Post on 10-Apr-2018
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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