r.n. g. patel institute of technology (mechanical

R.N. G. Patel Institute of Technology(Mechanical Engineering Department)

Heat treatment Processes

Dr. K. B. Rathod

Heat treatment Processes

Dr. K. B. Rathod

Most of the images are courtesy of the book “Material Science and Engg. an Introduction” byWilliam D. Callister, Jr. & David G. Rethwisch.Few were adopted from “ The Science and Engineering of Materials” by Donald. R. Askeland.Images collected from Wikipedia and google images were also used.

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Heat Treatment

Temperature Transformation (TTT) Diagram:

TTT diagram is a plot of temperature versus the logarithm of time for asteel alloy of definite composition.

It is used to determine whentransformations begin andend for an isothermal heattreatment of a previouslyaustenitized alloy.

TTT diagram indicates whena specific transformationstarts and ends and it alsoshows what percentage of

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TTT diagram is a plot of temperature versus the logarithm of time for asteel alloy of definite composition.

It is used to determine whentransformations begin andend for an isothermal heattreatment of a previouslyaustenitized alloy.

TTT diagram indicates whena specific transformationstarts and ends and it alsoshows what percentage oftransformation of austenite ata particular temperature isachieved.

Heat Treatment

Temperature Transformation (TTT) Diagram: Depending on the type of heat treatment, time and temperature, final

microstructure of the steel, or any Iron carbon will be changed and so doesthe properties.

MartensiteT Martensite

BainiteFine Pearlite

Coarse PearliteSpheroidite

General Trends

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Stre

ngth

Duc

tility

MartensiteT Martensite

BainiteFine Pearlite

Coarse PearliteSpheroidite

General Trends

Heat Treatment

e Temperature Transformation (TTT) Diagram:

Iron-carbon alloywith Eutectoid(0.8 % C)composition.

A: Austenite

P: Pearlite

B: Bainite

M: Martensite

e Temperature Transformation (TTT) Diagram:

Iron-carbon alloywith Eutectoid(0.8 % C)composition.

A: Austenite

P: Pearlite

B: Bainite

M: Martensite

Example1:

Iron-carbon alloy witheutectoid composition.

Specify the nature of the finalmicrostructure (% bainite,martensite, pearlite etc) for thealloy that is subjected to thefollowing time–temperaturetreatments:

Bainite,100%

Temperature Transformation (TTT) Diagram


Alloy begins at 760˚C and hasbeen held long enough toachieve acomplete and

austenitichomogeneousstructure.

Treatment (a) Rapidly cool to 350˚C Hold for 104seconds Quench to roomtemperature

Temperature Transformation (TTT) Diagram

Example2:



Martensite,100%

Example2:



Alloy begins at 760˚C and hasbeen held long enough toachieve a complete and

austenitichomogeneousstructure.

Treatment (b) Rapidly cool to 250˚C Hold for 100seconds Quench to roomtemperature

Austenite,100%


Bainite, 50%

Example3:

Iron-carbon alloy with eutectoidcomposition.

Specify the nature of the finalmicrostructure (% bainite,martensite, pearlite etc) for the

is subjected to thetime–temperature

alloy thatfollowingtreatments:

Austenite,100%

Almost 50%Pearlite,50%Austenite

Bainite, 50%

alloy thatfollowingtreatments:

Alloy begins at 760˚C and hasbeen held long enough to achievea complete & homogeneousaustenitic structure.

Treatment (c) Rapidly cool to 650˚C Hold for 20 seconds Rapidly cool to 400˚C Hold for 103 seconds Quench to room temperature

Final:50%Bainite,50%Pearlite

Critical Cooling RateCritical Cooling Rate

In Figure the cooling rates A and B indicate two rapid cooling processes.The end product of both cooling rates will be martensite. Cooling rate B isknown as the Critical Cooling Rate, which is represented by a cooling curve thatis tangent to the nose of the TTT diagram.Critical Cooling Rate is defined as the lowest cooling rate which produces 100%Martensite.

Importance of CCRImportance of CCR

Important when hardening the steels. To obtain fully martensiticstructure, the cooling rate must be more than CCR.

Actually it determines the type of structure obtained on cooling.Slow cooling will produce pearlite and higher cooling rate can formmartensite.

The final properties of the steel will depend upon the magnitude ofthe CCR.

Important when hardening the steels. To obtain fully martensiticstructure, the cooling rate must be more than CCR.

Actually it determines the type of structure obtained on cooling.Slow cooling will produce pearlite and higher cooling rate can formmartensite.

The final properties of the steel will depend upon the magnitude ofthe CCR.

Factors Affecting CCRFactors Affecting CCR

Composition of steel: The CCR varies with the carbon content of steel.It is minimum when C is around 0.9%.

Temperature of Hardening: The higher is the hardening temperature,the more uniform becomes the austenite, the lower is the critical coolingtime, and more will be the critical cooling rate.

Purity of Steel: The purer is the steel, the lesser will be the CCR.

Composition of steel: The CCR varies with the carbon content of steel.It is minimum when C is around 0.9%.

Temperature of Hardening: The higher is the hardening temperature,the more uniform becomes the austenite, the lower is the critical coolingtime, and more will be the critical cooling rate.

Purity of Steel: The purer is the steel, the lesser will be the CCR.

Heat Treatment

Heat treating is a group of industrial and metalworking processes usedto alter the physical, and sometimes chemical, properties of a material.The most common application is metallurgical.

Heat treatment involves the use of heating or chilling, normally toextreme temperatures, to achieve a desired result such as hardening orsoftening of a material.

Heat treatment techniques include:

Annealing,

Case hardening,

Precipitation strengthening,

Tempering, and

Quenching.11

Heat treating is a group of industrial and metalworking processes usedto alter the physical, and sometimes chemical, properties of a material.The most common application is metallurgical.

Heat treatment involves the use of heating or chilling, normally toextreme temperatures, to achieve a desired result such as hardening orsoftening of a material.

Heat treatment techniques include:

Annealing,

Case hardening,

Precipitation strengthening,

Tempering, and

Quenching.

Heat Treatment

Annealing: Annealing

Annealing involves heating the material to a predetermined temperatureand hold the material at the temperature and cool the material to the roomtemperature slowly. The process involves:

1) Heating of the material at the

elevated or predetermined

temperature

2) Holding the material (Soaking) at

the temperature for longer time.

3) Very slowly cooling the material

to the room temperature.3

Annealing: Annealing

Annealing involves heating the material to a predetermined temperatureand hold the material at the temperature and cool the material to the roomtemperature slowly. The process involves:

1) Heating of the material at the

elevated or predetermined

temperature

2) Holding the material (Soaking) at

the temperature for longer time.

3) Very slowly cooling the material

to the room temperature.

Heat Treatment

Annealing:

The various purpose of these heat treatments is to:

1) Relieve Internal stresses developed during solidification,

machining, forging, rolling or welding,

2) Improve or restore ductility and toughness,

3) Enhance Machinability,

4) Eliminate chemical non-uniformity,

5) Refrain grain size, &

6) Reduce the gaseous contents in steel.

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machining, forging, rolling or welding,

2) Improve or restore ductility and toughness,

3) Enhance Machinability,

4) Eliminate chemical non-uniformity,

5) Refrain grain size, &

6) Reduce the gaseous contents in steel.

Heat Treatment

Annea ling: Process Anneal ing

In this treatment, steel (or any material) is heated to a temperaturebelow the lower critical temperature, and is held at this temperature forsufficient time and then cooled.

Cooling rate is of littleimportance as the process isbeing done at sub criticaltemperatures.

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In this treatment, steel (or any material) is heated to a temperaturebelow the lower critical temperature, and is held at this temperature forsufficient time and then cooled.

Cooling rate is of littleimportance as the process isbeing done at sub criticaltemperatures.

The purpose of thistreatmenthardness

is to reduceand to increase

ductility of cold-worked steelso that further working maybe carried easily.

Heat Treatment

Annea ling: Process Anneal ing

This process is extensively used in the treatment of sheets and wires.

Parts which are fabricatedby cold forming such asstamping, extrusion, upsettingand drawing are frequentlygiven this treatment as anintermediate step.

Scaling or oxidation can be

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This process is extensively used in the treatment of sheets and wires.

Parts which are fabricatedby cold forming such asstamping, extrusion, upsettingand drawing are frequentlygiven this treatment as anintermediate step.

Scaling or oxidation can beprevented orthis process

minimized byspecially if

annealed at lower temperaturesor in non-oxidizing areas.

Heat Treatment

Annealing: Stress Relieving

As the name suggests, this process is employed to relieve internalstresses. No microstructural changes occur during the process.

Internal stresses are those stresses which can exist within a body in theabsence of external forces. These are also known as residual stresses arelocked-in stresses.

These stresses are developed in operations like:

Solidification of castings, welding, machining, grinding, shot peening,

surface hammering, cold working, case hardening, electroplated coatings,

precipitation and phase transformation.

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As the name suggests, this process is employed to relieve internalstresses. No microstructural changes occur during the process.

Internal stresses are those stresses which can exist within a body in theabsence of external forces. These are also known as residual stresses arelocked-in stresses.

These stresses are developed in operations like:

Solidification of castings, welding, machining, grinding, shot peening,

surface hammering, cold working, case hardening, electroplated coatings,

precipitation and phase transformation.

Heat Treatment


These internal stresses under certain conditions can have adverse effects:example: Steels with residual stresses under corrosive environment fail with stresscorrosion cracking.

These stresses also enhance the tendencyof steels towards warpage and dimensionalinstability.

Fatigue strength is reduced considerablywhen residual tensile stresses are presentin steel.

The problems associated with internalstresses are more difficult in brittlematerials than in ductile materials.

Stress –Corrosion Cracki1n5g

These internal stresses under certain conditions can have adverse effects:example: Steels with residual stresses under corrosive environment fail with stresscorrosion cracking.

These stresses also enhance the tendencyof steels towards warpage and dimensionalinstability.

Fatigue strength is reduced considerablywhen residual tensile stresses are presentin steel.

The problems associated with internalstresses are more difficult in brittlematerials than in ductile materials.

Stress –Corrosion Cracki1n5g

Heat Treatment

The process of stress relieving consists of heating materials uniformly to atemperature below the lower critical temperature, holding at this temperature forsufficient time, followed by uniform cooling.


Uniform cooling is of utmostimportance as non-uniform coolingwill itself result in the development ofwill itself result in the development ofinternal stresses. Thus the very

Stress –Corrosion Cracking

purpose of stress relieving will be lost.

Plain carbon steels and low alloysteels generally temperature is limitedto 600 oC. Higher temperature is usedfor high alloy steels.

The extent of the stresses relieveddepend upon the temperature employedand holding time.

Heat Treatment

Annealing: Norm a lizing

Normalizing is similar to full annealing, except steel is generally cooledin still air.

The normalizing consists ofheating steel to about 40-55 oCabove critical temperature(Ac3 or Accm), and holding forproper item and then coolingin still air or slightly agitatedair to room temperature.

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Normalizing is similar to full annealing, except steel is generally cooledin still air.

The normalizing consists ofheating steel to about 40-55 oCabove critical temperature(Ac3 or Accm), and holding forproper item and then coolingin still air or slightly agitatedair to room temperature.

In some special cases,cooling rates can be controlledby either changing airtemperature or air volume.

Heat Treatment


After normalizing, the resultant micro-structure should be pearlitic.

Since the temperature involvedin this process is more than thatfor annealing , the homogeneity

results in better dispersionof austenite increases and it

of

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results in better dispersionof austenite increases and it

offerrite and Cementite in the finalstructure.

Results in better dispersion offerrite and Cementite in the finalstructure.

The grain size is finer in normalized structure than in annealed structure.

Heat Treatment

Normalized steels are generally stronger and harder than fully annealedsteels.


Steels are soft in annealedcondition and tend to stick duringmachining. By normalizing, anoptimum combination of strengthand softness is achieved, whichresults in satisfactory level ofMachinability in steels.

Normalizing is the effectiveway to eliminate the carbidenetwork.

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condition and tend to stick duringmachining. By normalizing, anoptimum combination of strengthand softness is achieved, whichresults in satisfactory level ofMachinability in steels.

Normalizing is the effectiveway to eliminate the carbidenetwork.

Heat Treatment

Normalized treatment is frequently applied to steel in order to achieveany one or more of the objectives, namely:

To refine the grain structure,

To obtain uniform structure,

To decrease residual stresses,

To improve Machinability.


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Normalized treatment is frequently applied to steel in order to achieveany one or more of the objectives, namely:

To refine the grain structure,

To obtain uniform structure,

To decrease residual stresses,

To improve Machinability.

Heat Treatment

Hardening:

Hardening and Hardness are two very different things. One is a processof heat treatment and other is a extrinsic property of a material.

Hardening is a heat treatmentprocess in which steel is rapidly cooledfrom austenitising temperature. As aresult of hardening, the hardness andwear resistance of steel are improved.

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process in which steel is rapidly cooledfrom austenitising temperature. As aresult of hardening, the hardness andwear resistance of steel are improved.

Hardening treatmentconsists of

generallyhardening

temperature,heating to

holding at thattemperature, followed by rapid coolingsuch as quenching in oil or water or saltbaths.

Heat Treatment

Hardening: The high hardness developed by this process is due to the phasetransformation accompanying rapid cooling. Rapid cooling results in thetransformation of austenite at considerably low temperature into non-equilibrium products.

The hardening temperature depends on chemical composition. For plaincarbon steels, it depends on the carbon content alone. Hypoeutectoid steelsare heated to about 30 – 50 oC above the upper critical temperature, whereaseutectoid and hyper eutectoid steels are heated to about 30 – 50 oC abovelower critical temperature.

Ferrite and pearlite transform to austenite at hardening temperature forhypoectectoid steel. This austenite transforms to martensite on rapidquenching from hardening temperature. The presence of martensiteaccounts for high hardness of quenched steel.

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Hardening: The high hardness developed by this process is due to the phasetransformation accompanying rapid cooling. Rapid cooling results in thetransformation of austenite at considerably low temperature into non-equilibrium products.

The hardening temperature depends on chemical composition. For plaincarbon steels, it depends on the carbon content alone. Hypoeutectoid steelsare heated to about 30 – 50 oC above the upper critical temperature, whereaseutectoid and hyper eutectoid steels are heated to about 30 – 50 oC abovelower critical temperature.

Ferrite and pearlite transform to austenite at hardening temperature forhypoectectoid steel. This austenite transforms to martensite on rapidquenching from hardening temperature. The presence of martensiteaccounts for high hardness of quenched steel.

Heat Treatment

Hardening:Hardening is applied to cutting tools and machine parts where high hardness andwear resistance are important.

The Process Variables:

Hardening Temperature: The steel should be heat treated to optimum austenitisingtemperature. A lower temperature results lower hardness due to incompletetransformation t austenite. If this temperature is too high will also results lowerhardness due to a coarse grained structure.

Soaking Time: Soaking time at hardening temperature should be long enough totransform homogenous austenite structure. Soaking time increases with increase insection thickness and the amount of alloying element.

Delay in quenching: After soaking, the steel is immediately quenched. Delay inquenching may reduce hardness due to partial transformation of austenite.

Type of quenching medium also has a profound effect, which will be discussed briefly2.3

Hardening:Hardening is applied to cutting tools and machine parts where high hardness andwear resistance are important.

The Process Variables:

Hardening Temperature: The steel should be heat treated to optimum austenitisingtemperature. A lower temperature results lower hardness due to incompletetransformation t austenite. If this temperature is too high will also results lowerhardness due to a coarse grained structure.

Soaking Time: Soaking time at hardening temperature should be long enough totransform homogenous austenite structure. Soaking time increases with increase insection thickness and the amount of alloying element.

Delay in quenching: After soaking, the steel is immediately quenched. Delay inquenching may reduce hardness due to partial transformation of austenite.

Type of quenching medium also has a profound effect, which will be discussed briefly2.3

Heat Treatment

Hardening:

The main purpose of hardening tool steel is to develop high hardness.This enables tool steel to cut other metals. High hardness developed bythis process also improves wear resistance. Gears, shafts and bearings.Tensile strength and yield strength are improved considerably y hardeningstructural steels.

Because of rapid cooling, highinternal stresses are developed inthe hardened steel. Hence thesesteels are generally brittle.Hardening in general is followedby another treatment known astempering which reduces internalstresses and makes the hardenedsteel relatively stable,

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Hardening:

The main purpose of hardening tool steel is to develop high hardness.This enables tool steel to cut other metals. High hardness developed bythis process also improves wear resistance. Gears, shafts and bearings.Tensile strength and yield strength are improved considerably y hardeningstructural steels.

Because of rapid cooling, highinternal stresses are developed inthe hardened steel. Hence thesesteels are generally brittle.Hardening in general is followedby another treatment known astempering which reduces internalstresses and makes the hardenedsteel relatively stable,

Heat Treatment

Tempering: Hardened steels are so brittle that even a small impact will causefracture. Toughness of such a steel can be improved by tempering.However there is small reduction in strength and hardness.

Tempering is a sub-critical heattreatment process used to improvethe toughness of hardened steel.

Tempering consists of reheatingof hardened steel to a temperaturebelow Lower critical temperatureand is held for a period of time, andthen slowly cooled in air to roomtemperature.

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Tempering: Hardened steels are so brittle that even a small impact will causefracture. Toughness of such a steel can be improved by tempering.However there is small reduction in strength and hardness.

Tempering is a sub-critical heattreatment process used to improvethe toughness of hardened steel.

Tempering consists of reheatingof hardened steel to a temperaturebelow Lower critical temperatureand is held for a period of time, andthen slowly cooled in air to roomtemperature.

Heat Treatment

Tempering: At tempering temperature, carbon atoms diffuses out and form finecementite and softer ferrite structure left behind. Thus the structure oftempered steel consists of ferrite and fine cementite.

Thus tempering allows to precipitatecarbon as very fine carbide and allow themicrostructure to return to BCC

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Tempering: At tempering temperature, carbon atoms diffuses out and form finecementite and softer ferrite structure left behind. Thus the structure oftempered steel consists of ferrite and fine cementite.

Thus tempering allows to precipitatecarbon as very fine carbide and allow themicrostructure to return to BCC

function of the parts. Cutting tools

The temperatures are related to theare

tempered between 230 – 300 oC. If greaterductility and toughness are desired as in caseof shafts and high strength bolts, the steel istempered in the range of 300 – 600 oC.

Heat TreatmentTempering:Tempering temperatures are usually identified by the colour. Temperingtemperatures for tools and shafts along with temper colors.

Depending on temperatures, tempering processes can be classified as:

1)Low- temperature tempering(150 – 250 oC),

2)Medium – temperaturetempering (350 – 450 oC),

3)High – temperature tempering(500 – 650 oC).

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Tempering:Tempering temperatures are usually identified by the colour. Temperingtemperatures for tools and shafts along with temper colors.

Depending on temperatures, tempering processes can be classified as:

1)Low- temperature tempering(150 – 250 oC),

2)Medium – temperaturetempering (350 – 450 oC),

3)High – temperature tempering(500 – 650 oC).

Heat TreatmentTempering: Tempering temperatures are usually identified by the colour. Temperingtemperatures for tools and shafts along with temper colors.

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Heat TreatmentHardenability:The responsibility of a steel to a given hardening treatment is indicated bythe property known as Hardenability.

It is an index of the depth to whichthe martensite can be formed in agiven steel as a result of a givenhardening treatment.

The term Hardenability is used tomeasure the depth of hardnessachieved i.e. martensite introducedinto the steel section by quenchingthe steel from austenite state.

Greater the depth of hardness below the surface, higher will be the29

Hardenability:The responsibility of a steel to a given hardening treatment is indicated bythe property known as Hardenability.

It is an index of the depth to whichthe martensite can be formed in agiven steel as a result of a givenhardening treatment.

The term Hardenability is used tomeasure the depth of hardnessachieved i.e. martensite introducedinto the steel section by quenchingthe steel from austenite state.

Greater the depth of hardness below the surface, higher will be theHardenability of steel.

Heat Treatment

Hardenability:Hardenability of steel depends on composition of steel, method ofquenching and section of steel.

The addition of alloying elementsin steel decreases the critical coolingrate. Thus the Hardenability of alloysteels is more than that of the carbonsteels.

While in the oil quenching, thecooling rates are lower than waterquenching and thus the hardnessvalues are lower in case of oilquenched steels.

The larger section shows lower Hardenability because of their increase30

Hardenability:Hardenability of steel depends on composition of steel, method ofquenching and section of steel.

The addition of alloying elementsin steel decreases the critical coolingrate. Thus the Hardenability of alloysteels is more than that of the carbonsteels.

While in the oil quenching, thecooling rates are lower than waterquenching and thus the hardnessvalues are lower in case of oilquenched steels.

The larger section shows lower Hardenability because of their increasemass results in a lower overall rate of cooling.

Heat Treatment

rdenability: Jominy E nd Quench Test

The most simple and convenient method of determining the Hardenabilityis the Jominy EndQuench Test.

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The Jominy test involves heating a standard test piece of diameter 25 mmand length 100 mm to the austenite state, fixing it to a frame in a verticalposition and then quenching the lower end by means of a jet of water.

Heat Treatment

the test price, and hardnessmade

Hardenability: Jominy E nd Quench Test

The mode of quenching results in different rate of cooling along thelength of the test piece.

After a quenching, a flat of 0.38mm deep is ground along one side of

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the test price,measurements are

and hardnessmade along the

length of the test piece.

A bar of steel having goodHardenability shows higher hardnessreadings for greater distance from thequenched end.

Heat Treatment

Quenching: Quenching is a process of rapid cooling of materials from hightemperature to room temperature or even lower. In steels quenchingresults in transformation of austenite to martensite (a non-equilibriumconstituent).

During cooling, heat must be extracted ata very fast rate from the steel piece. This ispossible only when a steel piece is allowedto come in contact with some medium whichcan absorb heat from the steel piece with in ashort period.

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Quenching: Quenching is a process of rapid cooling of materials from hightemperature to room temperature or even lower. In steels quenchingresults in transformation of austenite to martensite (a non-equilibriumconstituent).

During cooling, heat must be extracted ata very fast rate from the steel piece. This ispossible only when a steel piece is allowedto come in contact with some medium whichcan absorb heat from the steel piece with in ashort period.

Under ideal conditions, all the heatabsorbed by the medium should be rejectedto the surroundings immediately.

Heat Treatment

Quenching:

The removal of heat during quenching is complex in the sense that heatis removed in three stages.

1) Vapor Blanket,

2) Nucleate Boiling,

3) Convection.

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1) Vapor Blanket,

2) Nucleate Boiling,

3) Convection.

Heat Treatment

temperature of the work- piece.

Quenching:Vapor Blanket (stage 1)

As soon as the work-piece comes into contact with a liquid coolant(quenchant), the surrounding quenchant layer is instantaneously heated upto the boiling point of the quenchant and gets vaporized due to the high

This acts as an insulator, preventingthe quenching oil from contacting themetal surface. As a consequence, the rateof cooling during this stage is slow.

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This acts as an insulator, preventingthe quenching oil from contacting themetal surface. As a consequence, the rateof cooling during this stage is slow.

At this stage the work piece is cooledonly by conduction and radiationthrough the vapor film.

Only the surface is cooledconsiderably prior to the formation ofvapor envelop.

Heat TreatmentQuenching:Nucleate Boiling (stage 2)

This second stage is also called as transport cooling stage or liquidboiling stage. The temperature of the work-piece comes down, throughvery slowly and the vapor blanket is no longer stable and collapses.

Metal surface comes into contact with the liquid/quenchant. Violent boiling quickly removes heatfrom the quenched component while formingbubbles and being pushed away, resulting in thecooler fluid coming into contact with the workpiece.

This happens till the temperature of the work piececomes down to the boiling point of the liquid.

Maximum cooling rate is achieved during this36

Quenching:Nucleate Boiling (stage 2)

This second stage is also called as transport cooling stage or liquidboiling stage. The temperature of the work-piece comes down, throughvery slowly and the vapor blanket is no longer stable and collapses.

Metal surface comes into contact with the liquid/quenchant. Violent boiling quickly removes heatfrom the quenched component while formingbubbles and being pushed away, resulting in thecooler fluid coming into contact with the workpiece.

This happens till the temperature of the work piececomes down to the boiling point of the liquid.

Maximum cooling rate is achieved during thisstage.

Unit 5Heat TreatmentQuenching:Convection (stage 3)

The third stage is called as the liquid cooling stage or the convection stage.

I starts when the temperature of thesurface becomes equal to the boilingpoint of the quenchant.

Cooling at this stage takes place viaconduction and convection processes.

The rate of cooling is the slowest atthis stage.

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Quenching:Convection (stage 3)

The third stage is called as the liquid cooling stage or the convection stage.

I starts when the temperature of thesurface becomes equal to the boilingpoint of the quenchant.

Cooling at this stage takes place viaconduction and convection processes.

The rate of cooling is the slowest atthis stage.

Heat TreatmentQuenching: Effect of Quenching M ediumQuenching medium has the profound effect on the final phase of thematerial. Quenching medium is directly related to the rate of the coolingof the material.

Some of the widely employed quenching media are water, aqueoussolutions, oils (mineral, vegetable and even animal oils), molten salts andair.

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Quenching: Effect of Quenching M ediumQuenching medium has the profound effect on the final phase of thematerial. Quenching medium is directly related to the rate of the coolingof the material.

Some of the widely employed quenching media are water, aqueoussolutions, oils (mineral, vegetable and even animal oils), molten salts andair.

Heat Treatmentnching: Effect of Quenching M edium (Water)

Water has maximum cooling rateamongst all common quenchantsexcept few aqueous solutions.

It is very cheap and easily disposedoff compared to other quenchants.

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Hence water is used for carbonsteels, alloy steels and non-ferrousalloys.

The layer if scale formed on thesurface during heating is also brokenby water quenching, thus eliminatingan additional process of surfacecooling.

Heat TreatmentOIL)enching: Effect of Quenching M edium (

Most of the Oils used as quenchantsare mineral oils. These are in generalparaffin based and do not possess anyfatty oils.

Quenching in oil provides slowercooling rates as compared to thoseachieved by water quenching.

The slower cooling rate reduces thepossibility of hardening defects.

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enching: Effect of Quenching M edium (

Most of the Oils used as quenchantsare mineral oils. These are in generalparaffin based and do not possess anyfatty oils.

Quenching in oil provides slowercooling rates as compared to thoseachieved by water quenching.

The slower cooling rate reduces thepossibility of hardening defects.

The temperature difference betweencore and the case of work piece is lessfor oil quenching than for waterquenching.

Heat TreatmentQunching: Effect of Quenching M edium ( A IR)

Many alloy steels are capable ofgetting hardened by cooling either in stillair or in a blast of air.

Such steels are popularly known as airhardening steels.

These steels are almost free from

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getting hardened by cooling either in stillair or in a blast of air.

Such steels are popularly known as airhardening steels.

These steels are almost free fromdistortionproblem of

problem. However, theoxidation during cooling

(quenching) may be encountered inpractice. Many grades of tool steels aresubjected to air hardening.

Cooling rates can be improved bymixing air and water.

Heat TreatmentQUenching: Effect of Quenching M edium :

Just the drastic water quench generates a fully martensite structure.

Although quenched in oil the austensite converts into suitably fine pearlite.

Accurate pearlite also results if the austenised eutectoid steel is air-cooled.

Though, if allowed to cool in furnace coarse pearlite is appearance.

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QUenching: Effect of Quenching M edium :

Just the drastic water quench generates a fully martensite structure.

Although quenched in oil the austensite converts into suitably fine pearlite.

Accurate pearlite also results if the austenised eutectoid steel is air-cooled.

Though, if allowed to cool in furnace coarse pearlite is appearance.

Heat Treatmentenching: Effect of Quenching M edium

Coarse Pearlite

- Smaller T:colonies arelarger

43Eutectoid Steel

- Larger T:colonies aresmaller

Figure: Microstructure resultingfrom Different Cooling Rates

Applied to Austenitized Samples of

Fine Pearlite

Heat Treatmenturface Hardening:In many situations hard and wear resistance surface is required with thetough core. Because of tough core the components can withstand impactload. The typical applications requiring these conditions include gearteeth, cams shafts, bearings, crank pins, clutch plate, tools and dies.

The combination of the these properties can be achieved by thefollowing methods:

1. Hardening and tempering the surface layers (surface hardening)

(i) Flame Hardening (ii) Induction Hardening

2. Changing the composition at surface layers (chemical heat

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urface Hardening:In many situations hard and wear resistance surface is required with thetough core. Because of tough core the components can withstand impactload. The typical applications requiring these conditions include gearteeth, cams shafts, bearings, crank pins, clutch plate, tools and dies.

The combination of the these properties can be achieved by thefollowing methods:

1. Hardening and tempering the surface layers (surface hardening)

(i) Flame Hardening (ii) Induction Hardening

2. Changing the composition at surface layers (chemical heat

treatment or case hardening)

(i) Carburising (ii) Nitriding (iii) Carburising and Cyaniding

Heat Treatmentrface Hardening: F l a m e Hardening

The flame hardening involves heating the surface of a steel to atemperature above upper critical point (850 oC) with a oxyacetylene flameand then immediately quenched the surface with cold water.

Heating transforms the structure of surface layers to austenite, and thequenching changes it to martensite.

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Heat Treatmentrface Hardening: F l a m e Hardening

The surface layers are hardened to about 50 – 60 HRC. It is less expensiveand can be easily adopted for large and complex shapes.

Flame hardened parts must be tempered after hardening. The temperingtemperature depends on the alloy composition and desired hardness.

46 The flame hardening methods are suitable for the steels with carboncontents ranging from 0.40 to 0.95% and low alloy steels.

Heat Treatmentface Hardening: Induction Hardening

Induction hardening involves placing the steel components within a coilthrough which high frequency current is passed. The current in the coilinduce eddy current in the surface layers, and heat the surface layers uotpaustenite state.

Then the surface is immediately quenched with the cold water totransfer the austenite to martensite. The principle of induction hardeningis:

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face Hardening: Induction Hardening

Induction hardening involves placing the steel components within a coilthrough which high frequency current is passed. The current in the coilinduce eddy current in the surface layers, and heat the surface layers uotpaustenite state.

Then the surface is immediately quenched with the cold water totransfer the austenite to martensite. The principle of induction hardeningis:

Heat Treatmentface Hardening: Induction Hardening

Advantages of induction hardening over flame hardening is its speedand ability to harden small parts; but it is expensive. Like flamehardening, it is suitable for medium carbon and low alloy steels.

Typical applications for induction hardening are crank shafts, camshafts, connecting rods, gears and cylinders.

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Unit 5Heat Treatment

gears, cams, bearings and clutch plates.

rface Hardening: Carburising

Carburising is carried out on a steels containing carbon less than 0.2%.It involves increasing the carbon contents on the surface layers upto 0.7 to0.8%.In this process, the steel is heated in contact with carbonaceous materialfrom which it absorbs carbon. This method is mostly used for securinghard and wear resistance surface with tough core carburising is used for

2 CO C + CO2

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gears, cams, bearings and clutch plates. 2 CO C + CO2

Heat TreatmentSurface Hardening: Carburising

The Following methods are used to diffuse carbon intosurface layers:

1) Pack (solid) Carburising,

2) Gas Carburising,

3) Liquid Carburising.

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Surface Hardening: Carburising

The Following methods are used to diffuse carbon intosurface layers:

1) Pack (solid) Carburising,

2) Gas Carburising,

3) Liquid Carburising.

Gas Carburising

Liquid Carburising

Heat TreatmentSurface Hardening: Nitriding

Nitriding involves diffusion of nitrogen into the product to form nitrides.The resulting nitride case can be harder than the carburized steel. Thisprocess is used for alloy steels containing alloying elements (Aluminum,Chromium and Molybdenum) which form stable nitrides.

Nitriding consists if heating a component in a retort to a temperature ofabout 500 to 600 oC. Through the retort the ammonia gas is allowed tocirculate. At this temperature the ammonia dissociates by the following

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reaction.

The atomic nitrogen diffuses into steel surface, and combines with thealloying elements (Cr, Mo, W, V etc) to form hard nitrides. The depth towhich nitrides are formed in the steel depends on the temperature and thetime allowed for the reaction. After the nitriding the job is allowed to coolslowly. Since there is no quenching involved, chances of cracking anddistortion of the component are less.

Surface Hardening: Nitriding

Nitriding involves diffusion of nitrogen into the product to form nitrides.The resulting nitride case can be harder than the carburized steel. Thisprocess is used for alloy steels containing alloying elements (Aluminum,Chromium and Molybdenum) which form stable nitrides.

Nitriding consists if heating a component in a retort to a temperature ofabout 500 to 600 oC. Through the retort the ammonia gas is allowed tocirculate. At this temperature the ammonia dissociates by the following

2 NH3 2N + 3H 2

Heat TreatmentSurface Hardening: Nitriding

The depth of nitrided case ranges from 0.2 to 0.4 mm and no machining isdone after nitriding.

Nitriding increase wear and corrosion resistance and fatigue strength of thesteel. Since nitriding is done at low temperature, it requires more time thancarburising, and also the capital cost if the plant is higher than carburising.

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Heat TreatmentSurface Hardening: Cyaniding

Similar to carbonitriding, cyaniding also involves the diffusion of carbonand nitrogen into the surface of steel. It is also called liquid carbonitriding.The components are heated to the temperature of about 800 – 900 oC in amolten cyanide bath consisting of sodium cyanide, sodium carbonate andsodium chloride.

After allowing the components in the bath for about 15 – 20 minutes, theyare quenched in oil or water. Cyaniding is normally used for low-carbonsteels, and case depths are usually less than 0.25 mm.

It produces hard and wear resistance surface on the steels. Because ofshorter time cycles, the process is widely used for the machine componentssubjected to moderate wear and service loads.

The process is particularly suitable for screws, small gears, nuts and bolts.53

Surface Hardening: Cyaniding

Similar to carbonitriding, cyaniding also involves the diffusion of carbonand nitrogen into the surface of steel. It is also called liquid carbonitriding.The components are heated to the temperature of about 800 – 900 oC in amolten cyanide bath consisting of sodium cyanide, sodium carbonate andsodium chloride.

After allowing the components in the bath for about 15 – 20 minutes, theyare quenched in oil or water. Cyaniding is normally used for low-carbonsteels, and case depths are usually less than 0.25 mm.

It produces hard and wear resistance surface on the steels. Because ofshorter time cycles, the process is widely used for the machine componentssubjected to moderate wear and service loads.

The process is particularly suitable for screws, small gears, nuts and bolts.53

r.n. g. patel institute of technology (mechanical

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