ISOTHERMAL TRANSFORMATION DIAGRAMS, QUENCHING AND HARDENING

Submitted by-N.Shasank (07326)Aditya Sahu(07327)Gaurav Sankhian(07328)Rajat Diwan(07329)Amit Chauhan(07330)Rajat Thakur(07331)Prashant Sharma(07332)Akshaydeep Singh(07129)

• MARTENSITE

It is formed when austenitized iron–carbon alloys are rapidly cooled (or quenched) to a relatively low temperature. Martensite is a nonequilibrium single-phase structure that results from a diffusion less transformation of austenite. It may be thought of as a transformation product that is competitive with pearlite and bainite. The martensitic transformation occurs when the quenching rate is rapid enough to prevent carbon diffusion. Any diffusion whatsoever will result in the formation of ferrite and cementite phases.

The needle shapedgrains are the martensite phase, andthe white regions are austenite that failed totransform during the rapid quench.

ISOTHERMAL TRANSFORMATION DIAGRAM

DEFINITION-

For an iron–carbon alloy of Eutectoid composition or for other compositions in which the temperature of the alloys held constant throughout the duration of the reaction such conditions of constant temperature are referred to on isothermal transformation diagrams, or time–temperature – transformation (T–T–T) diagrams.

PEARLITE- • The microstructure for this eutectoid steel that is slowly cooled through the eutectoid

temperature consists of alternating layers or lamellae of the two phases( and Fe3C) that form simultaneously during the transformation. In this case, the relative layer thickness is approximately 8 to 1.

• This microstructure is called pearlite because it has the appearance of mother of pearl when viewed under the microscope at low magnifications.

• At temperatures just below the eutectoid, relatively thick layers of both the -ferrite and Fe3C phases are produced; this microstructure is called coarse pearlite. With decreasing temperature, the carbon diffusion rate decreases, and the layers become progressively thinner. The thin layered structure produced in the vicinity of 540°C is termed fine pearlite.

●BAINITE

It is the micro constituent that is the product of austenitic transformation. It’s microstructure consists of ferrite and cementite phases and thus diffusional processes are involved in it’s formation. It forms as needles and plates depending upon temperature of transformation, which are only visible under electron microscopy. It is composed of needles of ferrite separated by elongated particles of Fe3C phase. Time temperature transformation of bainite transformation may also be represented on isothermal transformation diagram.

The isothermal transformation diagram for an iron-carbon alloy of eutectoid composition that has been extended to lower temperatures is shown in adjoining diagram. All three curves are C-shaped and have a “nose” at point N because the rate of transformation is maximum at that point. As given pearlite forms above the nose i.e. over the temperature ranges of about 540-727°c,whereas bainite is the transformation product between abot 215-540⁰c.Also it is notable that pearlitic and bainitic transformations are really competitive with each other and if some portion of an alloy has transformed to either pearlite or bainite,transformation to other micro constituent is not possible without reheating to austenite.

BAINITIC TRANSFORMATION- It requires the diffusion of carbon. For it we define upper bainite and lower bainite. Upper bainite forms between about 550-350 degrees Celsius and has feathery shaped ferrite. The feathery appearance arises from clusters of fine clusters of fine parallel ferrite laths. Whereas lower bainite forms below 350 degrees Celsius, ferrite is a plate like unit, inside which transition carbides such as Fe2.4c have precipitated at an angle of 55 degrees to the axis of ferrite plate. Sometimes ,two temperatures Bs and Bf are used to denote the start and finish of bainitic transformation. No bainite forms above Bs. The amount of bainite formed increases with decreasing temperature below Bs.

At Bf ,the austenite transforms to 100% bainite, if held long enough isothermally.. Ferrite nucleation is believed to be the first step in banite formation in contrast to cementite nucleation in pearlite. The ferrite plates may form by shear mechanism As the diffusion rate of carbon is slow, the distance over which it occurs is small, which explains submicroscopic size of crystal in banite.

Lower bainite in martensite matrix diagram of AISI steel that has been transformed at 3000 c.

Isothermal transformation diagrams

• For an iron–carbon alloy of eutectoid composition or for other compositions in which the temperature of the alloys held constant throughout the duration of the reaction such Conditions of constant temperature are referred to isothermal transformation diagrams, or time–temperature – transformation (T–T–T) diagrams.

PEARLITE

in iron-iron carbide eutectoid reaction -

• 0n cooling, austenite, having an intermediate carbon concentration, transforms to a ferrite phase and cementite

• The microstructure for this eutectoid steel that is slowly cooled through the eutectoid temperature consists of alternating layers or lamellae of the two phases( ferrite and cementite) that form a microstructure called pearlite because it has the appearance of mother of pearl when viewed under the microscope

Formation of pearlite

Photomicrograph of austenite Photomicrograph of

pearlite

Effect of Temperature on austenite-to-pearlitetransformation

For an iron–carbon alloy of eutectoid composition (0.76 wt% C), isothermal fraction reacted versus the logarithm of time for the

austenite to pearlite transformation.

Temperature plays an important role in the rate of the austenite - to- pearlitetransformation.

The temperature dependence for an iron – carbon alloy of eutectoid composition is indicated in [Figure ] which plots S-shaped curves of the percentage transformation versus the logarithm of time at three different temperatures.

• Demonstration of how an isothermal transformation diagram (bottom) is generated from percent transformation versus-logarithm of time measurements (top).

Upper portion gives the percentage transformation versus the logarithm of time at a particular temperature.

the time and temperature dependence of this transformation is in the bottom portion Here, the. vertical and horizontal axes are, respectively, temperature and the logarithm of time. Two solid curves are plotted; one represents the time required at each temperature for the initiation or start of the transformation; the other is for the transformationconclusion. The dashed curve corresponds to 50%of transformation completion.N0w here in this particular case the S-shaped curve [for 675C (1247F)], in the upper portion of above Figure illustrates how the data transfer is made to the bottom 0ne.

Characteristics -

The austenite-to pearlite transformation will occur only if an alloy is supercooled to below the eutectoid as indicated by the curves, the time necessary for the transformation to begin and then end depends on temperature.

The start and finish curves are nearly parallel, and they approach the eutectoid line asymptotically.

To the left of thetransformation start curve, only austenite (which is unstable) will be present,whereas to the right of the finish curve, only pearlite will exist.

In between, the austenite is in the process of transforming to pearlite, and thus both microconstituents will be present.

Demerits - This particular plot is valid only for an iron–

carbon alloy of eutectoid composition; for other compositions, the curves will have different configurations.

At temperatures just below the eutectoid(corresponding to just a slight degree of undercooling) very long times (on theorder of 105 s) are required for the 50% transformation, and therefore the reactionrate is very slow.

Demerits - This rate–temperature behavior is in

apparent contradiction of Equation which stipulates that rate increases with increasing temperature

By convention, the rate of a transformation r is taken as the reciprocal of time required for the transformation to proceed halfway to completion t0.5

Plot of fraction reacted versus the logarithm of time typical of many solid state transformations in which temperature is held constant.

MARTENSITE-TRANSFORMATION(ISOTHERMAL TRANSFORMATION DIAGRAM)

Martensite is formed when austenitized iron–carbon alloys are rapidly cooled (or quenched) to a relatively low temperature(in the vicinity of the ambient).

•Martensite is a nonequilibrium single-phase structure that results from a diffusion less transformation of austenite.

•The martensitic transformation occurs when the quenching rate is rapid enough to prevent carbon diffusion. Any diffusion whatsoever will result in the formation of ferrite and cementite phases.

•Martensitic transformation does not involve diffusion, it occurs almost instantaneously; the martensite grains nucleate and grow at a very rapid rate—the velocity of sound within the austenite matrix. Thus the martensitic transformation rate, for all practical purposes, is time independent.

The complete isothermal transformation diagram for an iron–carbon alloy of eutectoid composition:

Martensitic transformation is diffusion less and instantaneous. The beginning of this transformation is represented by a horizontal line designated M(start) .(Two other horizontal and dashed lines, labeled M(50%) and M(90%), indicate percentages of the austenite-to-martensite transformation.

The temperatures at which these lines are located vary with alloy composition but, nevertheless, must be relatively low because carbon diffusion must be virtually nonexistent.

The horizontal and linear character of these lines indicates that the martensitic transformation is independent of time; it is a function only of the temperature to which the alloy is quenched or rapidly cooled. A transformation of this type is termed an athermal transformation.

Consider an alloy of eutectoid composition that is very rapidly cooled from a temperature above 727ºC (1341ºF) to, say, 165ºC (330ºF). From the isothermal transformation diagram it may be noted that 50% of the austenite will immediately transform to martensite; and as long as this temperature is maintained, there will be no further transformation.

Isothermal transformation diagram for analloy steel

The presence of alloying elements other than carbon (e.g., Cr, Ni, Mo, and W) may cause significant changes in the positions and shapes of the curves in the isothermal transformation diagrams.

All common alloying element except cobalt shift the nose of C-curve to right, thus making it easier to quench the steel. They increase the harden ability of steel.

Shifting to longer times the nose of the austenite-to-pearlite transformation

The formation of a separate bainite nose. The shorten time at bainite nose determine the harden ability of steel.

An iron–carbon alloy of eutectoid composition:

An alloy steel

The nose of the austenite-to-pearlite transformation is shifted to longer times. Separate bainite nose is formed. These alterations may be observed by comparing Figures ,

which are isothermal transformation diagrams for carbon and alloy steels, respectively.

SPHEROIDITE TRANSFORMATION- If a steel alloy having either peritectic or bainitic microstructures is heated to and left at a temperature below eutectoid temperature for sufficiently long period of time –for e.g. at about 700⁰c for 18-24h,it results in another microstructure i.e. spheroidite, Instead of alternating ferrite and cementite lamellae or microstructures observed for bainite, the Fe3C phase appears as sphere like particles embedded in a continuous a-phase matrix. This transformation has occurred by further diffusion with no change in the compositions or relative amounts of ferrite and cementite phases. The driving force for this transformation is the reduction in a-Fe3C phase boundary area.

QUENCHING

QUENCHING

Heat treatment process involving rapid cooling from above the critical temperature

Useful to harden steel by formation of martensite. Rapid cooling may be done by water, air ,oil and dil.

sodium hydroxide solution. Cooling rate depends upon type of quenching

medium, temp. of quenching medium, surface condition of the part and size and mass of the part

Hardening by Quenching• Hardenability: Ability of an alloy to be hardened by

martensite formation.• Maximum cooling may be attained by cooling the material at a

rate equal to, or greater than, the critical cooling rate.• Hypoeutectoid steels are heated above the Ac3 temperature to

ensure full austenitization and to avoid the soft ferrite in the final microstructure. For hypereutectoid steels this is not necessary as the undissolved cementite on heating to just above Ac1 is itself very hard.

• A predominantly martensitic microstructure throughout the cross section depends mainly on three factors: (1) the composition of the alloy, (2) the type and character of the quenching medium, and (3) the size and shape of the specimen.

The iron- iron carbide equilibrium diagram

• A1: Eutectoid temp. for alloys to the left of eutectoid.

• A31: Eutectoid temp. for alloys to the right of eutectoid.

• A3: The curve separating austenite and austenite and ferrite.

The Jominy End-Quench Test

• A standard procedure that is widely utilized to determine hardenability

• With this procedure, except for alloy composition, all factors that may influence the depth to which a piece hardens (i.e., specimen size and shape, and quenching treatment) are maintained constant.

• A specimen (25.4 mm (1.0 in.) in diameter and 100 mm (4 in.) long) austenised at prescribed temperature and conditions is mounted in shown position.

• The lower end is quenched by a jet of water of specified flow rate and temperature.

Continued…..• After the piece has cooled to

room temperature, shallow flats 0.4 mm (0.015 in.) deep are ground along the specimen length and Rockwell hardness measurements are made for the first 50 mm (2 in.) along each flat.

• for the first 12.8 mm ( in.), hardness readings are taken at 1.6 mm ( in.) intervals, and for the remaining 38.4 mm (1 in.), every 3.2 mm ( in.).

• A hardenability curve is produced when hardness is plotted as a function of position from the quenched end.

Hardenability Curves• A typical hardenability curve is

represented in given figure.• The quenched end is cooled most

rapidly and exhibits the maximum hardness; 100% martensite is the product at this position for most steels.

• Cooling rate decreases with distance from the quenched end, and the hardness also decreases.

• With diminishing cooling rate more time is allowed for carbon diffusion and the formation of a greater proportion of the softer pearlite.

Quench Cracks

• Cracks formed on the surface of steel when thick cross-sections are quenched drastically.

• This reduces drastically the fatigue life of machine components.

• Origin due to a steep temperature gradient is produced from the surface to the centre of the steel resulting in

1.Thermal contraction of steel

2.Volume expansion due to transformation.

SURFACE HARDENING

a) CARBURIZING

b) CYANIDING

c) FLAME HARDENING

d) NITRIDING

e) INDUCTION HARDENING

CARBURIZING

One of the oldest and cheapest methods to have a hard wear resistant surface i.e. case.

NEED:

The water and the air present in the atmosphere cause decarburization as:

Fe₍c + H₂O → Fe + CO + H₂₎

Fe₍c + O₂ →Fe + CO₂₎

The surface, depleted of carbon, will not transform to martensite on subsequent hardening, and the steel will be left with a soft skin. For some tool applications, the stresses to which the part is subjected are maximum at the surface, so decarburization is harmful.

So, reverse process i.e. carburization is done.

CARBURIZING A low-carbon steel, usually about 0.20 percent carbon or lower, is placed

in an atmosphere that contains substantial amounts of CO and usual temperature is 1700 F. The following reaction takes place:⁰

Fe + 2CO → Fe₍c + CO₂₎

Fe₍c represents carbon dissolved in austenite.₎

Now a layer of carbon is formed on the surface. We know the core is of lower carbon content, so in order to reach equilibrium C-atoms will begin to diffuse inward. And the rate of diffusion depends on the diffusion coefficient and the carbon-concentration gradient. Under known and standard operating conditions, with the surface at a fixed carbon concentration, the form of the carbon gradient may be predicted, with reasonable accuracy, as a function of elapsed time. After diffusion has taken place for the required amount of time depending upon the case depth required, the part is removed from the furnace and cooled.

1. Pack Carburizing• In this method the articles to be carburized are packed in metal boxes or

pots surrounded by a suitable compound, rich in carbon. The boxes are sealed with clay to exclude air and are then placed in oven, where they are heated to 900-920 C, depending upon the composition of the steel. ⁰The carbon from the compound soaks into the surface of the hot steel to depth which depends upon the time that the box is left in the furnace. The internal remains unaffected while a thin case of high carbon content is formed on the surface.

• The steel is then removed from the box and reheated to a temperature just above critical temperature, followed by quenching in water, brine or oil. This refines the core.

• The steel is usually given second heat treatment at about 760-780 C.⁰

2. Liquid Carburizing

– When a fairly thin case is required a more economical process is to carburize the parts in a liquid bath. This consists of a container filled with molten salt, such as sodium cyanide, which is heated by electrical immersion elements or by a gas burner. Salt bath carburizing reduces distortion of the parts to a minimum, while equal heating being assured. There is freedom from sooting and oxidation problems as well and provides a uniform case depth.

– The only disadvantages are that cyanides are poisonous and some shapes cannot be handled because they will float.

3. Gas Carburizing

• In this case, metal is heated in a furnace into which a gas rich in such as methane, propane, butane, is introduced. Here it is necessary to maintain a continuous flaw of carburizing gas into the furnace and to extract the spent gas. Initial cost is high and process in economical only for large output.

• The horizontal rotary type furnace, used for smaller parts, in which the parts are rotated in stream of gas; while for large parts the vertical rotary furnace is used, in which gas is given a rotary motion so that it circulates around the parts.

CYNADING• Here liquid baths of sodium cyanide are used. The active hardening

agents of cyaniding baths i.e. C and N, are not produced directly from NaCN but first it decomposes to NaNCO i.e. sodium cyanate which decomposes as follows:

• 2NaCN + O₂→ 2NaNCO• 4NaNCO → Na₂CO₃ + CO + 2N• The proportion of N and C in the case depends upon the

composition and temperature of the bath. N content is higher in baths operating at lower temperature range and C content is higher for the higher ranges. And C content is lower than produced by carburizing, ranging from 0.5 – 0.8 percent.

• Exposure is for a shorter time and thin cases are produced up to 0.010 inches.

CARBONITRING• In this case steel is heated in a gaseous atmosphere of such

composition that C and N are absorbed simultaneously. The atmosphere used generally comprises of gases: carrier gas, enrichment gas and ammonia.

• Carrier gas is usually a mixture of nitrogen, hydrogen and carbon monoxide. It is supplied to furnace under positive pressure to prevent air infiltration thus making process easier to control.

• The enriching gas usually propane or natural gas is the main source for the carbon.

• The added ammonia breaks up to provide N to the surface of the steel.

• Cases up to 0.030 in. are produced.

FLAME HARDENING• This method does not change the chemical composition. Selected areas of

the surface of steel are heated into austenite range and then quenched to form martensite. So only steel capable of being hardened is used i.e. in the range of 0.30-0.50 percent carbon.

• Heat may be applied by a single oxyacetylene torch. It may be done as per following methods:

• 1. stationary• 2. progressing • 3. spinning• 4. progressive-spinning• In all procedures, provisions must be there for rapid quenching after the

required heating. This may be accomplished by water sprays etc. After then it needs to be relived of stresses by heating in the range of 350 – 400⁰C.

NITRIDING This is a process for case hardening of alloy steel in an atmosphere consisting of a mixture in suitable a proportion of ammonia gas and dissociated ammonia. The effectiveness of the process depends on the formation of nitrides in the steel by reaction of nitrogen with certain alloying elements. Although at suitable temperature and with the proper atmosphere all steels are capable of forming iron nitrides , the best results are obtained in those steels that contain one or more of major nitride-forming alloying elements. These are aluminum . Chromium and molybdenum. The nitrogen must be supplied in the atomic or nascent form ; molecular nitrogen will not react.

NITRIDING OPERATION

In nitriding process , nitrogen is introduced to steel by passing ammonia gas a muffle furnace containing the steel to be nitrided. The ammonia is purchased in tank as a liquid and introduced into the furnace as a gas at slightly greater than atmosphere pressure. With the nitriding operation at a temperature of 480 c to 540 c the ammonia gas partially dissociated into nitrogen and hydrogen gas mixture . The dissociation of ammonia is shown by following equation:

2NH3 = 2N + 3H2

• The operation of nitriding cycle is usually controlled so that the dissociation of the ammonia gas is held to approximately 30% but may be varied from 15% to 95% , depending upon operation conditions .

• The gas mixture leaving the furnace consists of hydrogen , nitrogen and undissociated ammonia. The undissociated ammonia which is soluble in water , is usually discharged into water and disposed in this manner.

• The free nitrogen formed by this dissociation is very active, uniting with the iron and other elements in the steel to form nitrides . These nitrides are more or less soluble en the form of a solid solution , or more likely are in a fine state of dispersion, imparting hardness to the surface of the steel. From the surface the nitrides defuse slowly and the hardness increases inwardly untill the unaffected core is reached . The depth of penetration depends largely upon the length of time spent at the nitriding temperature. Diffusion of these nitrides is much slower than diffusion of carbon in the carburizing operation so a much longer time is required to develop similar penetration.

MERITS & DEMERITS OF NITRIDING

• MERITS:-1. Greater resistance to wear and corrosion.

2. Less warping or distortion of parts treated.

3. Greater surface hardness.

4. Greater fatigue strength under corrosive conditions.

5. Higher endurance limit under bending stresses.

6. Better retention of hardness at elevated temperature .

• DEMERITS:-1. Medium use is expensive.2. High furnace cost due to long time of treatment.3. Necessity of using special alloy steels.4. Necessity of using high alloy containers to resist the

nitriding.

INDUCTION HARDENINGInduction hardening depends for its operation on localized heating produced by currents induced in a metal placed in rapidly changing magnetic field. The operation resembles a transformer in which primary or work coil is composed of several turns of copper tubing that are water cooled, and the part to be hardened is made the secondary of a high frequency induction apparatus. Five basic designs of work coil for use with high frequency units and the heat patterns developed by each shown in figure. These basic shapes are

(a).a simple solenoid for external heating

(b).a coil to be used internally for heating bores

(c).a “pie-plate” type of coil designed to provide high currents densities in a narrow band for scanning applications

(d).a single-turn coil for scanning a rotating surface, provided with a contoured half-turn that will aid in heating the fillet

(e).a “pancake” coil for spot heating .

INDUCTION HEATING & HARDENING MACHINE

Working of induction hardening process

When high frequency alternating current passes through the work coil a high frequency magnetic field is setup. This magnetic field induces high frequency eddy currents and hysteresis currents in the metal. Heating results from the resistance of metal doe to passage of these currents. The high frequency induced currents tends to travel at the surface of metal. This is known as skin effect. Therefore, it is possible to heat a shallow layer of steel without heating the interior. However, heat applied to surface tends to flow towards the center by conduction, and so time of heating is an important factor in controlling the depth of hardened zone. The surface layer is heated practically instantaneously to a depth which is inversely proportional to square root of frequency. The range of frequencies commonly used is between 10,000 and 500,000 Hz. Following table shows the effect of frequency on the depth of case hardness. Greater case hardening depth may be obtained at each frequency by increasing the time of heating.

TABLE: Effect of freq. on depth of Induction Hardness

FREQUENCY(Hz.) THEORETICLAL DEPTH OF PENETRATION OF ELECTRIC ENERGY

PRACTICAL DEPTHOF CASE HARDNESS

1,000 0.059 0.180 to 0.350

3,000 0.035 0.150 to 0.200

10,000 0.020 0.100 to 0.150

120,000 0.006 0.060 to 0.100

500,000 0.003 0.040 to 0.080

1,000,000 0.002 0.010 to 0.030

ADVANTAGES & DISADVANTAGES • ADVANTAGES:- The principle advantages are listed below:-

1. The time required for this heat treatment operation is less thereby increasing the labour productivity.

2. Deformation due to heat treatment is considerably reduced.

3. The articles which are induction heated have no scale effect.

4. The depth of hardness can be easily controlled by controlling the current.

5. The depth of hardness can be easily controlled by varying frequency of supply voltage.

• DISADVANTAGES:- Among the disadvantages are the cost of equipment, the fact that

small quantities or irregular-shaped parts cannot be handled economically costs. Typical parts that have been induction hardened are piston rods ,pump shafts , spur gears and cams.

PRECIPITATE HARDENING

Basic Definition

• The strength and hardness of some metal alloys may be enhanced by the formation of extremely small uniformly dispersed particles of a second phase within the original phase matrix; this must be accomplished by phase transformations that are induced by appropriate heat treatments, specifically Precipitation Hardening.

Other factors

• ‘‘Age hardening’’ is also used to designate this procedure because the strength develops with time, or as the alloy ages.

• Examples of alloys that are hardened by precipitation treatments include aluminium–copper, copper–beryllium, copper–tin, and magnesium-aluminium; some ferrous alloys are also precipitation hardenable.

• Mostly, precipitation hardening is commonly employed with high-strength aluminium alloys.

Mechanism

• Generally, the mechanism is best explained by means of a phase diagram.

• Though in practice, the precipitation hardenable alloys consist of 2 or more alloying elements, a binary phase diagram is used for simplified illustration.

• However, for successful hardening, 2 requisite features are mandatory – an appreciable solubility of one element in the other and a limitation on this which decreases with temperature.

• Age Hardening accomplished in 2 processes – Solution Heat Treating and Precipitation Heat Treating.

Solution Heat Treating…• In solution heat treatment, all

solute atoms are dissolved to form a singlephase solid solution.

• It features heating followed by rapid cooling/quenching so that β phase formation is prevented by diffusion as well.

• It is further comprehended by very slow rates of diffusion of β phase.

• Thus, it results in existence of a non-equilibrium state of α state supersaturated with β atoms.

Precipitation Heat Treating…• Precipitation Heat Treating

involves heating the α+β to a temperature suitable for faster diffusion rates of β phase followed by gradual cooling.

• With increase in the conc. of β phase, the strength of the alloy increases till a certain time (age), after which strength decreases gradually, called as overaging.

• The precipitation temperature and the ageing time at this temp. are crucial factors in determining the extent of precipitation and hence the strength of the alloy.

Strength vs Time Relation

Mechanism-An Example

• Studied most extensively on Aluminium-Copper alloys.• Studied best on alloy system of Al-Cu considering the phase

transformations in 96 wt% Al- 4 wt% Cu alloy.• With increasing concentration of copper in aluminium, the size

of precipitates of copper in aluminum matrix increases until an intermetallic compound is formed (CuAl2 ).

• Initially only a substitutional solid phase solution of copper in aluminum is present.

• As said, the mechanical properties of the alloy depend on the character of these precipitates.

Cu-Al Phase Diagram

Mechanism-Contd…• Initially an α phase is present which is a substitutional solid soln. of Cu in

Al. • Precipitation gradually starts with formation of copper atom clusters very

small in size and continues with gradual increase in size of precipitate particles.

• Intermediate metastable states are formed namely, θ”, θ’ and finally an intermetallic compound θ is formed.

• Strength is maximised at θ” phase with the subsequent stages formed as a result of overaging.

• The strengthening process is accelerated as the temperature is increased.• Ideally, temperature and time for the precipitation heat treatment should be

designed to produce a hardness or strength in the vicinity of the maximum.• Associated with an increase in strength is a reduction in ductility.

Lattice Transformation Factors• One more factor that makes

alloys amenable to age hardening.

• Distortions around the precipitates cause dislocation motions which are impeded thus making the alloy hard and strong.

• Softening and weakening during overaging is caused by resistance to slip in the lattice.

Miscellaneous Considerations

• A combination of the heat treatment processes employed for obtaining optimum strength characteristics.

• Ideally, alloys are first solution heat treated, quenched then cold worked and finally precipitation heat treated. This results in quite minimal loss of strength due to crystallization.

Characteristics

THANK YOU

• Submitted by-• N.Shasank (07326)• Aditya Sahu(07327)• Gaurav Sankhian(07328)• Rajat Diwan(07329)• Amit Chauhan(07330)• Rajat Thakur(07331)• Prashant Sharma(07332)• Akshaydeep Singh(07129)