heat treatment gc -08

8/6/2019 Heat Treatment Gc -08

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HEAT TREATMENT

G. CHOWDHURY Prof.(Met)


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Heat Treatment

Heat Treatment is the process of controlledheating and cooling of metals to alter theirphysical and mechanical properties without

changing the product shape.

Heat treatment sometimes takes placeinadvertently due to manufacturing processes

that either heat or cool the metal such aswelding or forming.


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Heat Treatment

DEFINATION:

A combination of heating & coolingoperation timed & applied to a metal or

alloy in the solid state in a way that willproduce desired properties.- Metal HandBook (ASM)


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Heat Treatment

Heat Treatment is often associated withincreasing thestrength of material

It can also be used to obtain certainmanufacturing objectives such as toimprove machining & formability, restoreductility etc.

Thus it is a very enabling manufacturingprocess that can not only helps other

manufacturing processeses, but

can also improve product performance byincreasing strength or other desirablecharacteristics.


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Heat Treatment

Steels are particularly suitable forheat treatments, since many phasetransformation are involved even inplain carbon steel and low-alloy steel.

Steels are heat treated for one of thefollowing reasons:

Hardening. Softening. Property modification.


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Hardening Heat Treatment

Hardening of steels is done to increase thestrength andwearproperties.

Pre-requisites for hardening issufficient

carbon andalloy content.

Sufficient Carbon - Direct hardening.

Otherwise- Case hardening

Case hardening is also done to harden asurface selectively leaving a tough core.


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Hardening Heat Treatment

Common Hardening Heat Treatments:

Quenching

Case carburizing

Case Nitriding Case Carbo-nitridingor Cyaniding

Flame hardening

Inductionhardening etc.


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Quenching

An act of

Heating to austenizing range, 30 500C above Ac3(Hypoeutectoid) or Ac1 (Hypereutectoid)

Holding sufficiently long for full transformation

Dipping in Quench Medium

Result

Formation of a hard & brittle structure known asMartensite.


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Mechanism ofQuenching

Gamma to alpha transformation takes place by aprocess of nucleation & growth and is timedependant.

Under slow cooling or moderate cooling rates, the

carbon atoms diffuse out of the austenite structure. With increase in cooling rate, insufficient time isallowed for carbon to diffuse out of the solution.

Although some movement of carbon atom takesplace, the structure can not be bcc, while the carbon

is trapped in solution. The resultant structure, Martensite is a

supersaturated solution of carbon trapped in a bodycentered tetragonal structure (bct).


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Quenching Contd..

Quenched steel is in a highly stressed condition and is toobrittle for any practical purpose. Quenching is alwaysfollowed by tempering to

Reduce the brittleness.

Relieve the internal stresses caused by hardening.

Hardening followed by tempering is intended for improvingthe mechanical properties of steel.

The chief aim in hardening of steel is to increase hardness &wear- resistance, retaining sufficient toughness at the sametime.


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Mechanism of Tempering

Tempering means subsequent heating to aspecific intermediate temperature and holdingfor specific time.

Tempering leads to the decomposition of

martensite into ferrite-cementite mixture andstrongly affects all properties of steel.

At low tempering temperature (up to 2000C or2500C), the hardness changes only to a smallextent and the true tensile strength and bending

strength are increased. This may be explained by the separation of

carbon atom from the martensite lattice and thecorresponding reduction in its stressed state andaccicularity.


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Mechanism of Tempering

A further increase in the tempering temperature

reduces the hardness, true tensile strength, and

proportional limit; yield point, while relative

elongation and reduction area increased.

This is due to formation offerrite and cementite

mixture.

At still higher temperature or holding time, the

spherodisation of cementite and coarsening offerrite grains leads to fall in hardness as well as

toughness.


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Quenching Contd..

Retained ferrite is detrimental to uniformproperties so heating beyond Ac3

Retained Cementite is beneficial as it is more

hard & wear resistant than martensite so

heating beyond Ac1, not ACM

Addition of C and Alloy elements shifts TTT

curve to right and changes the shape

Higher the carbon% - higher the hardness Higher the Alloy% - Higher the stability of M

Higher the degree of super cooling Higher

the amount of retained Austenite.


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Temper Embrittleness

A sharp fall in Impact strength when tempered at2500C to 4000C for extended hours is known astemper brittleness.

All steels, in varying degree, suffer from this.

Carbon steels display slight loss of toughness. The impact strength of alloy steel may be reducedby 50% to 60%

The reason fortemper brittleness is not yetsufficiently clear.

May be associated with the precipitation ofcarbides from the martensite in tempering.

May be due to decomposition ofretainedaustenite.

Temperature range of2500C to 4000C is avoided.


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Quenching Media

Quenching media with increased degree ofseverity of quenching

Normal Cooling, Forced Air or draftcooling, Oil, Polymer, Water and Brine

Material composition and weight of jobnormally decides the quenching medium

Aim is to have a cooling rate just bye-passing the nose of TTT curve for minimum

stress & warping during quenching. However, the cooling rate varies fromsurface to core: slower cooling towardscentre.


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Tempering contd..

Figure 1. Conventional quenching and tempering process


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Case Hardening

Objective is to harden the surface &

subsurface selectively to:

Obtain hard and wear-resistant surface

Obtain tough impact resistant core

Two get the best of both worlds

Case hardening can be done to all types of

plain carbon steels and alloy steels


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Case Hardening Contd..

Selectivity is achieved

a) For low carbon steels

By infus

ing carbon, boron or nitrogen in thesteel byheating in appropriate medium

Being Diffusion controlled process, Infusion isselective to

surface and subsurface

b) For medium & High carbon or Alloy steel By heating the surface selectively followed by Quenching


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Case Carburizing

Heating of low carbon steel in carburizingmedium like charcoal

Carbon atoms diffuse in job surface

Typical depth of carburisation; 0.5 to 5mm

Typical Temperature is about 9500C

Quenching to achieve martensite on surface

and sub-surface If needed, tempering to refine grain size and

reduce stresses


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Case Carbo-nitriding

Heating of low carbon steel containing Al in

cynide medium like cynide salt followed by

Quenching

Typical temperature is about 8500

C Nitrogen & Carbon atoms diffuse in job

Typical case depth 0.07mm to 0.5mm

Forms very hard & wear resistant complex

compounds, on surface & sub-surface

If needed, tempering to refine grain size and

reduce stresses


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Induction and Flame Hardening

Employed for medium & high carbon steel or alloy

steels

Local heating of the surface only either by flame or

induction current

Heating to austenizing range, 30 500C above Ac3(Hypoeutectoid) or Ac1 (Hypereutectoid)

Quenching in suitable quenching media If needed, tempering to refine grain size and reduce

stresses


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Softening Heat Treatment

Softening is done to: Reduce strength or hardness

Remove residual stresses

Restore ductility

Improve toughness Refine grain size

Change the electromagnetic properties of thesteel.

Restoring ductility or removing residualstresses is necessary when a large amountof cold working has been performed, such ascold-rolling operation or wiredrawing.


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Softening Heat Treatment

AnnealingIncomplete or Full

Normalizing

Tempering

Austempering

Martempering


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Incomplete Annealing

Dependingon working temperature, incomplete

annealingmay be sub-divided into:

Stress Relieving

Spherodising

Recrystallisationor Process annealing etc.


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Stress Relieving

To reduce residual stresses in large castings, weldedand cold-formed parts.

Such parts tend to have stresses due to thermalcycling or work hardening.

Parts are heated to temperatures of up to 600 -

650C (1112 - 1202F), and held for an extendedtime (about 1 hour or more) and then slowly cooledin still air.


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SpheroidizationSpheroidization is an annealing process used

for high carbon steels (Carbon > 0.6%) thatwill be machined or cold formedsubsequently.

This is done by one of the following ways:

Heat the part to a temperature just belowthe Ferrite-Austenite line, line A1 or belowthe Austenite-Cementite line, essentiallybelow the 727 C (1340 F) line.

Hold the temperature for a prolonged timeand follow by fairly slow cooling.

Or


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Spheroidization Contd.. Cycle multiple times between temperatures

slightly above and slightly below the 727 C (1340

F) line, say for example between 700 and 750 C

(1292 - 1382 F), and slow cool.

Or For tool and alloy steels heat to 750 to 800C

(1382-1472F) and hold for several hours followed

by slow cooling.


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All these methods result in a structure inwhich all the Cementite is in the form of smallglobules (spheroids) dispersed throughoutthe ferrite matrix.

This structure allows for improved machining

in continuous cutting operations such aslathes and screw machines.

Spheroidization also improves resistance toabrasion.

Spheroidization Contd..


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Process Annealing

Process Annealing is used to treat work-hardened parts made out of low-Carbon steels(< 0.25% Carbon).

This allows the parts to be soft enough toundergo further cold working withoutfracturing.

Temperature raised to just below the Ferrite-Austenite region, line A1on the diagram.

This temperature is about 727C (1341F) soheating it to about 700C (1292F) shouldsuffice.

Holding for sufficient time, followed by still

air cooling


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Process Annealing Contd..

Initially, the strained lattices reorient toreduce internal stresses (recovery)

When held long enough, new crystals grow(recrystallisation)

Since the material stays in the same phasethrough out the process, the only change thatoccurs is the size, shape and distribution of thegrain structure.

This process is cheaper than either fullannealing or normalizing since the material isnot heated to a very high temperature orcooled in a furnace.


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Full Annealing

The temperature is raised slowly to:About 50C (122F) above the Austenitictemperature line A3 in the case of Hypoeutectoid or eutectoid steels (steels with

0.8% Carbon).Held at this temperature for sufficienttime for all the material to transform intoAustenite or Austenite-Cementite as thecase may be.


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slowly cooled, preferably in the furnace at therate of about 30 C/hr to 200C/hr dependingon composition .

When temperature reaches below 500C,

taken out of furnace and cooled in roomtemperature air with natural convection.

The rate of heating for rolled stock or forgingsin heavy charges is as high as the given

furnaces can provide.

The holding time at the annealing temperaturefor these conditions is taken as one half to onehour per ton of the charge.

Full Annealing


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Full Annealing

Slow cooling in annealing enable the austeniteto decompose fully at lower degree of supercooling.

Higher the austenite stability, the slower thecooling must be to ensure full austenite

decomposition. Thus, alloy steels, in which austenite is very

stable should be cooled much slower thancarbon steel.

Complete phase recrystallisation ofhypereutectoid steel (full annealing) is required only incase when hot working (rolling or forging) wasfinished at high temperature resulting in coarsegrained structure.


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Full Annealing

The grain structure has coarse Pearlite withferrite or Cementite (depending on whether hypoor hyper eutectoid).

Complete phase recrystallisation of hypereutectoid steel (full annealing) is required only in

case when hot working (rolling or forging) wasfinished at high temperature resulting in coarsegrained structure.

If hot working is finished at a normaltemperature, incomplete annealing will besufficient.

Hypoeutectoid hot worked steel (rolled stock,sheet, forgings, castings etc) of carbon & alloysteels, may undergo full annealing.


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NormalizingRaising the temperature to 60C (140 F) above

line A3 or line ACM; fully into the Austenite range.

Held at this temperature to fully convert the

structure into Austenite

Removed from the furnace and cooled at roomtemperature under natural convection.

This results in a grain structure of fine Pearlite

with excess of Ferrite or Cementite.The resulting material is soft; the degree of

softness depends on the actual ambient

conditions of cooling.


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Difference of full annealing and normalizing

Normalising considerably cheaper than full annealing

since there is no added cost of controlled furnace cooling.

Fully annealed parts are uniform in softness (and

machinablilty) throughout the entire part; since the entirepart is exposed to the controlled furnace cooling.

Normalized parts, depending on the part geometry,

exhibit non-uniform material properties as the cooling is

non-uniform.Annealing always produces a softer material than

normalizing.

Normalizing Vs Annealing


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Austempering

Austempering is a specially designedquenching technique.

Job is quenched around 315 C (above Ms).

Held at this temperature for sufficient time to

Homogenize surface & core temperature. Undergo isothermal transformation from

Austenite to Bainite.

Bainite, being much finely spaced pearlite

structure is tough as well as hard enough Suitable for direct use in many applications


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Austempering Contd..

Austempering process.


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Martempering

Martempering is also a specially designed

quenching technique.

Job is quenched around 315 C (above MS).

Held at this temperature for sufficient time to

Homogenize surface & core temperature.

Further quenched to MSthrough MFThe structure is martensite

Needs to tempered to achieve desired

properties just like normal quenching &tempering

Advantage of Martempering over rapid

quenching is less distortion and tendency

to crack


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Martempering Contd..

Martempering process.


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Hardenability

Ability of a metal to respond to

hardening treatment

For steel, the treatment is Quenching toform Martensite

Two factors which decides hardenability

TTT Diagram specific to the composition

Heat extraction or cooling rate


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Hardenability Contd..

TTT Diagram

For low carbon steel, the nose is quite close totemperature axis

Hence very fast cooling rate is required toform Martensite

Causes much warp, distortion and stress

Often impossible for thick sections Carbon and Alloy addition shifts the nose to

right and often changes the shape


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Factors affecting cooling rate

Heating Temperature

Quenching bath temperature Specific heat of quenching medium

Job thickness

Stirring of bath to effect heat convection Continuous or batch process


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Hardenability is quantified as the depth uptowhich full hardness can be achieved

Amount of carbon affects both hardness of

martensite and hardenability Type and amount of alloying elements affect

mostly hardenability

The significance of alloying element is inlowering cooling rate for lesser distortion and

thick section


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Property Modification Treatment

These heat treatments are aimedeither to achieve a specific propertyor to get rid of a undesired property.

Examples are; Solution heat treatment

Precipitation hardening etc.


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Solution Heat Treatment

The term generally refers to taking all thesecondary phases into solution by heating andholding at a specific temperature

Except martensite, all other phases in steel arediffusion product

They appear or disappear in the primary

matrix by diffus

ion controlled process

Diffusion isTime & Temperature dependant


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In SS, when held at temperature range of

7000C 9000C, Cr combines with Carbon at GB

to form complex inter-metallic compounds

This depletes the GB of Cr resulting in loss of

corrosion resistance at GB

Become susceptible to Inter Granular

corrosion.


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Thissituation may occur due to high service

temperature or repair welding

Remedy is

Heat the job at 10500C

Hold till all the carbide redissolves in matrix

Fast cool to RT avoiding re-precipitation


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Precipitation Hardening

Hardening in steel is mainly due to martensite

formation during quenching

Common non-ferrous metals and alloys normally

dont respond to quenching

Precipitation or Dispersion hardening is a method

where finely dispersed second phase precipitates in

the primary matrix

These precipitations locks the movement of

dislocation causing increase in hardness


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Technique used for strengthening Al (Mg, Cu),Mg, Ti (Al, V) alloys and some variety of SS,Maraging Steel etc.

Also known as Dispersion orAge hardening

This technique exploits the phenomenon ofsupersaturation.

Nucleation occurs at a relatively hightemperature (often just below the solubility limit)

to maximise number of precipitate particles. These particles are then allowed to grow at

lower temperature in a process called aging..


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Diffusion's exponential dependence upon

temperature makes precipitation strengthening, like

all heat treatment, a fairly delicate process.

Too little diffusion (under aging), and the particleswill be too small to impede dislocations effectively;

too much (over aging), and they will be too large and

dispersed to interact with the majority of

dislocations

Typical dislocation size is 5-30 nm


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heat treatment gc -08

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