0.45 % C and 0.6%C.

Common BCC metals becomes brittle at low temperature high rates of strain. Many FCC

metals on the other hand, remain ductile even at low temperature. Polycrystalline HCP metals

are brittle, as a there are not enough slip systems to maintain grain boundary integrity.

Griffith’s Theory For Brittle Fracture:

According to Griffith, there are micro cracks in the metal that cause local concentration of

stress to values high enough to propagate to crack and eventually to fracture the metal.

According to Griffith, ‘A crack will propagate when the decrease in elastic strain energy is at

least equal to the energy required to create the new crack surface.’ In Griffith’s theory, an

energy method is employed to estimate the stress necessary to cause a crack to propagate.

Assumptions to Griffith’s Theory:

1. Consider lens (elliptical) shaped crack of length ‘2C’.

2. Material of ‘I’ unit thickness.

3. Crack run from the front to back face.

4. Longitudinal tensile stress ‘ ‘ is applied as shown in Fig.

5. The crack tends to increase its length to transverse direction.

6. If crack spreads,the surface area of crack increase , while the elastic strain energy stored

in the material decreases, because strains cannot be continuous across the cracked region.

7. ‘ ‘ is surface energy per unit area of the material.

8. No energy (elastic) is stored in cylindrical volume around crack.

9. Elastic energy is released when crack is introduced.

Consider simple cubic structure, within which there is a crack as shown in fig. We can think

of each atom in the crystal being rather like a sphere linked to neighboring atoms by springs.

The springs represent the bonds between the atoms. When the material is stretched, the

springs are stretched and elastic strain energy is stored in them.

For energy stored per unit volume is a stretched material is =1/2*Stress*strain. When bond

breaks, there will be a release of elastic energy, but breaking of bonds creates new surface.

The crack can be considered to propagate. When the released elastic energy is just sufficient

to provide the energy needed to create the new surfaces. The crack will not propagate if

the released elastic energy is not large enough to supply the required surface energy.

Suppose,we have an elliptical internal crack as shown in fig.The elastic strain energy that has

been released per unit volume by the crack.

= 1/2n*Stress*Strain

For each upper and lower surfaces of crack.

(1) Total elastic energy per unit volume (Ue) when crack introduced is,

Ue=( * * *e*2…. (1)

Where Ue= total elastic energy per unit volume

ϭ= Stress applied

e = Strain in the material

E = Young’s modulas

T = Unit thickness =1

C = length of crack

We know that, E = …. (2)

2’ at the end of equation (1) is for upper and lower surfaces.

Put value of strain (e) from equation (2) into equation (1), we have,

= *2

(t=1 unit thickness)

…. (3)

(2) Total surface energy ( U ):

Taking into consideration. The plastic deformation of the surfaces of crack requiring energy

‘p’ per unit area, the total energy required to create the crack is,

U =4C( …. (4)

Where ,

Energy required producing unit area of new surface.

Negative (-) sign of elastic energy indicates that, elastic energy stored in the

material is released as crack forms.

In fig U is plotted a function of ‘c’ , showing that, for a given value of ‘ϭ , U passes

through a maximum at a critical value of ‘c’ equal to ‘c*’. The larger is the value of ‘ϭ ,the

smaller is ‘c’ critical value can be found by setting d( U)/dc =0 As the applied tensile stress is

the external variable for a given material with the crack of length ‘2c’ , it is appropriate to

express the critical condition as a critical fracture stress ϭcrit

Equation (6) shows, the relationship between the length of the existing crack (2c). Energy ‘p’

Expends in plastic deformation and the stress required to fracture a metal.

Griffith’s theory in its original form is applicable only to a perfectly brittle material such as

glass. However, Griffith’s ideas have had great influence on the thinking about the fracture of

metals.

Fracture Toughness

Defects and cracks are present in all engineering materials. They may be introduced during

solidification or heat treatment stages of the material. The fracture resisting capacity of

machine component or engineering structures must be evaluated in the presence of

cracks or discontinuities is known as its fracture toughness.

One of the measures of fracture toughness of a material is the elastic strain release rate ‘G ’.

When the rate of release of elastic strain energy at the tip of crack reaches a high value, then

the crack propagates. The critical value is given by symbol ‘ Gc ’.

From Griffith type of approach, the fracture toughness is defined by the critical value of

parameter ‘Gc’. ‘Gc’ gives the value of the strain energy release per unit area of the crack

surface, when unstable crack extension (leading to fracture) takes place. For an

elastic crack of length 2c.

where γ = Energy required to produce unit area of new surface.

P = Plastic Deformation.

The Griffith’s equation can be written in the form of ‘Gc’ as follows,

Factors affecting on Fracture Toughness:

The fracture toughness of a material is affected by a numver of factors.

(i) Composition of the material:

Different alloys systems have different fracture toughness. E.g. aluminium alloys have less

fracture toughness than steel. Some alloying elements such as sulphur and phosphorus

reduce fracture toughness of steel.

(ii) Heat treatment:

Changes in temperature affect fracture toughness of material. Increase in temperature

reduces fracture toughness.

(iii) Service Conditions:

Service conditions such as temperature, corrosive environment and fluctuating load affect

fracture toughness. Increase in temperature, increase in corrosion environment and increase

in fluctuating load, reduces fracture toughness.

Fatigue

Fatigue is a form of failure that occurs in structures subjected to dynamic and fluctuating

stresses (e.g. bridges, aircraft and machine components). Under these circumstances it is

possible for failure to occur at a stress level considerably lower than the tensile or yield

strength for a static load. The term ‘fatigue’ is used because this type of failure normally

occurs after a lengthy period of repeated stress or strain cycling. Fatigue is important in as

much as it is the single largest cause of failure in metals, estimated to comprise

approximately 90% of all metallic failures; polymers and ceramics(except for glasses) are

also susceptible to this type of failure. Furthermore, fatigue is catastrophic and insidious,

occurring very suddenly and without warning.

Fatigue failure is brittle like in nature even in normally ductile metals, in that there is very

little, if any, gross plastic deformation associated with failure. The process occurs by the

initiation and propagation of cracks, and ordinarily the fracture surface is perpendicular to the

direction of an applied tensile stress

A fatigue failure can usually be recognized from the appearance of the fractured surface. Fig.

shows a smooth region, due to the rubbing action as the crack propagated through the section.

A rough region, where the member has failed in a ductile manner when the cross section was

no longer able to carry load. Frequently the progress of the fracture is indicated by a series of

rings or ‘bench marks’ progressing inward from the point of initiation of the failure.

Three basic factors are necessary to cause fatigue failure. These are:

(i) A maximum tensile stress of sufficiently high value.

(ii) A large enough variation or fluctuation in the applied stress.

(iii) A sufficiently large number of cycles of the applied stress.

In addition, there are a lots of other variables, such as stress concentration, corrosion,

temperature, overload, metallurgical structure, residual stresses and combines stresses, which

tends to alter the conditions for fatigue.

Stress Cycles

It is advantageous to define briefly the general types of fluctuating stresses which can cause

fatigue. Fig serves to illustrate typical fatigue stress cycles. Fig (a) illustrates a ‘completely

reversed cycle’ of stress of sinusoidal form. For this type of stress cycle the maximum and

minimum stresses are equal. Tensile stress is considered positive and compressive stress is

negative.

Fig(b) illustrates a ‘repeated stress cycles’ in which the maximum stress σmax and minimum

stress σmin are not equal in this illustration they are both tension, but a repeated stress cycle

could just as well contain maximum and minimum stresses of opposite signs or both in

compression.

Fig(c) illustrates a complicated stress cycle which might be encountered in a part such as an

aircraft wing which is subjected to periodic unpredictable overloads due to gusts.

A fluctuating stress cycle can be considered to be made up of two components, a mean or

steady, stress ‘σm’ and an alternating or variable stress ‘σa’. We must also consider the range

of stress ‘σr’.As shown in Fig (b), the range of stress is the algebraic difference between

maximum and minimum stress in a cycle.

σr= σmax –σmin

The alternating stress,

σa = σr/2 = (σmax – σmin)/2

The mean stress is the algebraic mean of the maximum and minimum stress in the cycle.

σm = (σmax + σmin) /2

Mechanism of fatigue failure

Crack initiation and propagation

The process of fatigue failure is characterized by three distinct steps: (1) crack initiation,

wherein a small crack forms at some point of high stress concentration; (2) crack

propagation, during which this crack advances incrementally with each stress cycle; and (3)

final failure, which occurs very rapidly once the advancing crack has reached a critical size.

Cracks associated with fatigue failure almost always initiate (or nucleate) on the surface of a

component at some points of stress concentration. Crack nucleation sites include surface

scratches, sharp fillets, keyways, threads, dents etc.

Crack Growth:

The crack formed on surface then develops slowly into the material in a direction roughly

perpendicular to the main tensile axis. Ultimately the cross-sectional area of the member will

have been so reduced that it can no longer withstand the applied load and ordinary tensile

fracture will result.

Fracture:

A fatigue crack ‘front’ advances a small amount during each stress cycle and each increment

of advance is shown on the fracture surface as a minute ripple line. These ripple lines radiate

out from the origin of fracture as a series o approximately concentric arcs. Few ripples much

larger than the rest and shows the general path which the crack has followed.

Fig shows mechanism of fatigue failure in three stages. The crack propagates slowly from the

source, the fracture surfaces rub together due to pulsating nature of the stress and so the

surface becomes burnished. Fatigue failures in metals are very easy to identity. The fatigue

cracks are not result of brittle fracture but of plastic slip.

Fatigue Testing

The method of testing the metal for fatigue was developed by Wohler. Fig shows a typical

experiment set up for Wohler’s set up. The specimen is in the form of cantilever and loaded

at one end through ball bearing. It is rotated by means of high speed motor to which a counter

is attached to count the number of rotations. At any instant, the upper surface of the specimen

is under tension and the lower surface is under compression, with the neutral axis.

In one rotation, the specimen undergoes two cyclic fluctuations of stress. The number of

cycles to cause failure will vary with the applied stress,lower will be the cycles to cause

failure. Similarly, if the stress is lowered, more number of cycles will be required to cause

failure.

Fatigue Limit or Endurance Limit:

When a series of stress is plotted against the number of cycles to failure at each stress on a

semi log scale,for iron, steel and ferrous alloys, a stress is reached below which fracture does

not occur for infinite number of cycles .This value of stress is called fatigue limit or

endurance limit. It is shown in fig (a). However, for non-ferrous metals and alloys like Al,

Mg, Cu and their alloys no such definite limit exists and as the stress is decreased, the

number of cycles to failure progressively goes on increasing. These materials do not have a

definite limit and hence it is customary to report fatigue strength (or endurance strength)

instead of fatigue limit. It is shown in fig (b).

Endurance or fatigue strength is the maximum stress that can be applied repeatedly over a

specified number of cycles without fracture (usually 108 cycles).

Factors affecting fatigue life:

a) Mean Stress

The dependence of fatigue life on stress amplitude is represented on the S-N plot. Such data

are taken for a constant mean stress σm , often for the reversed cycle situation (σm=0). Mean

stress, however, will also affect fatigue life; this influence may be represented by a series of

S-N curves, each measured at a different σm, as depicted schematically in the fig.. As

may be noted increasing the mean stress level leads to an decrease in fatigue life.

b) Surface effects

For many common loading situations, the maximum stress within a component or structure

occurs on its surface. Consequently most cracks leading to fatigue failure origin at surface.

positions, specifically at stress amplification sites. Therefore it has been observed that fatigue

life is especially sensitive to the condition and configuration of the component surface.

c) Design factors

The design of a component can have a significant influence on its fatigue characteristics. Any

notch or geometrical discontinuity can act as a stress raiser and fatigue crack initiation site;

these design features includes grooves, holes, keyways, threads and so on. The sharper

the discontinuity (i.e. smaller the radius of curvature), the more severe the stress

concentration. The probability of fatigue failure may be reduced by avoiding (when

possible) these structural irregularities, or by making design modifications whereby sudden

contour changes leading to sharp corners are eliminated.

d) Corrosion fatigue

The simultaneous action of cyclic stress and chemical attack is known as corrosion fatigue.

Corrosive attack without superimposed stress produces pitting of metal surface. The pits acts

as notches and produce a reduction in fatigue strength. When corrosion and fatigue occur

simultaneously, the chemical attack greatly accelerates the rate at which fatigue cracks

propagate. Materials which show a definite fatigue limit when tested in air at room

temperature shows no indication of a fatigue limit when the test is carried out in a corrosive

environment. When fatigue test is carried out in air not affected by the speed of testing, over

a range from 10 to 200 Hz, when the test is carried out in corrosive environment there is

definite dependence on testing speed. Since corrosive attack is a time-dependant

phenomenon, the higher the testing speeds, the smaller the damage due to corrosion. In usual

method, corrosion-fatigue test is carried out by continuously subjecting specimen in

combined influences of corrosion and cyclic stress until failure occurs.

A number of methods are available for minimizing corrosion-fatigue damage. In general,

material is protected from corrosive environment by metallic and non-metallic coatings

which are successful methods. Nitriding is effective in minimizing corrosion fatigue.

Fatigue failure can be produced by fluctuating thermal stresses under conditions where

no stresses are produce by mechanical causes. Thermal stresses result when the change

in dimensions of a member as a result of temperature changes. For the simple case of a bar

with fixed end supports, the thermal stress developed by a temperature change ’ΔT’ is

σ =α.E.ΔT

where α = linear thermal coefficient of expansion

E = elastic modulus

If failure occurs by one application of thermal stress, the condition is called thermal shock.

However, if failure occurs after repeated applications of thermal stress, of a lower magnitude,

it is called thermal fatigue. It exists at high temperature. Austenitic stainless steel is sensitive

to this phenomenon because of its low thermal conductivity and high thermal expansion.

f) Pre-Stressing

Pre-stressing is the process of loading an engineering component under controlled conditions

to a cyclic stress for a fixed number of cycles prior to any possibility of fatigue failure. When

magnitude of pre-stressing is lower than the operating stress level then it is known as under-

stressing.

Under controlled conditions when the magnitude of stress is higher than operating stress level

the condition is called overstressing. The number of pre-stressing cycles is always lesser than

the number of cycles needed to cause fatigue failure. Pre-stressing is desirable under

conditions of under-stressing. An under stressed component always exhibits best fatigue

resistance.

For an over-stressed component the failure resistance is lesser compared to a component that

has been under-stressed. Thus under stressing cause significant improvement in fatigue

behavior by strengthening the weak regions and enhancing their dynamic response to

operating stress

g) Notch effect

Fatigue strength of material is reduced by presence of notch in the material. In machine

elements it contains fillets, keyways screw threads and holes. Fatigue cracks in structural

parts usually start at such geometrical irregularities.

The effect of fatigue is generally studied by specimen containing a ‘V’ notch or a circular

notch. The effect of notches on fatigue strength is determined by comparing the S-N curves

of notched and unnotched specimens.

Creep:

Introduction

Materials are often placed in service at elevated temperatures and exposed to static

mechanical stresses (eg. Turbine rotors in jet engines and steam generators that

experience centrifugal stresses, and high pressure steam lines). Deformation under such

circumstances is termed creep. Defined as the time-dependent and permanent deformation of

materials when subjected to a constant load or stress, creep is normally an undesirable

phenomenon and is often the limiting factor in the lifetime of a part. It is observed in all

material types; for metals it becomes important only for temperatures greater than about

0.4Tm (Tm = absolute melting temperature).

Effect of Temperature on Mechanical behavior of Materials

When temperature of material increases the mobility of atoms increases rapidly thus ,

changes mechanical properties of material. High temperature will also result in greater

mobility of dislocations by the mechanism of dislocation climb. The equilibrium

concentration of vacancies increase and new deformation may form at high temperature. In

some metals, the slip system changes or additional slip system are introduced with

increasing temperature. At high temperature, cold-worked metals will recrystallize and

undergo grain coarsening while age hardening alloys may overage by prolonged exposure at

high temperature and loose strength.

Successful use of metals at high temperatures involves a number of problems but

modern technology demands better high temperature strength and oxidation resistance.

Creep Data Presentation

For a long time the principal high-temperature applications were associated with steam power

plants, oil refineries and chemical plants. The operating temperature in equipments such as

boilers and steam turbine exceeds 500oC. With the introduction of gas turbine engine,

requirements developed for materials to operate in critically stressed parts, like turbine

buckets at temperatures around 800oC.

An important characteristic of high-temperature strength is that it must always be considered

with respect to some time scale. The tensile properties of most engineering metals at room

temperature are independent of time for practical purposes.

High temperature strength data is commonly measured in creep strength or rupture strength.

Creep strength is defined as the stress at a given temperature which produces a steady-state

creep rate of fixed amount: rate in range 10-11

to 10-8

per second are typical i.e., 0.000004 to

0.004 percent per hour. Or in an alternate way creep strength can be defined as the stresses to

cause strain of 1 percent at given temperature. Rupture strength refers to the stress at a given

temperature to produce a life to rupture of certain amount usually in 1000,10000 or 1,00,000

hours.

It frequently is important to be able to extrapolate creep or stress-rupture data into regions

where data are not available. Therefore, common methods of plotting creep data are based on

plots which yield reasonable straight lines. Fig. shows the common method of presenting the

influence of stress on steady-state or minimum creep rate.

Note that a log-log plot is used, so that extrapolation of one log-cycle represents a tenfold

change. A change lope of the line will sometimes occur. It has been shown that the c\value of

the minimum creep rate depends on the length of time the creep test has been carried out. It

has been shown for long-time creep test (t>10000 hours) that the creep strength based on 1

percent creep strain is essentially equal to creep strength based on true minimum creep rate.

Creep Testing

The specimen to be tested is placed in the electric furnace where it is heated to

given temperature. The usual method of creep testing consists of subjecting the specimen at

constant tensile stress at constant temperature and measuring the extent of deformation or

strain with the time. The typical creep testing machine is shown in Fig. Even though, it

appears to be simple, it requires considerable laboratory equipment, great care and precision

in performance. The time of each test may be a matter of hours, weeks, months or even years.

Creep is also determined in compressing, shear and bending.

The data is presented by plotting creep curve as deformation (or strain) verses time at

constant temperature and stress. The test specimen may be circular, square or rectangular in

cross-section. Either a continuous record of deformation with time or sufficient number of

deformation reading with time should be taken over the entire period of test. The strain is

measured by strain gauge.

The strain generally lies between 0.1 to 1.0% and the period of testing does not exceed

10,000 hours. If creep is continued until fracture occurs, the test is called as ‘creep-rupture’

test. It determines the time necessary for fracture of the test piece

Mechanism of Creep Failure

Creep is a deformation process in which three main features to be involved are:

(1) The normal movements of dislocations along slip planes.

(2) Process ‘dislocation climb’ which is responsible for rapid creep at temperatures Above

0.5 Tm.

(3) Slipping at grain boundaries.

(4) Following mechanisms are known to be responsible for creep in crystalline Materials.

(5) Dislocation climb.

(6) Vacancy diffusion.

(7) Grain boundary sliding.

1) Dislocation Climb:

In the primary stages of creep, dislocations move quickly at first but soon become piled-up at

various barriers. At temperatures in excess of 0.5 Tm, thermal activation is sufficient to

promote a process known as ‘dislocation climb’. This would bring into use new slip planes

and so reduce the rate of work hardening.

In addition to plastic deformation by dislocation movement, deformation by a form of slip at

grain boundaries also occurs during the secondary stage of creep. These movements possibly

leads to the formation of ‘vacant sites’, that is lattice positions from which atoms are missing

and thus in turn makes possible ‘dislocation climb’.

In the tertiary stage of creep micro-cracks are initiated at grain boundaries due to the

movement of dislocations. In some cases, there is migration of vacant site, as a result necking

and consequent rapid failure follows.

In the low temperature region of creep, the cross-slip continues with the aid of thermal energy

and causes further plastic strain as function of time. In edge dislocation, burger’s vector and

‘t’ vector are perpendicular to each other. They cannot cross-slip like screw dislocations.

However, if the temperature I high enough for an appreciable diffusion rate of vacancies,

these may diffuse to the edge dislocations or away from them, making them to climb up or

down.

(2)Vacancy Diffusion:

The diffusion of vacancies control creep rate. In this mechanism, grain boundary acts as a

source and sinks for vacancies. The mechanism depends on the migration of vacancies from

one side of a grain to another. Referring to fig a grain ABCD is under stress ‘P’, the atoms

moved from faces ‘BC’ and ‘AD’, along the path shown and the grain creeps in the direction

of stress. Movement of atoms creating vacancies on faces ‘AB’ and ‘DC’ and destroying

themon the other faces.

(3) Grain Boundary Sliding:

The sliding of neighbouring grains with respect to the boundary that separates them .Fig

shows that, grain boundaries lose their strength at lower temperature than the grain

themselves. This effect arises from non-crystalline structure of the grain boundaries.

Creep Curve

Fig. illustrates the idealized shape of a creep curve. The slope of this curve (dε/dt ) is referred

to as the creep rate.

A typical creep test consists of subjecting specimen to a constant load or stress while

maintaining the temperature constant; deformation or strain is measured and plotted as a

function of elapsed time. Most tests are the constant load type, which yield information of an

engineering nature; constant tests are employed to provide a better understanding of the

mechanisms of creep.

Figure is a systematic representation of the typical constant load creep behaviour of metals.

Upon application of the load there is an instantaneous deformation, has indicated in the

figure, which is mostly elastic. The resulting creep curve consists of three regions, each of

which has his own distinctive strain-time feature. Primary or transient creep occurs first,

typified by a continuous decreasing creep rate; that is, the slope of the curve diminishes with

time. This suggests that the material is experiencing an increase in creep resistance or strain

hardening deformation becomes more difficult as the material is strained. For secondary

creep, sometimes termed steady-state creep, the rate is constant; that is, the plot becomes

linear. This is often the stage of creep that is of the longest duration. The constancy of creep

rate is explained on the basic of a balance between the competing processes of strain

hardening and recovery, recovery being the process whereby a material becomes softer and

retains its ability to experience deformation. Finally, for tertiary creep, there is an

acceleration of the rate and ultimate failure. This failure is frequently termed rupture and

results from micro structural and/or metallurgical changes; for example, grain boundary

separation, and the formation of internal cracks, cavities, and voids. Also, for tensile loads, a

neck may form at some point within the deformation region. These all lead to a decrease in

the effective cross-sectional area and an increase in strain rate.

Analysis of classical Creep Curve:

Andrade’s work on analysis of classical creep curve is focused on topic of creep. He

considered that the constant stress creep curve represents the superposition of two separate

creep processes which occur after the sudden strain which results from applying the load. The

first component of the creep curve is a ‘transient creep’ with a creep rate decreasing with

time. Added to this is a constant-rate ‘viscous creep’ component. Fig. shows Andrade’s

analysis of the classical creep curve.

Andrade found that the creep curve could be represented above the following empirical

equation.The various stages of the creep curve shown in fig. require further explanation. It is

generally considered that the creep curve has three stages. In British terminology the

instantaneous strain designed by in figure is often called the first stage of creep, so that with

this nomenclature the creep is considered to have four stages such as

(1) Instantaneous strain.

(2) Primary of transient creep

(3) Secondary or steady state creep.

(4) Tertiary of viscous creep.

21. Creep resistant materials and creep fracture :-

Creep resistant materials:

Creep resistant materials are required for structural and machine components used at elevated

temperatures. They should be capable of withstanding these temperatures without undergoing

creep beyond the specified limit, which may cause dimensional changes beyond permissible

limit used in the design.

The following are the requirements of a creep resistant material:

(i) It should have high melting points because, the creep becomes significant above

0.4Tm(Tm =melting point). If melting point is high material can be used at higher

temperature, e.g. iron, nickel and cobalt.

(ii) It should have course grained structure. The grain boundary region becomes quasi-

viscous at creep temperature. Since in course grained materials grain boundary area is less, so

that less amount of quasi-viscous region is formed with a less tendency to flow, reducing the

creep deformation.

(iii) It should be precipitation hardenable. It should have fine insoluble precipitates at the

operating temperature. If coherent precipitates are present maximum creep resistance is

obtained e.g. in nickel base and iron-nickel base super alloys coherent precipitates of Ni3 (Al,

Ti) is formed.

(iv) Dispersion hardening improves creep resistance.

(v) It should have high oxidation resistance i.e. the oxide film should follow either a

logarithmic or a cubic law of growth.

Creep fracture:

At lower temperatures grain boundaries are stronger and at high temperatures it becomes

weak as compared to grains. The temperature at which the strength of grain boundary is equal

to the strength of the grain is called Equi-cohesive Strength.

Material always fails at weak region by initiating crack. When crack occurred below equi-

cohesive temperatures, fracture is transgranular or transcrystalline i.e. the crack moves

through the grains. When crack occurred above equi-cohesive temperature, fracture is

intergranular or intercrystalline and it moves along the grain boundaries as shown in figure.

Module 3 : Theory of Alloys

& Alloy Diagrams

1. Introduction to Alloy: Pure metals are rarely used for engineering purposes except where high electrical

conductivity, high ductility or good corrosion resistance are required. These

properties are generally at a maximum value in pure metals, but mechanical properties

such as tensile strength, yield point and hardness are improved by alloying. In the

most cases general procedure in making an alloy is the first to melt the metal having

the higher melting point and then to dissolve in it the metal with the lower melting

point, stirring the mixture so that a homogeneous liquid solution is formed.

The definition of the alloy is given below:

Alloy: Alloy is a mixture of two or more elements having metallic properties. The

element present in the largest proportion is a metal and the others can be metal or

non-metal The element which is present in the largest amount is called as the base metal or

parent metal or solvent and the other elements are called as alloying elements or

solute.

2.The Solid Solution: It is an alloy in which the atoms of solute are distributed in the solvent and has the

same structure as that of the solvent. Solid solutions have different compositions with

similar structures and are like liquid solutions such as sugar in water.

Solid solutions are of two types: (i) Substitutional and (ii) Interstitial.

Substitutional solid solution means the atoms of B element i.e.solute are substituted at

the atomic sites of A element that is solvent. Depending on the distribution of B

atoms in A, substitutional solid solutions are classified into two types:

(A) Regular or Ordered and(B) Random or Disordered In a regular solid solution, the substitution of B atoms in A is by a definite order Page 2

(Fig.(a)) while there is no definite order or regularity in random solid solution.

(Fig.(b))

Complete regularity throughout the structure is possible only when the two metals are

mixed in some fixed proportion like 1:1, 3:1, etc. Substitutional solid solution

Formations favored if the atomic sizes of the two metals are nearly equal. Ordered

substitutional solid solution alloys in general are hard and require more energy for

plastic deformation then disordered substitutional solid solution alloys.

In Interstitial solid solutions, the atoms of B occupy the interstitial sites of A (Fig.5.2). This type

of solid solution formation is favored when the atomic size of B is very much small as c ompared

to the atomic size of A. The elements which can form interstitial solid solution with iron are carbon, boron, oxygen, hydrogen and nitrogen.

In general, solid solutions are soft, ductile and malleable and therefore, they can

be easily cold rolled, pressed or worked.

Intermediate phases:

When an alloy element (solute) is added to a given metal (solvent) in such an amount

that the limit of solid solubility is exceeded, a second phase appears with the solid

solutions. This second phase maybe another solid solution or an intermediate phase.

Page 3

These intermediate faces differ in composition as well as crystal structure from the

Parent metals and hence their properties are also different. These phases may have Either narrow or wide ranges of homogeneity and may or may not have simple chemical formula . Some intermediate faces have a fixed composition and they are called intermetallic compounds.

In general, intermetallic compounds are hard and brittle and have high melting points. The inter

mediate phases in which the ratio of number of free electrons to the number of atoms is constant

are called electron compounds and they exhibit similar characteristics.

3.HUME - ROTHERY'S Rules of solid solubility

In formation of solid solutions, the solubility limit of solute in the solvent is governed by certain factors. These are known as HUME - ROTHERY'S Rules of solid solubility. They are as below Relative- size Factor

The size factor is favorable for solid solution formation when the difference in atomic radius is

less than about 15 percent, if the relative size factor is greater than 8 percent but less than 15

percent, the alloy system usually shows a minimum. If the relative size factor is greater than 15

percent , solid formation is very limited. Chemical-affinity factor

The greater the chemical affinity of two metals, the more restricted is their solid solubility and

greater is their tendency towards compound formation. Generally the farther apart the elements

are in the periodic table, the greater is their chemical affinity. Relative-valance factor

If the solute metal has different valance from that of the solvent metal, the number of valance

electrons per atom, called the electron ratio, will be changed. Crystal structures are more

sensitive to a decrease in the electron ratio than to an increase. In other words, a metal of lower

valance tends to dissolve more of a metal of higher valance than vice versa. Crystal-structure factor

Complete solid solubility of two elements is never attained unless the elements have the same type of crystal lattice structure.

Page 4

4. Allotropic forms of IRON

Some metals change their crystal structures when there is change in temperature, this tendency of

metals is called as allotropic. Iron is allotropic in nature. Iron is in liquid form above 1539 C

liquid is converted into δ-iron(ferrite) which is in the body centered cubic structure. During

cooling process at 1400C δ-iron is converted into γ-iron which is face centered structure. Below 910C γ-iron(austenite) is converted into α-iron(ferrite), which is again a body centered structure.

α-iron is non magnetic upto 768C. If it cools below 768 C, it is in the BCC structure but

magnetic in nature upto room temperature.

Iron changes its crystal structure from bcc to fcc, this change in crystal structure is called

as allotropy. These allotropic transformation is shown in figure. Influence of carbon in IRON-CARBON alloying

Steels are alloys of Iron and carbon. The theoritical range of carbon in steels is from 0.008-

2.00%. The properties of steel depends on its carbon content. In general, as the carbon increases

there is increase in propotional stress, elastic stress, ultimate tensile strength, hardness, wear

resistance, resilience and hardenability and decreases in ductility, malleability, formability,

machinability and toughness.

When carbon percentage is more than 2%, material becomes hard and brital. Its ductility and

malaebility decreases. These materials cannot be forged, rolled, drawn or pressed into desired

shape, but casting process is applicable on these materials hence it is called as cast iron.

Carbon percentage controls the mechanical properties of steel and cast irons. Figure shows

stress-strain curve for 0.2%, 0.5%, 0.8% and 2.0% carbon steel. As increase in carbon

percentage, there is increases in yeild stress and changes all mechanical properties related to this.

When there is increase in carbon percentage, there is decrease in plastic region and at 2.0%

Page 5

carbon there is no plastic region and material fails at the end of elastic limit. When carbon

perceentage is more than 2%, it is called as cast iron. Due to increase in carbon percentage in

cast iron it becomes brittal and fails at the end of elastic region.

It means material will not show any permanent deformation before fracture. 5. IRON- IRON CARBIDE EQUILIBRIUM DIAGRAM :

The temperature at which the allotropic changes take place in iron is influenced by alloying

elements, the most important of which is carbon. The iron carbon alloy system is shown in fig.

This is the part between pure iron and an interstitial compound, iron carbide(Fe3C) containing

6.67 percent carbon by weight. Therefore we call this diagram as Iron-iron carbide equilibrium

diagram. This is not a true equilibrium diagram, since equilibrium implies no change of phase

with time. It is a fact, however, that the compound iron carbide will decompose in to iron and

carbon(graphite). This decomposition will take a very long time at room temperature, and even at

727°C it takes several years to form graphite. Iron carbide is called a metastable phase.

Therefore, the Iron-iron carbide diagram, even though it technically represents metastable

conditions, can be considered as representing equilibrium changes, under conditions of relatively

slow heating and cooling.

Page 6

The properties of alloy steels can also be discussed with the help of Fe-C (Or Fe3C)

equilibrium diagram by keeping in mind the influence of alloying elements on the Fe-C diagram.

It is shown in fig. It is essential to study the Fe-C equilibrium or phase diagram in detail.

CRITICAL TEMPERATURE LINES:

The temperature at which the phase change occurs during heating and cooling are called

critical temperatures. These temperatures are denoted by symbols and their meaning during

heating under equilibrium conditions are given below:

(1) A0(210°C):

At this temperature line cementite changes from magnetic to non magnetic character.

(2) A1(727°C):

At this temperature line pearlite transforms to austenite during heating. It does not depend

on the carbon content in the alloy. This transformation occurs at constant temperature of 727°C

called eutectoid temperature. Page 7

(3) A2:

At this temperature line ferrite changes its property from magnetic to non-magnetic during

heating. It is called as curie temperature.This line is at 0°C and 768oC temperature upto A3 line

then it is parallel to A3 line upto 0.8%C and 727oC temperature and then it is constant at 727°C

temperature upto 6.67 % carbon.

(4) A3:

At this temperature line the last trace of free ferrite gets dissolved to form 100% austenite. It

decreases from 910oC temperature and 0 % C to 727°C temperature and 0.8%C and then it is

constant at 727°C temperature upto 6.67%C.

For hypereutectoid steels, all A1,A2 and A3 lines coincide with the eutectoid temperature

and hence from 0.8%C to 6.67%C. It is also designated as A3,2,1.

(5)Acm:

At this temperature line,last trace of free cementite gets dissolved to form 100% austenite .

It is a function of carbon. It increases from 727°C temperature and 0.8% C to 1147°C

temperature and 2.0%C. It is also called upper critical temperature for cementite.

(6)A4:

At this temperature line, last trace of austenite gets dissolved to form 100% delta-ferrite. It

is a function of carbon. It increases from 1400°C temperature and 0%C to 1492°C temperature

and 0.1%C.

6. TRANSFORMATION IN Fe-Fe3C: Peritectic Transformation : In general, Peritectic transformation is denoted as:

Cooling

S1 + L S2

Heating where S1 and S2 are two different solids and L is liquid. Page 8

In Fe-Fe3C equilibrium diagram Peritectic reaction as shown in above fig. occurs at 1492°C

temperature and 0.18% carbon. The transformation is in the above figure: When δ -ferrite of 0.1%C combines with liquid having 0.55% C and forms γ (austenite) of

0.18%C, the amounts of δ (ferrite) and L can be found out by applying lever rule. The above

lever arm, shows the lever arm for calculating percentage of δ (ferrite) and liquid.

Lever arm shows at 0.1%C, δ- ferrite is 100%, at 0.55% C liquid is 100%and at 0.18% C, γ is 100%. Here 0.18% C is the pivot of lever arm. % amount of δ = (opposite arm length /total arm length) X 100

=((0.55-0.18)/(0.55-0.1))X 100

=82.2% % amount of liquid =(opposite arm length /total arm length) X 100 Page 9

=((0.18-0.1)/(0.55-0.1))X100 = 17.8%

For hypoperitectic steels (steels of carbon content less than 0.18%) the transformation is as follows :

1492o C

δ + L δ + γ

Eutectoid transformation : In general eutectoid transformation is denoted as:

Constant temp.

S1 S2 + S3

Where S1, S2 and S3 are different solid phases.

Fig above shows eutectoid region of Fe-Fe3C diagram. Eutectoid transformation in Fe-Fe3C

diagram is at 727°C temperature and 0.18% C and is as follows:

Page 10

Austenite (γ) of 0.8%C decomposes at 727°C temperature and forms mixture of ferrite

(0.025%C) and Fe3C (6.67%C). This eutectoid mixture is called as pearlite. Pearlite consists of

alternate lamillae of ferrite and cementite.

Fig above shows lever arm for calculating amount of ferrite and cementite in pearlite at room temperature. % amount of ferrite =(opposite arm length / total arm length) X100

=((6.67-0.8)/(6.67-0.008))X100

=88.10% %amount of cementite = ((0.8-0.008)/(6.67-0.008)) X 100

=11.9%

Austenite is not stable below 727°C temperature. When temperature is reduced below 727°C it is

converted into eutectoid mixture of α-ferrite and cementite which is called as pearlite. Which are

stable forms below 727°C. It is clear that ferrite lamilla is 7.4 times thicker than cementite

lamilla in the pearlite structure of 0.8%C.

Eutectic Transformation :

When two metals are completely soluble in their liquid state but completely insoluble in their solid state is called as eutectic. General reaction of eutectic transformation is: Constant

L S1 + S2

Page 11

Material Technology Theory of Alloys & Alloy Diagrams Module 3

Temperature

Where S1 and S2 are two different solids and L is liquid.

In Fe-Fe3C diagram as shown in Fig. below Eutectic reaction occurs at 1147°C temperature and

4.3%C and is as follows: Liquid of 4.3% C transforms at constant temperature of 1147°C into Austenite (γ) of 2.0%C and

cementite of 6.67 % C. This Eutectic mixture of Austenite and Cementite is called as ledeburite.

Austenite from ledeburite not stable below 727°C and transformed into Pearlite. At room

temperature, structure consist of pearlite and cementite. This mixture is called as Transformed

Ledeburite.

The amount of pearlite and cementite in transformed ledeburite at room temperature can be calculated by using lever rule. Fig above shows lever arm at room temperature. Page 12

% Amount of pearlite =(opposite arm length /total arm length) X 100

=((6.67-4.3)/(6.67-0.8)) X 100

=40.4% % Amount of cementite =((4.3-0.8)/(6.67-0.8)) X 100

=59.6%

Cementite is hard and pearlite is also fairly hard and therefore transformed ledeburite is also hard and brittle.

7.Slow cooling of steel 0.3 % C

Steel portion of iron –iron carbide equilibrium is of greatest interest and the various changes takes place during very slow cooling.

The changes in the structures are describe below separately for 0.3% C steel (hypoeutectoid) &

1.0% C Steel (Hypereutectoid). Slow cooling of 0.3% carbon steel from liquid state to room temperature.

Fig Shows steel portion of Iron-Iron Carbide Equilibrium Diagram. In this Diagram vertical Line

is drawn at 0.3% C steel from liquid state to Room temperature. The structural changes of this steel are marked at each point during cooling as follows: Point 1:

At point-1, iron carbon alloy is in liquid state. Point 2:

Upto point 2 alloy is in liquid state. At point 2 solidification process starts and some part of

liquid is trying to convert into δ-ferrite. Point 2-3: Just below point 2, liquid is trying to convert into δ-ferrite .This process continues till point 3,

At point 3 there is hyperperitetic reaction and it is possible to calculate exact percentage of liquid

and δ-ferrite at 1492oC. Lever arm for application of lever rule at 0.3% carbon and

1492oC constant temperature is shown in the below figure(b).

Page 13

% amount of δ= x100

= x100

= 55.56% % amount of liquid= x100

= x100

=44.44%

At point 3, at 1492o C there is 55.56% δ-ferite and 44.44% liquid steel in 0.3% C steel.

Page 14

Point 3-4:

At point 3, ’γ’ start separating out at grain boundaries of ‘δ’ ferrite From point 3-4 there is liquid and γ (austenite) and solidification process continued. Point 4-5: Point 4 is on solidus line. Upto this point solidification process continued and end on the point 4.

Just below point 4, there is 100% austenite. From point 4-5, there is no change in structure but

only decrease in temperature, from point 4 to 5, there is 100% γ (austenite).

Point 5-6:

At point 5,α starts separating out at the grain boundaries of γ, as temperature decrease the

amount of α ferrite increases.Point 5 is on critical temperature ‘A3’,where last trace of α

ferrite get converted into γ during heating.

Point 6 is on eutectoid line i.e. hypoeutectoid portion. At 727oC constant temperature it is

possible to apply lever rule for calculating exact % of α-ferritte and pearlite. Lever arm is

shown for application of lever rule at 0.3% C and 727oC constant temperature.

% amount of α= x100

= x100 Page 15

=64.5%

% amount of pearlite = x100

= x100

=35.5%

At point 6, at 727oC constant temperature, there is 64.5% α ferrite and 35.5% pearlite at 0.3% C

Steel Point 6-7:

Below point 6 , γ is not stable and tries to transform into some stable form. Stable form

available at equilibrium cooling just below 727oC is pearlite .So that, γ transform at

constant of 727oC to a laminar mixture of ferrite and cementite called as pearlite by

eutectoid transformation process.

Cooling from point 6-7 do not result in significant change in structure.The same structure is

observed at room temperature. At point 7 lever rule can be applicable to calculate exact

percentage of α –ferrite and pearlite at room temperature. Figure shows lever arm at 0.3% C and at constant room temperature

% amount of α= x100

= x100

=63.1%

% amount of pearlite = x100

=

x100 =36.9%

Page 16

At room temperature ,there is 63.1% α-ferrite and 36.9% pearlite.

8. Slow Cooling of 1.0% Carbon Steel from Liquid State to Room Temperature:

Fig. shows steel portion of Fe-Fe3C equilibrium diagram.

In this diagram vertical line is drawn at 1.0% C steel from liquid state to room temperature. The

structural changes for this steel are marked at each point during cooling as follows : Point 1:

At point 1, the iron-carbon alloy is in the liquid state . Point 2:

From point 1 to point 2 is in the liquid state .At point 2, solidification process starts and

liquid is trying to solidify into γ - phase. Page 17

Point 2-3:

Just below point 2, solidification process starts and liquid is converted into solid phase

‘γ ‘. At point 3, liquid is converted into 100% solid this point is on the solidus line. Point 3-4:

At point 3 ,liquid is converted into 100% γ (austenite). There is no change in structure

from point 3 to point 4. Point 4-5:

At point 4, Fe3C starts separating out on the grain boundaries of austenite. As the

Page 18

temperature decreases, amount of Fe3C increases and austenite decreases. Fe3C is intermetallic

compound.

Point 5 is on eutectoid line and 7270C constant temperature. The Lever rule can be

applicable at this constant temperature to calculate exact percentage of pearlite and Fe3C. Figure

above shows lever arm at 1.0%C steel and 7270C constant temperature.

% Amount of pearlite= Oppsite arm length/Total arm length x 100

= (6.67-1.0)/(6.67-0.8) X 100

= 96.6%

% Amount of Fe3C =(1.0-0.8)/(6.67-0.8) X 100

=3.4%

At point 5, as per lever rule, there is 96.6% pearlite and 3.4% Fe3C.

Point 5-6:

Just below point 5, γ is not stable and by eutectoid transformation convert into

pearlite. Pearlite is lamillar mixture of α-ferrite and cementite. Cooling from point 5 to point 6

does not result in significant change in structure. The same structure is observed at room

temperature.

Microstructure Changes from Liquid State to Room Temperature :

The microstructure observed at various points from liquid state to room temperature

by referring figure below:

For other steels, the sequence of structural changes are very much similar to above; only the amounts of phases will be different for each steel. Page 19

CAST IRON 9. INTRODUCTION:-

Cast irons, like steels, are basically alloys of iron and carbon. In the relation to the iron-iron

carbide diagram, cast irons contain a greater amount of carbon than that necessary to saturate

austenite at the eutectic temperature. Therefore, cast irons contain between 2 and 6.67 percent

carbon. Since high carbon content tends to make the cast iron very brittle, most commercially

manufactured types are in the range of 2.5 to 4 percent carbon.

The ductility of cast iron is very low, and it cannot be rolled, drawn, or worked at room

temperature. Most of the cast irons are not malleable at any temperature. However, they melt

readily and can be cast into complicated shapes which are usually machined to final dimensions.

Since casting is the only suitable process applied to these alloys, they are known as cast irons.

Although the common cast irons are brittle and have lower strength properties than most steels,

they are cheap, can be cast more readily than steel, and have other useful properties. In addition,

by proper alloying, good foundry control, and appropriate heat treatment, the properties of any

type of cast iron may be varied over a wide range.

Page 20

TYPES OF CAST IRONS:

The best method of classifying cast iron is according to the metallographic structure. There are

four variables to be considered which lead to the different types of cast iron, namely the carbon

content, the alloy and impurity content, the cooling rate during and after freezing and the heat

treatment after casting. These variables control the condition of the carbon and also its physical

form. The carbon may be combined as iron carbide in cementite or it may exist as free carbon in

graphite. The shape and distribution of the free carbon particles will greatly influence the

physical properties of the cast iron. The types of cast irons are as follows: (1) Gray cast iron. (2) Meehanite cast iron. (3) Nodular cast iron. (4) White cast iron. (5) Malleable cast iron. (6) Alloy cast iron.

10. WHITE CAST IRON

In these cast irons, all the carbon is present in the form of combined Carbon (i.e. Cementite )

and there is no free carbon (i.e. graphite).The appearance of a fractured surface is white because

of absence of graphite and hence the name is "white cast iron". Since there is no graphitization,

the solidification of a white cast iron and the resulting microstructural changes can be exactly

indicated by the Fe-Fe3C equilibrium diagram. The graphitization is suppressed by controlling

the chemical composition and cooling rate.

A) Cooling of a hypoeutectic cast iron with 3.0% carbon: The above cast iron is indicated at 3.0% carbon on Fe-Fe3C equilibrium diagram i.e. Fig. and cooling from the molten state is explained below.

(i) From 1 to 2, the alloy is in the liquid state and there is no change in the state of the alloy. Page 21

(ii) Just below 2, austenite starts separating out from the liquid and further cooling will increase

the amount of austenite. This continues upto 3. This primary or proeutectic austenite is in the

dendritic form since it is separating out from the liquid state. The amount of austenite according

to lever rule at just above 3 will be :

Amount of austenite (of 2.0% carbon) = = 56.5%

The remaining 43.5% alloy still exists in the liquid state with 4.3% Carbon. The microstructure is as shown below. (iii) At 3, the above liquid of eutectic composition solidifies at constant

temperature (11470C ) and forms an eutectic mixture of austenite and cementite called ledeburite

as indicated below :

Page 22

1147

Liquid γ + Fe3C

(of 0.8% C ) (of 2.0% C) (of 6.67% C )

Since ledeburite is formed at high temperature, its structure is coarse. The microstructure is shown in Fig. (iv) From 3 to 4, almost there is no change in the morphology of the existing structure. However, due to decrease in solubility of carbon in austenite, it ( i.e. both primary and eutectic austenite)

rejects Fe3C to the adjacent eutectic cementitic areas. Due to this, the amount of Fe3C increases.

The Fe3C which separates out from point 3 to 4 is called proeutectoid cementite.

(v) At 4, all the austenite (i.e. primary as well as eutectic) transforms isothermally to an eutectoid mixture of ferrite or cementite i.e. pearlite.

727

γ α + Fe3C (of 0.8% C) (of 0.025% C) (of 6.67% C)

The microstructure is shown in fig. below

(vi) From 4 to 5, there is almost no change in microstructure except slight increase in the amount

of Fe3C due to the decrease in solubility of carbon in ferrite from 0.025% to 0.008%.

Composition and properties of white Cast iron

Due to presence of all the carbon in the combined form, white cast irons contain large

amount of cementite and hence they are brittle and hard. The hardness of these cast irons depend Page 23

on the carbon content and increases with increase in Carbon, making the cast irons excessively

brittle with higher amounts of carbon and unsuitable for engineering applications. Therefore

majority of the white cast irons are hypoeutectic in carbon with the following composition range.

C - 2.3 to 3.0% ,Si - 0.5 to 1.3% , S - 0.06 to 0.1 % , P - 0.1% to 0.2% ,Mn - 0.5 to 1.0%

These cast irons are hard ( 350 to 500 BHN), strong in compression (140 to 175 kg/mm2), and

resistant to abrasive wear. They are difficult to machine and therefore, their finishing to the final

size is done by grinding.

Typical applications of white cast iron include wearing plates, road roller surface, pump

liners, mill liners, grinding balls, dies and extrusive nozzles. They are also used for the

production of malleable castings. They are not employed for structural parts because of their

excessive brittleness. Advantages : Higher hardness, higher brittleness, high wear resistant, good compressive strength. Diadvantages :

1. It contains a large amount of cementite as a continuous interdendritic network. So it

makes cast iron hard and wear resistant but extremely brittle. 2. It is difficult to machine to their final size and only grinding process is applicable.

3. Lack of machinability. 11. Malleable Cast Irons

These cast irons are produced from white cast iron castings by a malleablizing heat

treatment (malleablizing anneal). The heat treatment consist of heating the white castings slowly (to avoid cracking) to a temperature between eutectoid and eutectic temperatures,

usually at around 900o (800 – 950

oC) and holding at this temperature for a long time (24 hours

to several days) followed by cooling to room temperature. The above heat treatment cycle is shown in fig below.

Page 24

Due to heating to 900oC (i.e. at point 1), the structure of cast iron consists of austenite

and cementite. Cementite being a metastable phase, decomposes to austenite and graphite(i.e.

temper carbon graphite) with a long holding time. The above graphitization of cementite gives

rise to rough, ragged irregular nodules or spheroids called as rosettes of temper carbon graphite.

Therefore, at point 2 the structure consists of rosettes of temper carbon graphite in a matrix of

austenite. Cooling to room temperature with moderate cooling rate results in the transformation

of austenite to pearlite at eutectoid temperature and therefore at 3, the microstructure shows

rosettes of temper carbon graphite in the matrix of pearlite. However, if the cooling rate is slow

(or if the silicon in the white casting is more), the cementite from pearlite may also decompose

giving ferrite and graphite and the structure at room temperature i.e. at 3’ may show rosettes of

temper carbon graphite in the matrix of ferrite. For intermediate conditions, the matrix may be

pearlite and ferrite.

11.4.4 Types of malleable Cast Iron: Depending on the microstructure malleable cast irons are classified as:

(1) Pearlitic malleable

(2) Ferritic malleable

(3) Ferrito-pearlitic malleable

(1) Pearlitic Malleable Cast Iron: It is harder (200 to 275 BHN), stronger and slightly more brittle as well as machine. These

cast irons are used in manufacturing of (1) Crankshafts

(2) Axles

(3) Gears

(4) Links

(5) Railway electrification Systems

(2) Ferritic MalleableCast Iron: It is soft (80 to 100 BHN).Graphite is in round shape and well distributed. It is ductile,

tough and gives impact strength. It is used for (1) Pipe fittings

(2) Valves

(3) Farm equipments

(4) Bearing blocks

(3) Ferrito-Pearlitic Malleable Cast Iron: It is also called as bull’s eye structure. They are having intermediate properties to show of

ferritic and pearlitic malleable cast irons. Page 25

12. Gray Cast Irons

The cast irons containing graphite in the form of flakes are called gray cast irons and show gray fracture.

The graphite in these cast irons is formed during freezing, in contrast to the formation of

graphite in malleable cast irons which forms during subsequent heat treatment. The graphite

flakes interrupt the steel like matrix to a large extent and hence these cast irons are brittle and

relatively weak in tension as compared to malleable cast irons. However, gray cast irons are the

cheapest of all the ferrous alloys and easiest to cast due to their high castability resulting from

low melting point, good fluidity of melt and low shrinkage during solidification.

Properties Of Gray Cast Irons:

(i) Excellent machinability

(ii) Good compressive strength

(iii) Good bearing properties

(iv) Fairly good corrosion resistance.

These properties together with the low cost makes them suitable for large number of

applications. However, they suffer from few defects such as growth and fire-cracks or heat

checks. Growth i.e. permanent expansion occurs when heated to about 400oC.This results in loss

of strength with increased brittleness. Fire-cracks or heat checks occur in the form of cracks due

to repeated local heating and cooling to a temperature of the order of 550oC; and due to high

thermal gradients between the surface and the interior, it leads to failure of the component. These

drawbacks may be reduced by the addition of certain alloying elements like chromium,

molybdenum and nickel, and grain refines to the gray cast irons. Page 26

The composition of the alloy is adjusted in such a way that all the proeutectic (from

hypereutectic alloys) and eutectic cementite decomposes as soon as it forms and eutectoid

cementite does not decompose. This is done by controlling carbon and silicon in the alloy. If the

amount of these two elements is more, eutectoid cementite may also decompose with the normal

cooling rates as encountered in sand moulds. This leads to ferritic matrix and the cast iron

becomes soft and extremely weak thereby becoming unsuitable for engineering applications.

Therefore, a control over the composition must be exercised in such a way that all the proeutectic

and eutectic cementite must decompose – obviously this leads to the decomposition of all

proeutectoid cementite – and no eutectoid cementite should decompose. This is necessary to

obtain pearlitic matrix in the gray cast irons. The matrix can also be controlled by controlling the

cooling rate through the eutectoid transformation region. These cast irons have the following

composition range.

C - 3.2 to 3.7%

Si – 2.0 to 3.5%

S – 0.06 to 0.1%

P – 0.1 to 0.2%

Mn – 0.5 to 1.0%

Their properties depend on composition, and size and distribution of graphite flakes but are

roughly in the following range.

T.S. - 15 to 40 kg/mm2

Hardness - 150 to 300 BHN Elongation - less than 1%

A typical microstructure of a pearlitic gray cast is shown in the fig. Gray cast irons are widely used for machine bases, engine frames, drainage pipes, elevator

and industrial furnace counter weights, pump housings, cylinders and pistons of I.C. engines, fly wheels, etc.

Types of Gray Cast Iron: Depending upon the distribution of graphite flakes, A.F.A. (American Foundry

Association) and A.S.T.M. (American Society for Testing Materials) jointly have classified gray

cast irons into five types such as type ‘A’ to type ‘E’, as shown in fig.

Page 27

Since graphite is very soft and weak, interdendritic segregation of graphite is not desirable

because the cast iron becomes excessively brittle. Therefore, both the types D and E are not

desirable and the more regular pattern of type E graphite is still more undesirable than type D.

This type of Distribution is observed when the alloy solidifies as hypoeutectic alloy with coarse

primary dendrites of austenite.

Type C shows superimposed flakes and is a mixture of free extremely large and straight graphite

flakes with larger number of much finer flakes. Due to the large amount of graphite and long

flakes (superimposed one over the other) , this type is also not desirable. This is observed when

the alloy solidifies as hypereutectic alloy.

Type B graphite is observed in the mottled region that lies between the white and gray. This type of graphite distribution also increases the brittleness and hence is undesirable.

Type A graphite is most desirable because of uniform distribution and random orientation of

graphite flakes, since they do not seriously interrupt the continuity of the pearlitic matrix. This

type of distribution is observed when the alloy solidifies as an eutectic alloy. The various undesirable types of graphite flakes distribution can be avoided as below: Page 28

Type C graphite can be prevented by avoiding the hypereutectic compositions i.e. by reducing the carbon content of cast iron by addition of steel scrap.

Type B can be avoided by using compositions high enough in carbon and silicon so that the cast iron doesn't solidify as mottled at the cooling rates encountered during its freezing.

Type D and E can be avoided by using slow cooling slow rates during freezing. This results in

complete divorcement giving A - type of distribution with slightly coarse with graphite flakes.

This can also be achieved by reducing the austenite dendrite size by the addition of certain grain

refiners or inoculants which result in fine graphite flakes.

Once type A distribution is obtained, The properties of gray cast iron depend on the length of

graphite flakes. Short graphite flakes create less interruption in the continuity of steel like

matrix and hence give better mechanical properties. Graphite flake sizes are described in terms

of flake size numbers according to table established joint by A.F.A and A.S.T.M. Microstructures of Gray cast iron:

Gray cast iron shows various microstructures depending on cooling rate. If cooling rate

is fast it shows ‘pearlitic gray cast iron’ and if cooling rate is very slow pearlite from cast iron

decomposes into ferrite. Microstructure at slow cooling rate gives ‘ferritic gray cast iron’. If

cooling rate is moderate it shows combined structure of ferrite and pearlite gray cast iron and is

called as ferrito-pearlitic gray cast iron. The structures are shown in fig. Page 29

13. NODULAR CAST IRONS

These cast irons contain graphite in the form of nodules or spheroids. Due to this, the

interruption in the steel like matrix is less as compared to the interruptions produced by flake

graphite of gray cast irons. This increases the tensile strength, ductility and toughness. They are

also called as spheroidal graphite cast irons (S.G.C.I.) or ductile cast irons.

They are produced from gray cast irons by the addition of small quantity of certain elements

called as nodulizing elements such as magnesium, cerium, barium , lithium or zirconium. The

most common addition to gray cast iron for the production of nodular castings is magnesium.

The addition of Mg (0.06 to 0.08%) is done to the gray cast iron melt usually in the ladle just

prior to pouring into the moulds. Any delay in pouring results in distortion of nodular shape of

graphite and reduction in properties of these cast irons. The effect of nodulizing elements is

purely temporary and is lost due to long holding. Remelting of nodular cast iron produces gray

cast iron, unless fresh nodulizing addition is done.

All the nodulizing elements including Mg have low density and are highly chemically reactive. If

they are added in the pure form, they simply float to the top of the bath and burn or decompose at

the surface. Therefore, they are usually added in the form of master alloys.

Also, these elements have strong affinity for sulphur and they scavenge sulphur from the molten

bath as an initial step in producing nodular graphite. The additions are expensive and hence for

effective utilization of these elements, the original gray iron melt must contain less amount of

sulphur (<0.03%). The sulphur (and also phosphorus) is reduced and melt with soda ash(sodium

carbonate).

When nodulizing element is added to the molten bath and stirred, large amount of gas is evolved

and also gets dissolved in the melt. This dissolved gas gives rise of blow holes in the solidified

casting. Also, the contraction of nodular cast iron freezing is considerably greater than that of

ordinary gray cast iron. Due to this design of the mould is necessary to avoid shrinkage cavities

in the solidified casting.

Page 30

The mechanism by which graphite nodules are formed is not known with certainity. It is

postulated that the addition of nodulizing elements may be affecting the surface favouring the

nodule formation.

The matrix of a nodular cast iron can be controlled by controlling composition i.e. carbon &

silicon or alloying elements or by cooling rate. A typical microstructure of a nodular cast iron is

as shown.

The range of properties of these cast irons is as shown below: T.S. - 38 TO 80 kg/mm^2 Elongation – 6 TO 20% Hardness – 100 TO 300 BHN

It is widely used for crankshafts, gears, punch dies, sheet metal dies, furnace doors, pipes, pistons, cylinder blocks and heads, and bearing blocks.

Page 31

MODULE 04 – CASE & SURFACE HARDENING PROCESSSES

Surface hardening

In many cases such as crankshafts, camshafts, gears etc., hard and wear resistant surfaces is

required with tough core to withstand impact loads such a requirement is difficult to achieve

by using steel of uniform composition low carbon steel are tough but cannot be hardened.

High carbon steel can be hardened but are not so tough also, they are very much likely to

distort or crack during hardening because of their poor hardenability and difficult to machine

even before hardening.

The surface at the core of the component should have opposite properties the core should be

relatively soft to support the hard case, otherwise material become brittle. The core should be

tough to provide resistance to notch propagation. If a crack develops on the surface ,it should

be stopped when it reach the core , instead of continuing through the core.

Case of hardening steels

Case-hardening may be defined as a process for hardening a ferrous material in such a

manner that the surface layer known as case, is substantially harder than the remaining

materials known as core.

Such a problem is either solved by:

(i) Increasing carbon on the surface of the low carbon(0.1-0.2%C) or low carbon low alloy

steel and subsequently heat treating the heat component in a specific manner to produce hard

and wear resistant surface (i.e..case) and tough centre (i.e. .core)

(ii) Introducing nitrogen in the surface of tough steel so as to produce hard nitrided case with

no subsequent heat treatment.

(iii) Introducing carbon and nitrogen in the surface of tough steel and subsequently heat

treating to produce hard and wear resistant case or

(iv) Hardening the surface without change of composition of surface. These methods are

discussed below.

The methods of increasing carbon and nitrogen contents at the surface of ferrous alloys are

(i) Carburizing

(ii) Nitriding

(iii) Carbonitriding

(iv) Cyaniding

Carburizing

The method of increasing the carbon on the surface of steel is called carburizing. It consists

of heating the steel in the austenitic region in the contact with carburizing medium, holding at

this temperature for a sufficient period and cooling during temperature. In the austenite

region, the solubility of carbon is more and hence the carbon from medium diffuses into the

steel i.e.in the austenite. High carbon content content on surface does not mean high hardness

on the surface, unless the carbon is present in martensitic form. Hence after carburizing,

hardening treatment is necessary to bring the carbon in martensitic form. Therefore, the

hardening heat treatment that follows the carburizing operation is as important as carburizing

itself. Carburizing is also known as Cementation, case carburizing and case hardening.

Depending on medium used for carburizing, it is classified into following types:

(i) Solid (or Pack or Box) carburizing

(ii) Gas carburizing

(iii) Liquid carburizing

Solid carburizing

The components to be carburized are packed with a carbonaceous material in steel or cast

iron boxes and sealed with clay. If these boxes are not properly sealed, air comes in contact

with the carbonaceous material and the medium simply burns without any carburizing. The

usual carbonaceous material consists of hard wood charcoal, and an energizer such as barium

carbonate or some other carbonate like sodium or calcium carbonate. A typical composition

consists of

53-55% charcoal, 30-32% coke and remaining carbonates of Ba, Ca and Na. These boxes are

heated to some temperature in the austenitic region and kept at this temperature until the

desired degree of penetration is obtained. Carburizing is obtained by following reactions.

O2 (from box) + C (from the medium) → CO2

And BaCO3→BaO+CO2

2CO→ C + CO2

↓

(Dissolves in austenite)

CO is an active carburizing agent. It tries to diffuse into steel and while doing so, it reverts to

carbon dioxide and carbon. This is dissolved in austenite. The liberated CO2 combines with

carbon from medium and forms CO. The process goes on repeating and carburizing continues

by the above indirect process i.e .through the formation of CO.

The maximum carbon at the surface and the case depth depend on the temperature of

carburizing and the time of holding. Higher the temperature, higher is the carbon at the

surface and more is the case depth. However , higher the temperature lead to excessive grain

austenitic region, coarsening and are not recommended at the given temperature/increase in

holding time increases the case depth without changing the maximum Carbon at the surface.

At the usual temp. of carburizing(925-950 C),the case depth varies from 1.0mm to 2.5 mm

for total carburizing times of 6 to 15 hours.

Pack carburizing is principally used to produce relatively thick carburized cases (1mm or

more) where extreme uniformity in carbon content is not essential.

Advantages of Pack Carburizing:

(1) More case depth (1-25mm), more grinding allowance

(2) Less dimensional changes during carburizing.

(3) Process is cheap.

Disadvantages of Pack carburizing

(1) Process is batch type and dirty.

(2) Case carburized layer is not uniform.

(3) Requires very long cooling cycle time (6-15Hrs)

(4) Surface finish is poor.

Gas Carburizing:

Here the components are heated in austenitic region in the presence of carbonaceous gas such

methane, ethane, propane, or butane diluted with a carrier gas such as flue gas.

These gases decompose and carbon diffuses into the steel. To maintain a constant and

uniform rate of carbon diffusion, gas composition must be controlled along with proper

circulation of gas in the furnace chamber. This is necessary to avoid dead spots (spots where

carbon diffusion stops) or the formation of spot on the components. Gas carburizing produces

extremely uniform cases with shorter times because no boxes and solid carburizer need to be

heated as is required in pack carburizing. Also the process can be performed at somewhat

lower temperatures (900-925° C) than those normally used for pack carburizing

Gas carburizing is commonly used to obtain relatively thin cases of high uniformity. Case

depth from 0.2 to 0.5mm can be obtain in 1 to 2 hrs at temperature of 900 C.

Advantages of Gas Carburizing:

1) The time required for process is very less as compared to pack carburizing.

2) The surface finish of product is superior.

3) Automation of process is possible for mass production.

4) Process is clean.

5) Case carburized layer is uniform.

6) Case depth can be controlled accurately.

7) Less labour cost as compared to pack carburizing.

Disadvantages of Gas Carburizing:

1) High skilled labours are required to maintain the necessary case depth.

Liquid Carburizing:

In this method, carburizing is done by immersing the steel components in a carbonaceous

fused salt bath medium, the components are kept in wire basket and wire basket is immersed

in cyanide rich mixture maintained at a temperature of 950°C. At this constant temperature

components are hold for 5 min. to 1 hour for making uniform carbon layer on the surface.

The liquid carburizing process is shown in figure.

Liquid Carburizing Medium:

The liquid carburizing carried out in baths containing,

Sodium cyanide = 20 to 50 %

Sodium carbonate = up to 40 %

Sodium chloride (or barium chloride) = Balance.

In the process of alkaline earth salts, the access of oxygen to the bath is reduced and if the

operating temperature is kept high, the formation of cyanate (NaCNO) is inhibited. Under

these conditions, the reaction proceeds chiefly by the cyanamide shift as below:

BaCO3 + 2 NaCN → Ba(CN)2 + Na2CO3

Ba(CN)2 → BaCN2 + C

Advantages of Liquid Carburizing:

1) Rapid heat transfer and requires less holding time.

2) Low distortion.

3) Rapid absorption of carbon and nitrogen.

4) Uniform case depth.

5) Reduced time for steel to reach upto carburizing temperature.

6) Case depth between 0.1 to 0.5 mm.

Disadvantages of Liquid Carburizing:

1) Sodium cyanide is highly poisonous and hence care should be taken during its storage, use

and disposal.

2) Molten cyanide can explode when it is in contact with water. Care should be taken that all

work should be dried before it is placed into the liquid bath.

3) Cyanide containing water must be carefully disposed or made non-poisonous before

its disposal.

HEAT TREATMENT AFTER CARBURIZING

If carburizing has been correctly carried out, the core will still be low carbon content (0.1 to

0.2%), while the case contains carbon not more than 0.8%. If the carbon content of the case is

higher than 0.8%, network of primary cementite will coincide with the grain boundaries of

the original austenite giving rise to intercrystalline brittleness. If case containing carbon 1.0%

or more, after quenching material becomes soft due to more retained austenite.

After the component has been carburized, heat-treatment will be necessary for increasing

hardness of case (surface) and increasing toughness of core (center).

The heat treatment after carburizing is carried out in three stages,

(1) Core Refining Process

(2) Case Refining Process

(3) Tempering Process

(1) Core Refining Process

The heat treatment is necessary to increase toughness of core. In this process carburized

components are heated to 8800C temperature. At this temperature coarse ferrite/pearlite

structure present in the core is replaced by fine austenite crystals. The components are

then water quenched so that at room temperature a fine ferrite/bainite/martensite structure is

obtained in the core.

Due to core refining process case shows coarse brittle martensite structure. To reduce

brittleness of martensite and make it fine structure further treatment is necessary.

(2) Case Refining Process

In this process, the components are heated upto 7600C, so that the coarse martensite of the

case changes to fine grained austenite. After quenching fine grains of austenite transformed

into fine martensite in the case.

At the same time martensite produced in the core by core refining process is reduced due to

heating quickly up to 7600C and then quenched without soaking. This gives largely ferrite

bainite structure in the core. It gives more toughness and shock-absorbing property in the

core.

(3) Tempering Process

Due to heating and cooling of components for core refining process and case refining

process, there are developments of internal stress in the component which can be eliminated

by tempering process. Fine martensite produced in the case of components gives hardness

and brittleness. This brittleness from the martensite structure also reduced by tempering

process.

In tempering process, components are heated between 160 to 2200C temperature; it is held for

1 to 4 hours and cooled in air up to room temperature.

FACTORS CONTROLLING DEPTH OF CASE

In Carburizing process, active carbon produced from the decomposition of carburizing

atmosphere is absorbed by the surface of steel. The absorbed carbon is diffused towards the

interior of the part. Production, absorption and diffusion of carbon are greatly affected by,

(1) Temperature of Carburizing

(2) Time required for Carburizing

(3) Composition of the carburized atmosphere

(1) Temperature of Carburizing

The temperature most commonly used for carburizing is 920 to 9400C. This

temperature produces a completely austenitic phase in steel and the absorption and diffusion

rates are sufficiently high. Fig shows effect of temperature on controlling case depth in

carburizing process.

(2) Time Required for Carburizing

The time of carburizing process increases the depth of penetration of carbon towards the

interior of the part. The relation between the case-depth and the time of carburizing is given

by the following equation

Case-Depth=k√t

Where,

k=constant

t=time of carburizing in hours

The value of ‘k’ increases with increasing temperature of carburizing. Carburizing time can

vary from 2-36 hours depending upon the required case depth. In addition to this, it may take

several hours to bring the work up to the operating temperature. The effect of carburizing

time on case depth is shown in figure.

(3) Composition of the carburized atmosphere

In a carburized part, the carbon content varies along the cross-section of the component. It is

maximum at the surface and decreases towards the interior of the part. The variation of

carbon along the cross-section in a carburized part is called carbon concentration gradient and

typical variation is shown in figure.

CYANIDING :

Cyaniding is a process by which carbon and nitrogen content at the surface of steel are

increased .It is also called as Liquid Carbo-nitriding.

In this process the components are heated in the furnace in the temperature ranging from 760

to 8700 C. .It is immersed for 30 to 100 mins in the bath for addition of carbon and nitrogen

on the surface of steel components and cooled upto room temperature. The Cyaniding process

is shown in the figure.

Liquid Medium for Cyaniding:

The commonly used cyanide baths consists of sodium carbonate and sodium chloride .The

sodium chloride and sodium carbonate are added to provide fluidity and control the melting

point of the bath .Some typical compositions of cyanide baths are given in the table below.

The melting point of bath varies from 540 to 6200C, depending upon its composition.

Constituents Baths

I II III IV

Sodium cyanide,% 97 75 45.3 30

Sodium

carbonate,%

2.3 3.5 37 40

Sodium chloride,% 0.7 21.5 17.7 30

Chemical reaction:

Active carbon and nitrogen are not produced directly from sodium cyanide .Molten

cyanide decomposes in the presence of air at the surface of the bath to produce sodium

cyanide, which in turns decomposes to provide carbon and nitrogen as per following

chemical reactions:

2NaCN + O2 ---> 2NaCNO

4NaCNO --> Na2CO3+2NaCN + CO + 2N add on component

2CO ---> CO2 + C

Proper selection of the bath temperature and composition can regulate the relative amount of

carbon and nitrogen in cyanide case, lower bath temperatures are favoured for minimizing

distortion during quenching, while higher temperatures are selected for faster penetration.

Advantages of Cyaniding:

(1) Case depth of a cyanided part varies from 0.025 to 0.25 mm.

(2) Carbon and nitrogen contents varies between 0.4 to 0.7% at surface and decrease rapidly

towards the core.

(3) Process is fast, requires less time as compared to Carburizing process.

(4) A rapid rate of penetration.

Disadvantages of cyaniding:

(1) Regular checking and adjustments of the bath compositions is necessary to obtain uniform

case depth.

(2) Sodium Cyanide is poisonous and hence care should be taken during its storage, use and

disposal

(3) Molten cyanide can explode when it is in contact with water. Care should be taken that all

work should be dried before it is placed into the liquid bath.

(4) Cyanide containing water must be carefully disposed or made non-poisonous before its

disposal.

Applications of Cyaniding:

Cyaniding is used to produce a file-hard, wear resistant surface on the steel. It is particularly

useful for parts requiring a very thin hard case for screws, small gears , nuts and bolts.

Nitriding :

Nitriding is accomplished by heating the steel in contact with a source of atomic nitrogen at

a temperature of about 5500 C. The atomic nitrogen diffuses into the steel and combines with

iron and certain alloying elements present in the steel and forms respective nitrides. These

nitrides increase the hardness and wear resistance of steels. Molecular nitrogen does not

diffuse into the steel and hence is completely ineffective as a nitriding medium. The atomic

nitrogen source can be a molten salt bath containing NaCN similar to that used in liquid

carburizing without the addition of alkaline earth salts or dissociated ammonia and

accordingly the process is called liquid nitriding or gas nitriding.

In liquid nitriding, nitriding occurs by the formation and decomposition of cyanide by the

same reactions as given in liquid carburizing. Since the temperature of nitriding is less

(5500C), carbon cannot diffuse into the steel because of absence of austenite and hence only

nitrogen diffuses into the steel.

The gas nitriding is very much similar in operation to that of gas carburizing. The atomic

nitrogen produced due to the dissociation of ammonia (2NH3----->2N + 3H2 ) diffuses into

the steel because of absence of austenite and hence only nitrogen diffuses into the steel. For

good results, control over dissociation rate of ammonia and its circulation is necessary.

The steel components are hold at a constant temperature 5500C for 24 to 72 hours for addition

of atomic nitrogen layer on surfaces. It is cooled upto room temperature. The nitriding

process is shown in figure.

Advantages of Nitriding:-

1. Alloy steels after nitriding gives higher hardness and does not require any hardening heat

treatment as require for carburized components.

2. Nitriding reduces notch sensitivity and gives more fatigue life of components.

3. Nitrided layers give high hardness, better corrosion resistance and excellent bearing

properties.

4. Nitrided cases have higher hardness (1000-1200 VPN) than the carburized and hardened

cases. The high hardness is maintain up to 6000C temperature.

5. It is very clean process.

Disadvantages of Nitriding:-

1. Nitriding is not suitable for plain carbon steels.

2. Nitrided cases are relatively thin, usually less than 0.5mm.

3. Time required for nitriding process is very long (24-72 hours).

4. The white layer on surfaces of nitrided components should be removed by precision

grinding or lapping which is difficult and expensive.

5. No heat treatment is required after nitriding. Therefore, the core properties should be

adjusted before the components are nitrided.

Carbonitriding

In this process, both carbon and nitrogen are diffused into the surface of steel. The source of

carbon and nitrogen may be fused salt bath or a gaseous medium containing CH4, C2H6, etc.

(carburizing gases) with 5% to 10% ammonia. The temperature of the process is between 750

to 850oC. The phases present in steel at this temperature are ferrite and austenite. Nitrogen

diffuses in ferrite and carbon diffuses in austenite.

Nitrogen absorption at the surface of steel retards carbon diffusion so much that, within an

hour or so, further increase in case depth becomes extremely slow. Therefore the treatment

times for carbonitriding are usually less than one hour and correspondingly the case depths

are also smaller (0.075 to 0.25 mm). To transform the carburized areas into martensite, the

carbonitrided steel is always quenched from the carbonitriding temperature in oil or water.

Nitrogen and carbon diffusion increases the hardenability of surface steel and hence in many

cases oil quenching is sufficient to produce martensite.

Carbonitriding Process:

In this process the components are heated in the air tight muffle furnace in the temperature

range of 750 to 900oC. It is hold for 30 minutes to 6 hours at the constant temperature for

addition of carbon and nitrogen on the surface of steel components. It is quenched from this

temperature in oil or water upto room temperature. The carbonitriding process is shown in the

figure.

Carbonitriding Medium:

Depending on the medium used in this process, the process is called liquid carbonitriding or

gas carbonitriding.

Liquid carbonitriding is very much similar to liquid nitriding. It contains sodium cyanide (20-

30%). These processes have following differences with cyaniding.

a) Liquid carbonitriding contains alkaline salts which are not used in cyaniding.

b) Cyaniding containing higher percentage of sodium cyanide.

c) The cyaniding case contains higher nitrogen and lower carbon as compared to liquid

carbonitriding.

d) Cyanided case depths are less.

Gas carbonitriding contains gaseous medium containing CH4, C2H6 etc. (carburizing gases)

with 5-10% ammonia (NH3).

Carbonitriding Reactions:

The carbonitriding process is carried out slightly above A1 (727oC) temperature. The phases

present in the steel at this temperature are ferrite and austenite. Nitrogen is a ferrite stabilizer,

it is more stable in ferrite region. If temperature is lower, the nitrogen diffusion is prompted

and process becomes only nitriding.

Carbon is austenite stabilizer and is more stable in austenite region. If temperature is

increased than A3, the process becomes only carburizing. The carbonitriding reactions in

gaseous process

are,

CH4 ---- > 2H2 + C (on surface of steel components.)

2NH3 --- > 3H2 + 2N (on surface of steel components.)

Gaseous carbonitriding is carried out in muffle furnace and fan is used to circulate

gas continuously around the steel components.

Advantages of Carbonitriding:

a) Gives better service life.

b) Process is fast, require less time.

c) Carbonitriding gives hard, wear resistant, temperature resistant, high fatigue and high

impact properties on the surface of steel components.

d) Process is less expensive as compared to carburizing.

e) Gives uniform case depth.

f) Case depth varies from 0.075 to 0.75 mm.

Disadvantages of Carbonitriding:

a) It is applied only when case depth is upto 0.75 mm.

b) Temperature should be between A1 (727oC) and A3 (910

oC), otherwise at low temperature

there will be only nitriding and at higher temperature there will be only carburizing process.

c) It is last process because of addition of nitrogen, so that core refining process should be

carried out before carbonitriding.

d) This process is not suitable for plain carbon steel because of nitrogen, it gives white layer

(Fe4N), which is hard and brittle.

Applications of Carbonitriding:

This process is suitable for alloy steel containing nickel, chromium, molybdenum with carbon

contents upto 0.25%.

FLAME HARDENING:

Flame hardening is heat treatment process in which the surface of medium carbon steel is

heated rapidly above the transformation temperature i.e. austenitic temperature by high

temperature flame and then quenched by water spray to convert austenite into martensite. The

process is shown in fig.

In flame hardening, the high temperature flame is obtained by oxyacetylene flame, which can

generate temperature up to 30000C. The surface of component becomes hard by above

process but core remains soft.

TYPES OF FLAME HARDENING:

The flame hardening can be done in different ways:

1. SPOT HARDENING.

2. PROGRESSIVE.

3. SPINNING.

4. PROGRESSIVE – SPINNING.

1. SPOT HARDENNING:

In this method a spot or local area of the component is heated by one or more flames

followed by quenching in water. Both work and torch are stationery. This method is used for

hardening of small parts.

2. PROGRESSIVE:

In this method, a work is stationery and heating (torch) and quenching (water spray) device

are move over the component surface at a controlled rate. This method is used for large parts

such as ways of lathes, large gears, etc.

3. SPINNING:

In this method, a torch is stationery and work rotates. It is used for parts having a rotational

symmetry in which the flames are held against a rotating work piece and when heating is

complete, the part is quenched by water spray or by completely immersed in water. This

method is used for surface hardening of precision gears, pulley, splines, shafts etc.

4. PROGRESSIVE-SPINNING:

In this method heating torch moves and work piece also rotate and heating is followed by

quenching in water or by water spray.

The success of many flames hardening application depends largely on the skill of the

operation. The major operating variables are as follow:

(1) Distance between the gas flame and the work surface.

(2) Gas pressure and ratios.

(3) Rate of travel of flam -head or work.

(4) Type, volume and application of quench.

Flame hardening causes less distortion than conventional hardening and due to high heating

rate, oxidation and decarburization are minimum.

Advantage of flame hardening:

1. Large machine parts can be surface hardened economically.

2. Involves portable equipment.

3. There is little scaling and decarburization.

4. Provide precise control of case properties.

Disadvantage of flame hardening:

1. There is possibility of overheating and thus damaging the part.

2. It is difficult to produce hardened zone less than 1.5mm in depth.

3. Success of process depends on skill of operator.

APPLICATION OF SURFACE HARDENING:

Lathe ways, large gears, pulleys, shafts, spline shafts can be hardened by flame

hardening process.

Induction Hardening:

Here heating is done within thin layer of surface metal by using high frequency induced

currents. The component is heated by means of an inductor coil (heating coil) which consists

of one or several turns of water-cooled copper tube. High frequency alternating currents

flowing through the inductor generate alternating magnetic field. This electromagnetic field

induces eddy currents of the same frequency in the surface layers which rapidly heat the

surface of the component. Within a short period of 2 to 5 minutes the temperature of surface

layer comes to above the upper critical temperature of that steel. The high frequency induced

currents chiefly flow through the surface layer, a phenomenon is known as Skin effect. The

layer through which these currents flow is inversely proportional to the square root of

frequency of induced currents and hence, the depth of hardened layer can be controlled by

controlling the frequency supply voltage. The usual range of frequency of frequency is from

1000 Hz to 1, 00,000 Hz and the hardened depths obtained are from 0.5 to 6 mm. After the

necessary temperature is attained, the component is quenched by water spray usually without

removing from the inductor coil. Due to very fast heating and no holding time, the austenite

grain size is very fine which results in fine grained martensite.

Steels with carbon content from 0.4 to 0.5% are most suitable for induction hardening.

However, case carburized components can also be hardened. Some of the examples are

crank- shafts, camshafts, axles, gears, rolls of rolling mills, boring bars, brake drums,

overhead travelling crane wheels etc.

Some of the advantages of the process are as below:

1) Fast heating and no holding time leads to increase in production rates.

2) No scaling and decarburization.

3) Less distortion because of heating of only surface.

4) Easy control over the depth of hardening by control of frequency and/or Time of

Holding.

The process has few drawbacks as below:

1) Irregular shaped parts are not suitable for induction hardening.

2) Because of high cost of induction hardening unit, the process is not suitable for small scale

production.