Unit I – Ferrous Materials

Materials: Inanimate solids used by an engineer in the practice of his profession

Fig. 1.1

As it is clear from the fig. 1.1 that performance of the material depends upon structure of the

materials, its properties and method of processing.

Classification of Materials

Metals and Alloys: Cu, Ag & Au characterized by free electron. Alloys are combination

of one metal with one or more element and this combination behaves like metal. Metal

like behavior is characterized by electrical resistivity. Only in this class of material

resistivity increases with increase in temperature while in others it decreases with

increasing temperature. Characteristics are owed to non-localized electrons (metallic

bond between atoms) i.e. electrons are not bound to a particular atom. They are

characterized by their high thermal and electrical conductivities. They are opaque, can be

polished to high luster. The opacity and reflectivity of a metal arise from the response of

the unbound electrons to electromagnetic vibrations at light frequencies. They are

relatively heavier, strong, yet deformable.

Ceramics and Glasses: They are Oxides and silicates. Very hard and brittle.

Characterized by their higher resistance to high temperatures and harsh environments

than metals and polymers. Typically good insulators to passage of both heat and

electricity. Less dense than most metals and alloys. They are harder and stiffer, but brittle

in nature. E.g.: Glass, Porcelain etc.

Polymers: Commercially called plastics; noted for their low density, flexibility and use

as insulators. Mostly are of organic compounds i.e. based on carbon, oxygen and other

nonmetallic elements. Consists large molecular structures bonded by covalent and van

der Waals forces. They decompose at relatively moderate temperatures (100- 400 C).

Application: packaging, textiles, biomedical devices, optical devices, ceramics household

items, toys, etc. E.g.: Nylon, Teflon, Rubber, Polyester, etc.

Composites: Consist more than one kind of material; tailor made to benefit from

combination of best characteristics of each constituent. Available over a very wide range:

natural (wood) to synthetic (fiberglass). Many are composed of two phases; one is matrix

– which is continuous and surrounds the other, dispersed phase. Classified into many

groups: (1) depending on orientation of phases; such as particle reinforced, fiber

reinforced, etc. (2) depending on matrix; metal matrix, polymer matrix, ceramic matrix.

E.g.: Cement concrete, Fiberglass, special purpose refractory bricks, plywood, etc.

Semiconductors: Their electrical properties are intermediate when compared with

electrical conductors and electrical insulators. These electrical characteristics are

extremely sensitive to the presence of minute amounts of foreign atoms. Used in many

applications in electronic devices over decades through integrated circuits. It can be said

that semiconductors revolutionized the electronic industry for last few decades.

Properties of Engineering Materials

Identity of material which describes its state and behavior under specified conditions.

Physical Properties: physical state of material such as its texture, appearance, mass, density,

melting point, boiling point etc.

Chemical Properties: reactivity of a material which is the rate at which material changes its

chemical identity such as corrosion rate, oxidation rate etc.

Mechanical Properties:

Resistance against deformation in particular under static and dynamic mechanical loading

such as hardness, toughness, elastic modulus etc.

Behavior of material in terms of deformation and resistance to deformation.

Strength: ability of material to resist external forces without rupture

Hardness: resistance against abrasion/scratching/indentation. Vickers, Brinell, Rockwell

and Knoop tests are performed to measure hardness.

Toughness: Measure of material to withstand shock and the extent of plastic deformation

before rupture. Toughness is total energy absorbed till fracture and is represented by area

of stress strain curve till fracture. Izod and Charpy tests are performed to measure

toughness.

Resilience: total energy absorbed till elastic limit and is given by area of stress strain

diagram till elastic limit.

Phase Diagram

A diagram showing conditions of an alloy at which thermodynamically distinct phases can occur

at equilibrium.

Common components of a phase diagram are lines of equilibrium or phase boundaries, which

refer to lines that mark conditions under which multiple phases can coexist at equilibrium. Phase

transitions occur along lines of equilibrium.

Triple points are points on phase diagrams where lines of equilibrium intersect. Triple points

mark conditions at which three different phases can coexist. For example, the water phase

diagram has a triple point corresponding to the single temperature and pressure at which solid,

liquid, and gaseous water can coexist in a stable equilibrium.

The solidus is the temperature below which the substance is stable in the solid state.

The liquidus is the temperature above which the substance is stable in a liquid state. There may

be a gap between the solidus and liquidus; within the gap, the substance consists of a mixture of

crystals and liquid.

Component: pure metals and/or compound of which an alloy is composed.

Phase: a homogenous portion of system having uniform physical and chemical characteristics.

Microstructure: microstructure is characterized by the number of phases present, their

proportions and the manner in which they are distributed or arranged. It depends on alloying

element present, their concentration and the heat treatment of the alloy.

Phase Equilibrium: equilibrium is achieved when free energy is minimum in some specified

conditions of temperature, pressure & concentration. When equilibrium is achieved then

characteristics of the system do not change with time but persists indefinitely, that is, the system

is stable.

Equilibrium phase diagram

A diagram shown on temperature vs composition (of alloy) axes. Three informations can be

drawn from a phase diagram:

i) Phases Present

ii) Composition of Phases

iii) Phase amounts

Tie Line drawn on a phase diagram is a constant temperature line. Ends of tie line give

composition of phases.

A phase diagram is shown in the above figure. Tie-line LS is drawn at point O. Ends of tie line L

and S gives composition of phases. So composition of liquid phase is Wl and composition of

solid phase at O is Ws.

Amount of phases present is given by lever rule.

Amount of liquid phase = OS/LS = (Ws – W0)/(WS – Wl)

Amount of solid phase = LO/LS = (W0 – Wl)/(WS – Wl)

Iron-Carbon Phase Diagram

Pure iron upon heating experiences two changes in crystal structure before it melts.

α(ferrite) γ(Austenite) δ(Ferrite)

till 912°C BCC 912-1394°C FCC 1394-1538°C BCC

BCC - Body Centered Cubic Structure

FCC – Face Centered Cubic Structure

Thus the Fe-C phase diagram can be divided into two portions. First is before 6.7% of C which is

called iron rich portion and second is beyond 6.7% of C which is called carbon rich portion.

Since all steels and cast irons fall before 6.7% of carbon so only that portion is used. So this

diagram can also be called as iron-iron carbide phase diagram.

Cementite: at 6.7 wt% of carbon iron carbide (Fe3C) is formed which is called cementite. It is

very hard and brittle and its presence greatly enhances strength of steels. Cementite is stable at

room temperature but if heated between 650-700°C for several years, it transforms to α-ferrite

and carbon but this decomposition is very sluggish. Rate of this reaction can be increased by

addition of silicon which is used in making of cast iron.

Ferrite (α): Virtually pure iron with body centered cubic crystal structure (bcc). It is stable at all

temperatures up to 910°C. The carbon solubility in ferrite depends upon the temperature; the

maximum being 0.022% at 723°C. It is relatively soft and is magnetic below 768°C.

Austenite(γ): it is not stable below 727°C. Maximum solubility of C in austenite phase is 2.14%

at 1147°C. Austenite is non-magnetic.

δ-Ferrite: it is same as α-ferrite except the temperature ranges at which it exists.

Eutectic Reaction: In a eutectic reaction one liquid upon cooling transforms into two solids.

L γ + Fe3C

On Fe-C diagram this reaction occurs at 1147°C and 4.3% of C.

Eutectoid Reaction: In a eutectoid reaction a solid upon cooling changes to two solids.

γ α + Fe3C

So austenite on cooling changes to a mixture of ferrite and cementite. This reaction occurs at

727°C and 4.3% of C.

Different microstructures of Fe-C alloys

Pearlite: When eutectoid composition of austenite is cooled slowly below 727°C, pearlite is

formed. So it is a eutectoid mixture of ferrite and cementite, the two phases is formed as

alternating layers. It has the properties intermediate between the soft, ductile ferrite and hard,

brittle cementite.

The iron-iron carbide phase diagram can be divided into two parts called “hypoeutectoid steels”

(steels with carbon content to the left of eutectoid point [0.83% carbon]) and “hyper eutectoid

steels” which have carbon content to the right of the eutectoid point. Iron containing very low

percentage of carbon (0.002%) called very low carbon steels will have 100% ferrite

microstructure (grains or crystals of ferrite with irregular boundaries). Ferrite is soft and ductile

with very low mechanical strength. When the carbon content is somewhat increased then the

microstructure at ambient temperature becomes a mixture of what is known as „pearlite and

ferrite‟. Hence we see that ordinary structural steels have a pearlite + ferrite microstructure.

However, it is important to note that steel of 0.20% carbon ends up in pearlite + ferrite

microstructure, only when it is cooled very slowly from higher temperature during manufacture.

When the rate of cooling is faster, the normal pearlite + ferrite microstructure may not form,

instead some other microstructure called bainite or martensite may result.

Bainite: Bainite has series of strips or needles of ferrite separated by elongated particles of

cementite phase. Bainitic and Perlitic transformation are competitive with each other and one

phase once formed cannot be changed into other without reheating to form austenite.

Martensite: Formed when austenitized Fe-C alloys are rapidly cooled to a relatively low

temperature (in the vicinity of ambient). It is non-equilibrium single phase structure that results

from a diffusion-less transformation of austenite. At very high cooling rate FCC austenite

experiences a polymorphic transformation to a body centered tetragonal (BCT) martensite. It is

hardest and most brittle of all microstructures.

Types of metals and alloys

Metallic materials are broadly of two kinds – ferrous and non-ferrous materials. This

classification is primarily based on tonnage of materials used all around the world. Ferrous

materials are those in which iron (Fe) is the principle constituent. All other materials are

categorized as non-ferrous materials. Another classification is made based on their formability. If

materials are hard to form, components with these materials are fabricated by casting, thus they

are called cast alloys. If material can be deformed, they are known as wrought alloys. Materials

are usually strengthened by two methods – cold work and heat treatment. Strengthening by heat

treatment involves either precipitation hardening or martensitic transformation, both of which

constitute specific heat treating procedure. When a material cannot be strengthened by heat

treatment, it is referred as non-heat-treatable alloys.

Ferrous materials

Ferrous materials are produced in larger quantities than any other metallic material. Three factors

account for it: (a) availability of abundant raw materials combined with economical extraction, (b)

ease of forming and (c) their versatile mechanical and physical properties. One main drawback of

ferrous alloys is their environmental degradation i.e. poor corrosion resistance. Other

disadvantages include: relatively high density and comparatively low electrical and thermal

conductivities. In ferrous materials the main alloying element is carbon (C). Depending on the

amount of carbon present, these alloys will have different properties, especially when the carbon

content is either less/higher than 2.14%. This amount of carbon is specific as below this amount of

carbon, material undergoes eutectoid transformation, while above that limit, ferrous materials

undergo eutectic transformation. Thus the ferrous alloys with less than 2.14% carbon are termed as

steels and the ferrous alloys with higher than 2.14% carbon are termed as cast irons.

Steels

Steels are alloys of iron and carbon plus other alloying elements. In steels, carbon is present in

atomic form, and occupies interstitial sites of Fe microstructure. Alloying additions are necessary

for many reasons including: improving properties, improving corrosion resistance, etc. Arguably

steels are well known and most used materials than any other materials.

Mechanical properties of steels are very sensitive to carbon content. Hence, it is practical to

classify steels based on their carbon content. Thus steels are basically three kinds: low-carbon

steels (wt% C < 0.3), medium carbon steels (0.3 <% wt of C < 0.6) and high-carbon steels (% wt

of C > 0.6). Although steel alloy may contain as much as 2.14% C but in practice carbon

concentration rarely exceeds 1%.The other parameter available for classification of steels is

amount of alloying additions, and based on this steels are two kinds: plain carbon steels and

alloy-steels.

Low carbon steels: These steels are produced in the greatest quantities in comparison to other

alloys. Carbon present in these alloys is limited, and is not enough to strengthen these materials by

heat treatment; hence these alloys are strengthened by cold work. Their microstructure consists of

ferrite and pearlite, and these alloys are thus relatively soft, ductile combined with high toughness.

Hence these materials are easily machinable and weldable. Typical applications of these alloys include: structural shapes, tin cans, automobile body components, buildings, etc.

A special group of ferrous alloys with noticeable amount of alloying additions are known as HSLA

(high-strength low-alloy) steels. Common alloying elements are: Cu, V, Ni, W, Cr, Mo, etc. These

alloys can be strengthened by heat treatment, and yet they are ductile and formable. They possess

higher strengths than plain carbon steels and are more resistant to corrosion than plain carbon steels

which they have replaced in many applications where structural strength is critical like in bridges,

towers, support columns in high rise buildings and pressure vessels etc.

Medium Carbon Steels

These are stronger than low carbon steels. However these are less ductile than low carbon steels.

These alloys can be heat treated to improve their strength. Usual heat treatment cycle consists of

austenitizing, quenching, and tempering at suitable conditions to acquire required hardness. They

are often used in tempered condition having microstructure of tempered martensite. As

hardenability of these alloys is low, only thin sections can be heat treated using very high quench

rates. Ni, Cr and Mo alloying additions improve their hardenability. Typical applications include:

railway tracks & wheels, gears, other machine parts which may require good combination of

strength and toughness.

High carbon steels

These are strongest and hardest of carbon steels, and of course their ductility is very limited.

These are heat treatable, and mostly used in hardened and tempered conditions. They possess

very high wear resistance, and capable of holding sharp edges. Thus these are used for tool

application such as knives, razors, hacksaw blades, etc. With addition of alloying element like

Cr, V, Mo, W which forms hard carbides by reacting with carbon present, wear resistance of

high carbon steels can be improved considerably.

Alloy Steels

Low Alloy Steel

Steels are called alloy steels if total alloying element composition other than carbon is greater

than 1%. Elements are added to improve strength and toughness, to decrease or increase the

response to heat treatment, and to retard rusting and corrosion. Low alloy steel is generally

defined as having a 1.5% to 5% total alloy content. Common alloying elements are manganese,

silicon, chromium, nickel, molybdenum, and vanadium. Low alloy steels may contain as many

as four or five of these alloys in varying amounts.

Low alloy steels have higher tensile and yield strengths than mild steel or carbon

structural steel. Since they have high strength-to-weight ratios, they reduce dead weight

in railroad cars, truck frames, heavy equipment, etc.

Ordinary carbon steels, that exhibit brittleness at low temperatures, are unreliable in

critical applications. Therefore, low alloy steels with nickel additions are often used for

low temperature situations.

Steels loose much of their strength at high temperatures. To provide for this loss of

strength at elevated temperatures, small amounts of chromium or molybdenum are added.

High Alloy Steel: This group of expensive and specialized steels contains alloy levels in excess

of 10%, giving them outstanding properties.

Austenitic manganese steel contains high carbon and manganese levels, that give it two

exceptional qualities, the ability to harden while undergoing cold work and great

toughness. The term austenitic refers to the crystalline structure of these steels.

Stainless steels are high alloy steels that have the ability to resist corrosion. This

characteristic is mainly due to the high chromium content, i.e., 10% or greater. Nickel is

also used in substantial quantities in some stainless steels. Tool steels are used for cutting

and forming operations. They are high quality steels used in making tools, punches,

forming dies, extruding dies, forgings and so forth. Depending upon their properties and

usage, they are sometimes referred to as water hardening, shock resisting, oil hardening,

air hardening, and hot work tool steel.

Because of the high levels of alloying elements, special care and practices are required

when welding high alloy steels.

Effect of Alloying Elements

Manganese: Manganese is added to steel to improve hot working properties and increase

strength, toughness and hardenability. Manganese, like nickel, is an austenite forming element

and has been used as a substitute for nickel in the Austenitic stainless steels. Manganese

counteracts the brittleness caused by sulphur by forming manganese disulphide thus improving

hot working characteristics in presence of sulphur which gives lubrication thus ensuring good

surface finish of machined parts.

Chromium: Chromium is added to the steel to increase resistance to oxidation. This resistance

increases as more chromium is added. 'Stainless Steel' has approximately 11% chromium and a

very marked degree of general corrosion resistance when compared with steels with a lower

percentage of chromium. When added to low alloy steels, chromium can increase the response to

heat treatment, thus improving hardenability and strength.

Nickel: Nickel is added in large amounts, over about 8%, to high chromium stainless steel to

form the most important class of corrosion and heat resistant steels. These are the austenitic

stainless steels, where the tendency of nickel to form austenite is responsible for a great

toughness and high strength at both high and low temperatures. Nickel also improves resistance

to oxidation and corrosion. It increases toughness at low temperatures when added in smaller

amounts to alloy steels.

Tungsten: it forms abrasive resistant carbide particles in tool steel. At large percentage it

improves hot hardness and hot strength so it is useful in making in cutting and hot working tools.

Molybdenum: Molybdenum, when added to chromium-nickel austenitic steels, improves

resistance to pitting corrosion especially by chlorides and sulphur chemicals. When added to low

alloy steels, molybdenum improves high temperature strengths and hardness. When added to

chromium steels it greatly diminishes the tendency of steels to decay in service or in heat

treatment.

Titanium: The main use of titanium as an alloying element in steel is for carbide stabilization. It

combines with carbon to form titanium carbides, which are quite stable and hard to dissolve in

steel, this tends to minimize the occurrence of inter-granular corrosion. When adding

approximately 0.25%-0.60% titanium, the carbon combines with the titanium in preference to

chromium, preventing a tie-up of corrosion resisting chromium as inter-granular carbides and the

accompanying loss of corrosion resistance at the grain boundaries.

Phosphorus: Phosphorus is usually added with sulphur to improve machinability in low alloy

steels, phosphorus, in small amounts, aids strength and corrosion resistance. Experimental work

shows that phosphorus present in austenitic stainless steels increases strength. Phosphorus

additions are known to increase the tendency to cracking during welding.

Sulphur: When added in small amounts sulphur improves machinability but it causes hot

shortness (cracking at grain boundaries during solidification while casting). Hot shortness is

reduced by the addition of manganese, which combines with the sulphur to form manganese

sulphide. As manganese sulphide has a higher melting point than iron sulphide, which would

form if manganese were not present, the weak spots at the grain boundaries are greatly reduced

during hot working.

Vanadium: vanadium minimizes grain growth tendencies thus steels having much finer grains

are formed.

Silicon: Silicon is used as a deoxidizing (killing) agent in the melting of steel, as a result, most

steels contain a small percentage of silicon. Silicon contributes to hardening of the ferritic phase

in steels and for this reason silicon killed steels are somewhat harder and stiffer than aluminum

killed steels.

Niobium (Columbium)Niobium is added to steel in order to stabilise carbon, and as such

performs in the same way as described for titanium. Niobium also has the effect of strengthening

steels and alloys for high temperature service.

Cobalt: Cobalt becomes highly radioactive when exposed to the intense radiation of nuclear

reactors, and as a result, any stainless steel that is in nuclear service will have a cobalt restriction,

usually approximately 0.2% maximum. This problem is emphasized because there is residual

cobalt content in the nickel used in producing these steels.

Aluminum (Al): used as a grain refinement agent especially in the form of aluminum nitride

particles. It is also deoxidizing agent used in killed steels. Also increases nitridability (Used in

nitriding* steels).

*Nitriding is a type of surface hardening method of steels which will be discussed in next unit.

Tool Steels

Tool steel refers to a variety of carbon and alloy steels that are particularly well-suited to be

made into tools. Their suitability comes from their distinctive hardness, resistance to abrasion,

their ability to hold a cutting edge, and/or their resistance to deformation at elevated

temperatures (red-hardness). Tool steel is generally used in a heat-treated state. Many high

carbon tool steels are also more resistant to corrosion due to their higher ratios of elements such

as vanadium and niobium.

With carbon content between 0.7% and 1.5%, tool steels are manufactured under carefully

controlled conditions to produce the required quality. The manganese content is often kept low to

minimize the possibility of cracking during water quenching.

Tool steels are made to a number of grades for different applications. Choice of grade depends

on, among other things, whether a keen cutting edge is necessary, as in stamping dies, or whether

the tool has to withstand impact loading and service conditions. In general, the edge temperature

under expected use is an important determinant of both composition and required heat treatment.

The higher carbon grades are typically used for such applications as stamping dies, metal cutting

tools, etc.

High Speed Steel

High speed steel, also known as high speed tool steel, is another type of alloy steel. Its carbon

content ranges from 0.7% to 1.5%. It generally contains one or more metals such as chromium,

vanadium, molybdenum, tungsten and cobalt. The first four of these elements are carbide

farmers. These carbides are very hard and wear resistant; therefore they make good cutting tools.

Cobalt is not carbide former but it increases the red hardness of the cutting tool. Thus the tool

retains its hardness at high temperature up to about 600°C while plain carbon tools start to soften

at about 232°C. High speed steel tools cost two to four times more than plain carbon steel tools.

Stainless steels

The name comes from their high resistance to corrosion i.e. they are rust-less (stain-less). Steels

are made highly corrosion resistant by addition of special alloying elements, especially a

minimum of 12% Cr along with Ni and Mo. Stainless steels are mainly three kinds: ferritic,

austenitic and martensitic steels. This classification is based on prominent constituent of the

microstructure. Typical applications include cutlery, razor blades, surgical knives, etc.

Ferritic stainless steels are principally Fe-Cr-C alloys with 12-14% Cr. They also contain small

additions of Mo, V, Nb, and Ni.

Austenitic stainless steels usually contain 18% Cr and 8% Ni in addition to other minor alloying

elements. Ni stabilizes the austenitic phase assisted by C and N. Other alloying additions include

Ti, Nb, Mo (prevent weld decay), Mn and Cu (helps in stabilizing austenite).

By alloying additions, for martensitic steels Ms (starting temperature of martensite formation on

TTT diagram) is made to be above the room temperature. These alloys are heat treatable. Major

alloying elements are: Cr, Mn and Mo.

Ferritic and austenitic steels are hardened and strengthened by cold work because they are not

heat treatable. On the other hand martensitic steels are heat treatable. Austenitic steels are most

corrosion resistant, and they are produced in large quantities. Austenitic steels are non-magnetic

as against ferritic and martensitic steels, which are magnetic.

High-temperature steel

Molybdenum has been the key alloying element used to develop creep-resistant ferritic steels for

service temperatures up to 530 °C. Products and components made of high-temperature steels

include seamless tubing for water boilers and superheaters, boiler drums, collectors, pumps and

pressure vessels for elevated temperature service.

Large steam turbines require creep-resistant (creep is deformation of material with time under

fixed load conditions. Creep increases as the service temperature is increased) steels for safe,

economical operation. The main application areas of creep-resistant steels are power generation

and petrochemical plants, which use all product forms. Steam turbines require large forgings and

castings, whereas pressure vessels, boilers and piping systems require tubes, pipes, plates and

fittings. In addition to high creep strength, other material properties like hardenability, corrosion

resistance, and weldability are also important. The relative importance of these properties

depends on the specific application. For example, large turbine rotors require steels with good

hardenability, whereas power plant tubing and piping must have good weldability. Even so, the

alloys used in these different applications employ the same mechanisms to improve creep

strength. Molybdenum in solid solution reduces the creep rate of steel very effectively.

Owing to its higher resistance to chemical and mechanical degradation at elevated temperatures,

high temperature steels is extensively demanded in the market. Due to their characteristics such

as corrosion resistance, oxidation resistance, hydrogen brittleness and creep resistance these are

ideal choice for high temperature working environments. The steel is classified on the basis of its

micro-structure, which can be ferritic-austenitic (duplex), ferritic, austenitic and martensitic. The

structure of steel grade is determined by its chemical composition.

Low Temperature Steel

Most steels (except austenitic stainless steels) become brittle at temperatures below -10°C. At the

ductile to brittle transition temperature the impact energy can decrease by a factor 10. Steels

containing Ni have a lower ductile to brittle transition temperature. The embrittlement

temperature decreases with increasing Ni percentage. So these steels are used in applications

where temperature is very low.

Advanced High Strength Steels

Advanced High-Strength Steels (AHSS) are complex, sophisticated materials, with carefully

selected chemical compositions and multiphase microstructures resulting from precisely

controlled heating and cooling processes. Various strengthening mechanisms are employed to

achieve a range of strength, ductility, toughness, and fatigue properties. These steels aren‟t the

mild steels of yesterday; rather they are uniquely light weight and engineered to meet the

challenges of today‟s vehicles for stringent safety regulations, emissions reduction, solid

performance, at affordable costs.

The AHSS family includes Dual Phase (DP), Complex-Phase (CP), Ferritic-Bainitic (FB),

Martensitic (MS or MART), Transformation-Induced Plasticity (TRIP), Hot-Formed (HF), and

Twinning-Induced Plasticity (TWIP). These 1st and 2nd Generation AHSS grades are uniquely

qualified to meet the functional performance demands of certain parts. For example, DP and

TRIP steels are excellent in the crash zones of the car for their high energy absorption. For

structural elements of the passenger compartment, extremely high-strength steels, such as

Martensitic and boron-based Press Hardened Steels (PHS) result in improved safety

performance. Recently there has been increased funding and research for the development of the

“3rd Generation” of AHSS. These are steels with improved strength-ductility combinations

compared to present grades, with potential for more efficient joining capabilities, at lower costs.

These grades will reflect unique alloys and microstructures to achieve the desired properties.

IS Designation of Steels

Examples:

15Mn2Mo44-0.15%C, 0.2%Mn, 11%Mo

25C5-0.25%C, 0.5%Mn Manganese Based

35C10S14-0.35%C, 1%Mn, 0.14%S

40Ni8Cr8V2-0.4%C, 2%Ni, 2%Cr, 0.2%V

22SMnPb36-0.22%C, 0.36%S

22S20-0.22%C, 0.2%S

Cast irons

Though ferrous alloys with more than 2.14 wt.% C are designated as cast irons, commercially

cast irons contain about 3.0-4.5% C along with some alloying additions. Alloys with this carbon

content melt at lower temperatures than steels i.e. they are responsive to casting. Hence casting is

the most used fabrication technique for these alloys. Furthermore some cast irons are very brittle

and casting is the most convenient fabrication technique.

Cementite is a meta-stable compound and under some circumstances it can be made to dissociate

or decompose to form α-ferrite and graphite.

Fe3C 3Fe(α) + C(graphite)

This tendency to form graphite is regulated by the composition and rate of cooling. Graphite

formation is promoted by the presence of Si in concentration greater than 1%. Also slower

cooling rates during solidification favor graphitization. For most cast irons, the carbon exist as

graphite and both microstructure and mechanical behavior depend on composition and heat

treatment.

Cast iron is of four types:

Gray Iron

Ductile or Nodular Iron

White Iron

Malleable Iron

Gray cast iron: These alloys consists carbon in form of graphite flakes, which are surrounded by

either ferrite or pearlite matrix. Because of presence of graphite, fractured surface of these alloys

look grayish, and so is the name for them. Alloying addition of Si (1-3wt.%) is responsible for

decomposition of cementite, and also high fluidity. Thus castings of intricate shapes can be

easily made. Due to graphite flakes, gray cast irons are weak and brittle. However they possess

good damping properties, and thus typical applications include: base structures, bed for heavy

machines, etc. They also show high resistance to wear.

Nodular (or ductile) Cast Iron: Alloying additions are of prime importance in producing these

materials. Small additions of Mg/Ce to the gray cast iron melt before casting can result in

graphite to form nodules or sphere-like particles. Matrix surrounding these particles can be either

ferrite or pearlite depending on the heat treatment. These are stronger and ductile than gray cast

irons. Typical applications include: pump bodies, crank shafts, automotive components, etc.

White cast iron: When Si content is low (< 1%) in combination with faster cooling rates, there

is no time left for cementite to get decomposed, thus most of the brittle cementite retains.

Because of presence of cementite, fractured surface appear white, hence the name. They are very

brittle and extremely difficult to machine. Hence their use is limited to wear resistant

applications such as rollers in rolling mills. Usually white cast iron is heat treated to produce

malleable iron so it is used as an intermediary in production of malleable cast iron.

Malleable cast iron: These are formed after heat treating white cast iron. Heat treatments

involve heating the material up to 800-900°C, and keep it for long hours, before cooling it to

room temperature. High temperature incubation causes cementite to decompose and form ferrite

and graphite. In these clusters of graphite exists in matrix of ferrite or pearlite. Thus these

materials are stronger with appreciable amount of ductility. Typical applications include:

railroad, connecting rods, marine and other heavy-duty services.

Recognized patterns of distribution of graphite flakes in grey cast iron Depending upon the size and distribution of graphite flakes, grey cast irons have been classified

into various types, such as, size one to size eight and type A to E. Various size numbers have

been obtained for different lengths of graphite flakes. The length of the graphite flakes can vary

from 0.01mm to 1mm. Large graphite flakes seriously interrupt the continuity of the pearlite

matrix, thereby reducing the strength and ductility of grey iron. Small graphite flakes are less

damaging and therefore generally preferred. Slow cooling of the hypoeutectic cast irons results

in coarser and a fewer number of graphite flakes. Increasing the amount of carbon and silicon

content increases the amount of graphite flakes and thus decreases the strength of the casting.

Similar to length of graphite flakes their orientation and distribution can vary from random

orientation and uniform distribution to preferred orientation and typical segregations. This results

in various types of grey cast iron ranging from A to E. The most desirable flake pattern in grey

iron is represented by the uniform distribution and random orientation of type A.

The best method of reducing the size and improving the distribution of graphite flakes is the

addition of a small amount of a material known as inoculant. Inoculating agents that have been

used successfully are metallic calcium, aluminum, titanium, zirconium, silicon carbide, calcium

silicide or combination of these. These inoculants probably promote the nucleation of primary

austenite resulting in small grains which reduces the size and improves the distribution of the

graphite flakes.

TYPE A TYPE B

Random flake graphite, uniform distribution Rosette flake graphite

TYPE C TYPE D

Kish graphite (hyper-eutectic compositions) Undercooled flake graphite

TYPE E

Interdendritic flake graphite (hypo-eutectic compositions)

Type A – random orientation of graphite flakes with uniform distribution, most preferred

structure

Type B (Rosette Flake Graphite) - this graphite flake structure may result when there is poor

inoculation and nucleation.

Type C (Kish Graphite) – this structure is typically found in hypereutectic grey irons where the

graphite flakes are the first to precipitate from the melt. These are randomly oriented and widely

varied in size. It greatly reduces the mechanical properties and produces a rough surface finish

on machining.

Type D - type D & E graphite flake structure are typically found where undercooling of melt is

the greatest e.g. edges, parting lines, thin sections etc. Type D is a very fine pattern of flakes

surrounding areas without graphite.

Type E – this structure mainly occurs in hypoeutectic steels. In this graphite flakes have

preferred orientation and appear in a quasi-regular pattern.

A.Abbas