Tratamientos Térmicos_Unidad II

8/8/2019 Tratamientos Trmicos_Unidad II

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Heat Treatment

Thirugnanam K

SEA Materials Engineering


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Fundamentals of Heat Treatmentfor Metallic Materials

IntroductionThe purpose of this presentation is to provide abasic understanding of the metallurgicalprocesses associated with the heat treatment of

metallic materials.


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Thermal Processes:

1. Shape change of materials

Such as forging, forming, extrusion, rolling, welding

and casting (foundry).

2. No shape change of materials

Such as heat treatment and coating.


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Liquid water and ice are familiar examples of how amaterial can exist in various forms. Steel also exists in

various forms, including several different solid forms.

Various forms of material


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What is Heat treatment?

Answer

: The controlled heating and cooling ofmaterials for the purpose of altering their structuresand properties.


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Purposes of performing a heat treatmentprocess on a metallic component

Modification of the microstructure for improvement ofmachining, cold forming processes.

Obtaining the required mechanical properties, such as

strength, toughness, hardness, wear resistance andfatigue life based on application.

Reduction in brittleness, residual stress, dimensionalinstability of components.

Surface protection from environment, such asoxidation, corrosion medium and stress corrosion.


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Fundamentals of Heat Treatment Processing Development forMetallic Components

When a material is selected per design requirements and application,the next step is how to have the designed parts satisfy theserequirements, such as mechanical properties (strength, hardness,toughness, residual stress state and fatigue strength) and

microstructure. They are achieved through a proper heat treatmentprocess.

How to develop a heat treatment process???

Tool one: Fe-Carbon phase diagram

For heating above the austenitizing temperature

Tool two: T-T-T or IT curve of selected material

Determination of cooling rate (reduction of cracking risk and

distortion) to obtain the required microstructure.


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Most alloying elements in the steels are Cr, Mo, Ni, Si, Mn, as wellas V and W.

All of them change the positions of the A1, A3 and Am boundariesand the eutectoid composition will be changed.

All important alloying elements decrease the eutectoid carbon content. For example, 1080 (0.8% C) steel iscalled hyper-eutectoid steel. However, H19 (0.3% C) is hyper-eutectoid steel due to addition of Cr (2%), W (8%)and V (1%). Eutectoid point shifts toward to left in the Fe-C diagram.

Austenite-stabilizing elements (Mn and Ni) decrease A1 temperature, i.e., expend gamma zone.

Ferrite-stabilizing elements (Cr, Si, Mo , W, V and Ti) increase A1 temperature, i.e., shrink gamma zone.

The effect of combination of alloying elements on the Fe-C diagram is very complicated (may expand or shrinkgamma zone).

Effects of alloying elements on the Fe-C phase diagram


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1. 1035 steel 1570 F (855 C)2. 1040 steel 1555 F (845 C)3. 4340 steel 1570 F (855 C)

4. 5130 steel 1570 F (855 C)5. 8620 steel 1600 1700 F (870 925 C for carburizing)6. H19 steel 2005-2200 F (1095 1205 C)7. D2 steel 1795-1875 F (980-1025 C)8. T2 steel 2300-2375 F (1260-1300 C)

Examples of austenitizing temperature development forsteel hardening


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Full TTT Diagram

The complete TTTdiagram for aniron-carbon alloy of

eutectoidcomposition.

A: austenite

B: bainite

M: martensite

P: pearlite


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Effect of the carbon content on the T-T-T- curve profile

1. Move the nose of T-T-T curve toward the lower-right direction.

2. Lowering Ms point temperature (martensitic transformation start temperature).

3. Increasing hardening ability or hardenability using same cooling rate.


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So Whats a CCT Diagram?

Phase Transformations and Production ofMicroconstituents takes TIME.

Higher Temperature = Less Time.

If you dont hold at one temperature and allow time to

change, you are Continuously Cooling.

Therefore, a CCT diagrams transition lines will bedifferent than a TTT diagram.


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Slow Cooling

Time in region

indicates amount ofmicroconstituent!


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Medium Cooling

Cooling Rate, R, is

Change in Temp /Time C/s


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Fast Cooling

This steel is very

hardenable 100%Martensite in ~ 1minute of cooling!


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Heat Treatment can be considered in termsof three aspects

1. In crystallographic change

2. In microstructure change

3. In mechanical and physical property change


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In crystallographic change

From BCC to FCC to BCT

BCC

FCC

BCT

BCC Body centered cubic

FCC Face centered cubic

BCT Body centered tetragonal


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In microstructure change

From pearlite to austenite to martensitethrough quenching (fast cooling)


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In mechanical properties, such as hardness

SAE 1050 SAE 4147


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Heat Treatment Process TreeHeat Treatment (Heating, Holding & Cooling)

Annealing Normalizing Through hardening Case hardening

Stress relief

annealing

Recrystallization

annealing

Spheroidizedannealing

Isothermal

annealing

Quenching &

tempering

Interrupted

quenching

Isothermal

Austempering

Martempering

Vacuum

Induction hardening

Flame hardening

Laser hardening

Carburizing

Carbonitriding

Nitrocarburizing


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FULL ANNEALING

It consists of heating steel toaustenitic region (790-900C),followed by slow cooling, preferably inthe furnace itself or in any good heat-insulating material.


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Full annealing

8620 ASR

AS Received


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Objectives of the Full Annealing

To improve ductility To facilitate cold working or machining

To remove internal stresses completely

To get enhanced magnetic and electrical properties

To promote dimensional stability

To refine grain structure

Disadvantage: The prolonged heat treatment cycle, involved in this process, makes it quite

expensive.


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ISOTHERMAL ANNEALING

In this process, hypoeutectoid steel is heatedabove the upper critical temperature (A3 -750-900C)and held for some time at this

temperature. The steel is then cooled rapidly to a temperature

less than the lower critical temperature (i.e. 600 -

700 C) After all the austenite is transformed into

lamellar pearlite, steel is cooled in air.


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Adv. of Isothermal Annealing

The time required is less compare to FullAnnealing.

Hence cheaper than full annealing

process.

Improves Machinability and also results in

a better surface finish by machining. Widely used for alloy steels.


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Disadv. of Isothermal Annealing

Used for hypoeutectoid steels only It is suitable only for small-sized components.

Heavy components cannot be subjected to this treatment because it isnot possible to cool them rapidly and uniformly to the holding

temperature at which transformation occurs.


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PROCESS ANNEALING

Steel is heated to a temperature below the lowercritical temperature (670-720C), and is held atthis temperature for sufficient time and then

cooled.


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Process Annealing

To reduce hardness and to increase ductility ofcold-worked steel so that further working may becarried out easily.

It is an intermediate operation and is sometimesreferred to as in-process annealing.

Mostly used in sheet and wire industries


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Spheroidise Annealing

Spheroidising is a heat treatment process whichresults in a structure consisting of globules orspheroids of carbide in a matrix of ferrite.


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Spheroidizing annealing

8620 ASRAs Received


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Purpose of Spheroidising

The majority of all spheroidising activity isperformed for improving the cold formability ofsteels.

The spheroidised structure is desirable whenminimum hardness, maximum ductility, or (inhigh carbon steels ) maximum machinability isimportant.


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Spheroidise Annealing

Low-carbon steels are seldom spheroidised formachining, because in the spheroidisedcondition they are excessively soft and gummy.

The cutting tool will tend to push the materialrather than cut it, causing excessive heat andwear on the cutting tip.


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Stress Relieving

The process of stress relieving consists ofheating steel uniformly to a temperature belowthe lower critical temperature (less than 600C),holding at this temperature for sufficient time,followed by uniform cooling.


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Stress Relieving

Sources of internal stresses solidification of castings welding

machining

grinding shot peening

surface hammering

cold working, bending

electroplated coatings


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NORMALISING

WHAT IS NORMALISING? Normalising is an austenitising heating cycle followed by coolingin still air or slightly agitated air.

Typically, the job is heated to a temperature about 50C above

the upper critical line of the iron-iron carbide phase diagram priorto cooling. (830 - 925C)


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NORMALISING


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PURPOSE OF NORMALISING

To improve Machinability To refine the grain structure

To homogenise the microstructure in order to

improve the response in hardening operation. To modify and refine cast dendritic structure

To reduce banded grain structure due to hot

rolling.


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NORMALISING Vs ANNEALING

Normalised steels are harder than annealed one.

Prolonged heat treatment time and higher energyconsumption make the annealing treatment moreexpensive than normalising.

Cooling rates are not critical for normalising as in thecase of annealing.

Annealing improves the machinability of medium carbon

steels, whereas normalising improves machinability oflow carbon steels.


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HARDENING

Certain applications demand high hardnessvalues so that the components may besuccessfully used for heavy duty purposes.

High hardness values can be obtained by aprocess known as Hardening.


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HARDENING

Hardening treatment consists of heating to austenitisingtemperature(815 - 870C), holding at that temperature,followed by rapid cooling such as quenching in water, oil,or salt baths.

The high hardness developed by this process is due tothe phase transformation accompanying rapid cooling.

The product of low temperature transformation ofaustenite is martensite, which is a hard microconstituentof steel.


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Conventional quenching & tempering


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HARDENING

Successful hardening usually means achievingthe required microstructure, hardness, strength,or toughness while minimising residual stress,distortion, and the possibility of cracking.


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Selection of Quenching Medium

Selection of a quenching medium depends onthe hardenability of the particular alloy, thesection thickness and shape involved and thecooling rates needed to achieve the desiredmicrostructure.

Hardenability: It is the ability of the steel to be transformed partially or

completely from austenite to martensite while quenching.


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Various Quenching Mediums

Gaseous Quenchants Helium, Argon and Nitrogen

Liquid Quenchants Oil

Oil with some additives Polymer Quenchants

Water

Brine Water


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Factors affecting Hardening

Chemical composition of steel Size and shape of the steel part

Hardening cycle (heating rate, hardening

temperature, holding time and cooling rate) Homogeneity and Grain size of austenite

Quenching Media

Surface condition of steel part.


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Hardening

High hardness developed by hardening enablestool steel to cut other metals.

It also improves wear resistance.

Tensile strength and Yield Strength areimproved by hardening.

This process is frequently used for chisels,

sledge, hand hammers, centre punches, shafts,collars and gears.


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Tempering

Tempering consists of heating hardened steelbelow the lower critical temperature, followed bycooling in air or at any other desired rate.


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Tempering

In the as-quenched martensitic condition, thesteel is too brittle for most applications.

The formation of martensite also leaves high

residual stresses in the steel. Therefore, hardening is almost always followed

by tempering.


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Tempering

The purpose of tempering is to relieve residualstresses and to improve the ductility andtoughness of the steel.

This increase in ductility is usually attained at thesacrifice of the hardness or strength.

Hardness decreases and toughness increasesas the tempering temperature is increased.


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Tempering

Dimensional Changes Martensite transformation is associated with an increase in volume.

During tempering, martensite decomposes into a mixture of ferrite andcementite with a resultant decrease in volume as temperingtemperature increases.


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Sub-Zero Treatment

Retained Austenite: In practice, it is very difficult to have a completely martensitic structure

by hardening treatment.

Some amount of austenite is present in the hardened steel.

This austenite existing along with martensite is referred to as Retained

Austenite.


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Sub-zero Treatment

Retained austenite is converted into martensiteby this treatment.

The process consists of cooling steel to sub-zerotemperature which should be lower than the M

ftemperature of the steel (-30 to -70C)

Tempering is done immediately to remove theinternal stresses developed by Sub-zerotreatment.


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Sub-zero Treatment

Increase in hardness

Increase in wear resistance

Increase in dimensional stability


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Martempering

In the conventional hardening process, the surface and

centre cool at different rates and transform to martensiteat different times.

In Martempering, the steel is quenched into a bath kept

just above Ms. After allowing sufficient time for thetemperature to become uniform throughout the cross-section, it is air-cooled through the martensitic range.

The transformation to martensite occurs more or less

simultaneously across the section.


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Adv. Of Martempering

Residual stresses developed duringmartempering is lower.

It also reduces or eliminates susceptibility

to cracking.


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Bainite

Upper (550-350C)

Rods of Fe3C

Lower (350-250C)

Fe3C Precipitates in Plates ofFerrite

It is still Ferrite and Cementite!Its just acicular.


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Austempering

Increased ductility, toughness and strength

Reduced distortion, which lessens subsequentmachining time, stock removal, sorting,inspection and scrap.

The shortest overall time cycle to throughharden within the hardness range of 35 - 55HRc, which results in savings in energy andcapital investment.


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Austempering

Limitation Limitation on size is necessary since the part is required to attain

uniform temperature of the quenching bath rapidly.

Therefore, only comparatively thin sections can be austemperedsuccessfully.

Austempered Ductile Iron (ADI)


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Austempered Ductile Iron (ADI)

Aus-ferrite

Actually, the definition is Isothermal temperature heat treatment of ductile iron (spheroidized iron)


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ADI Microstructures(a) As-received ductile iron.(B) Aus-ferrite at high T.(C) Aus-ferrite at low T.

B

A

C


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ADI grade and mechanical properties


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CASE HARDENING

There are situations in which the requirement issuch that the outer surface should be hard andwear resistant and the inner core more ductileand tougher.

Such a combination of properties ensures thatthe component has sufficient wear resistance togive long service life and at the same time has

sufficient toughness to withstand shock loads.


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CASE HARDENING

CARBURISING

CYANIDING

CARBONITRIDING

NITRIDING PLASMA NITRIDING

FLAME HARDENING

INDUCTION HARDENING


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CARBURISING

This is the oldest and one of the cheapestmethods of case hardening.

It is carried out on low carbon steels whichcontain from 0.10 - 0.25% carbon.

Carburising is carried out in the temperaturerange of 900 - 930 C

The surface layer is enriched with carbon upto0.7 - 0.9 %


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CARBURISING

In this process, carbon is diffused into steel byheating above the transformation temperatureand holding the steel for sufficient time in contactwith a carbonaceous material which may be a

solid medium, a liquid or a gas.

Followed by Quenching and Tempering.


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GAS CARBURISING

The Steel is heated in contact with carbonmonoxide and/or a hydrocarbon which is readilydecomposed at the carburising temperature.

Temperature : 870-950C

Gas carburising may be either batch orcontinuous type.


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Air

Natural gas

Mixer

Retort

En

dogas

Natural gas

Endo-gas Generator Carburizing furnace

N2


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GAS CARBURISING

Gas atmosphere for carburising is producedfrom liquid (methanol, iso-propanol) or gaseoushydrocarbons (propane and methane)

An endo-thermic gas generator is used to supplyendothermic gas.


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GAS CARBURISING

Approximate composition of the gas inflow intothe furnace is Nitrogen 40%

Hydrogen 40%

Carbon Monoxide 20%

Carbon Dioxide 0.3%

Methane 0.5%

Water vapour 0.8%

Oxygen in traces


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1. Heat and soak at carburizing temperature to ensure temperature uniformity

throughout steel.

2. Boost step to increase carbon content of austenite.

3. Diffusion step to provide gradual case/core transition.

4. Gas pressure or oil quench

CH4 + Fe=Fe(C) +2H2


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Atmosphere carburized surface

profile, showing the IGOVacuum carburized surface profile,showing a clear surface (no IGO)


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LIQUID CARBURISING

Popularly known as Salt bath carburising.

In this process, carburising occurs throughmolten cyanide (CN) in low carbon steel cast pottype furnace heated by oil or gas.

Bath temperature : 815 - 900C

Salt mixture consists of Sodium or Potassium Cyanide

Barium chloride


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LIQUID CARBURISING

CHEMICAL REACTION BaCl2 + 2NaCN ---> Ba(CN)2 + NaCl

Ba(CN)2 + Fe ---> Fe(C) + BaCN2


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SOLID CARBURISING

This method of carburising is also known as

pack carburising.

In this process, steel components to be heattreated are packed with 80% granular coal and

20% BaCO3 as energizer in heat resistant boxesand heated at 930C in electric chamber furnacefor a specific period of time depends on case

depth.


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SOLID CARBURISING

CHEMICAL REACTION Energizer decomposes to give CO gas to the steel furnace

BaCO3 ---> BaO + CO2 CO2 + C ---> 2CO

Carbon monoxide reacts with the surface of steel

2CO + Fe ---> Fe(C) + CO2

CARBONITRIDING


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CARBONITRIDING

The surface layer of the steel is hardened by

addition of both carbon and nitrogen.

This process is carried out a lower temperatures(in the range 800 - 870C) in a gas mixture

consisting of a carburising gas and ammonia.

A typical gas mixture contains about 15% NH3,5% CH4 and 80% neutral carrier gas.


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Air

Natural gas

MixerRetort

Endogas

Natural gas

Endo-gas Generator Carbonitriding furnace

NH3

N2

Air

Naturalga

MixerRetort

Endogas


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NITRIDING


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NITRIDING

Nitriding is carried out in a ferritic region below 590C.

So there is no phase change after nitriding.

Before nitriding, proper heat treatment should be givento steel components.

All machining and finishing operations are finishedbefore nitriding.

The portions which are not to be nitriding are covered bythin coating of tin deposited by electrolysis.

NITRIDING


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NITRIDING

Anhydrous ammonia gas is passed into the

furnace at about 550C, where it dissociates intonascent nitrogen and hydrogen.

Thus,

2NH3 ----> 2[N]Fe + 3H2 The surface hardness achieved varies from 900

to 1100 HV.

PLASMA NITRIDING


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PLASMA NITRIDING

Plasma nitriding is also known as ion nitriding process.

In this process, the steel component to be nitrided is keptat 450C in vacuum at a negative potential of the orderof 1000 volts with respect to chamber.

Then an appropriate mixture of N2 and H2 is passed at apressure of 0.2-0.8 m bar.

As a result, plasma formation of these gases takesplace.

Ion (Plasma) Nitriding Equipment

(Photos Courtesy of Surface Combustion)


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(Photos Courtesy of Surface Combustion)


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FLAME HARDENING


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FLAME HARDENING

Flame hardening is done by means of

oxyacetylene torch.

Heating should be done rapidly by the torch andthe surface quenched before appreciable heat

transfer to the core occurs.

Application For large work pieces

Only a small segment requires heat treatment When the part requires dimensional accuracy

Induction Hardening


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Induction Hardening

Here, an alternating current of high frequency

passes through an induction coil enclosing thesteel part to be heat treated.

The induced emf heats the steel.

Immediately after heating, water jets areactivated to quench the surface.

Induction hardening of camshaft


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duct o a de g o ca s a t

65

Induction hardeningInduction hardening of crankshaft


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20

25

30

35

40

45

50

55

60

0 0.02 0.04 0.06 0.08 0.1 0.12 0.14

Distance from surface, inch

Hardness,

HRC

(convertedbymicro)

Adv of Induction Hardening


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Adv. of Induction Hardening

Provides energy savings

Provides much higher heating rates

Ease of automation and control

Reduced floor space requirements Quiet and clean working conditions

Suitability for integration in a production line

Chrysler Engineering Standards Related to Heat Treatment Process


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PS-1: PS-1 HEAT TREATMENT - QUENCH AND TEMPER ANDAUSTEMPERDPS-2: PS-2 HEAT TREATMENT - GAS CARBURIZINGDPs-3: PS-3 HEAT TREATMENT - LIQUID BATH CASE HARDENINGPS-4: PS-4 HEAT TREATMENT - MISCELLANEOUSPs-5: PS-5 SELECTIVE HEATING SPECIFICATIONS - HEAT STAKING,INDUCTION BONDING, INDUCTION HARDENING & TEMPERING PROCESSES,LASER HEAT TREATING

PS-6: PS-6 HEAT TREATMENT ALUMINUM ALLOYSPS-7: PS-7 HEAT TREATMENT - FLAME HARDENINGPS-8: PS-8 HEAT TREATMENT - CARBONITRIDINGPS-9: PS-9 HEAT TREATMENT-AUSTEMPERED NODULAR ANDMALLEABLE IRON

http://adress2.tcc.chrysler.com/adress/


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Q & A

Thanks for your patient

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