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Lecture

By

Dr. Sahab Prasad

([email protected])

IOFS -01

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Alloyisdefinedasthecombination oftwo or morechemicalelements, one ofwhichMUSTbea metalandtheresultantproductshouldhave metalliccharacteristics.

Alloyscouldbe Binary, Ternary, Quaternaryandso ondependinguponthenumber ofelementspresent.

Alloysareclassifiedbased onthestructuretheyhavei.e. a] h omogeneousb] mixture

Alloys


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Phase : Aphaseissomethingwhichisphysically distinct,chemicallyhomogeneousandtosomeextentmechanicallyseparable(onthe microstructure level)

Possiblephasesin Solidstatea] Pure Metal b] Compound /intermediatealloyphasec] Solid Solution

Phases inAlloy Structure


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Compounds:

a] ValenceCompounds betweentwodissimilar metalsfollowingvalencerulese.g.CaSe, Mg2Sn,Cu2Se

b] InterstitialCompounds betweentransition metals Sc, Ti, Ta, W, Fe,WandC,H,B,O,Ne.g. TiC, TaC, Fe4N, TiH, Fe3C

c] ElectronCompounds Some ofthealloy systemsshowsimilarlatticestructureatornearcompositionshavingadefiniteelectron /atomratios e.g.

AgCd, Ag5Cd8, AgCd3



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Solid Solutions: Asolutioninsolidstate.Substitutional: Wheresoluteatomssubstitute /replacethesolventatomsinthesolvent

lattice.Canhaveeither ofthefollowing a] Limited Solubility

b] Unlimited Solubility (e.g. Au-Ag,Cu-Ni, Sb-Bi)

Unlimited SolubilitysolidsolutionsfollowHume-Rothery Guide-lines /factorsviz.Crystallatticefactor,Relativesizefactor,Chemicalaffinityfactor & Relativevalencefactor

Interstitial: Wheresoluteatoms occupyinterstitial(inbetween)sitesinthesolventlattice.Restrictedsolublities.Importantexamplesinclude Ferrite, Austenite



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Disordered Solid Solution

Substitutional Solid Solution


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Ordered Solid Solution

Substitutional Solid Solution


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Interstitial Solid Solution


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CoolingCurves


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For the pure Metals & mostof the Compounds

For the Solid - solutions

Liq

s

Liq

Liq+s

s

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Number ofPhases,

AmountofPhases,Type ofPhases &Form ofPhases

Sowe mustknow a] theconditionsunderwhichthesephasesexist b] theconditionsunderwhich achangewill occur

Greatdeal ofinformationregardingphasechangesisaccumulatedandrecordedin

graphicalformintheform ofwhatisknownasPhase /Equilibrium /ConstitutionDiagrams.

AlloyPhase Diagrams


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ISOMORPHOUS SYSTEMS


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Have complete solubility in the liquid andsolid states.

Range of freezing temperature

Depression / Elevation of freezing point of

pure component

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Construction ofPhase Diagram


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By taking a series of cooling curves for the same system over arange of compositions the liquidus and solidus temperatures foreach composition can be determined allowing the solidus andliquidus to be mapped to determine the phase diagram.

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Construction ofPhase Diagram


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By removing the time axis from the curves and replacing itwith composition, the cooling curves indicate thetemperatures of the solidus and liquidus for a givencomposition.

This allows the solidus and liquidus to be plotted toproduce the phase diagram.

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Phase Diagram


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For any given point (x, T) the phasediagram can answer the following:

1. What phases are present?

2. What are the phase compositions?

3. What are the relative amounts of

the phases (phase proportions orphase fractions)?

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Cu-Ni BinaryPhase Diagram


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Point A:60 wt% Ni at 1100C

Q: Phase present?

Ans: E

Q: Phase composition ?

Ans: 60 wt% Ni

Q: Phase amount ?Ans: 100%

Point B:35 wt% Ni at 1250C

Q: Phases present?

Ans: E + L

Q: Phase compositions ?

Rule - I

Q: Phase amounts ?Rule - II

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Composition ofphases in the two-phase region

Rule - I

CL

= 31.5 wt% Ni

CE= 42.5 wt% Ni

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Amount ofphases in the two-phase region

Rule - IIEL

Tie-Line: A lever

Alloy composition C0: Fulcrum

fL: weight at liquidus point

fE: weight at solidus point

The lever is balanced)()( 00 CCfCCf LL !

1! ff

armlevertotal

armleveroppositef !

!

E

E 0

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Development of Microstructure during Solidification


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727.01

273.011

3

3243

3235

!!

!!

!

ff

f

L

Single phasepolycrystalline E

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The Eutectic System


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The Eutectic System


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Tin BismuthAlloySystem

The Eutectic System / Reaction


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EUTECTIC means

EASY / LOWEST

MELTING

LIQUID SOLID1 + SOLID2

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The Eutectic Transformation


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Eutectic Reaction & EutecticAlloy


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Invariantreaction

L cool183C E F62

wt%Sn18

wt%Sn97

wt%Sn

Eutectic mixture

375 X

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HypoeutecticAlloy


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Amount of proeutectic E at a temperature just below 183C

Tie line just below 183C (green)

5.044

22

1862

4062!!

!Eprof 5.05.01 !!FEeutf

18 9762

Eutectic mixture EF

Proeutectic orPrimary E

= Amount of E at a temperature just above 183C

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HypoeutecticAlloy


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Amount of total E and total F at a temperature just below 183C

Tie line just below 183C(red)

72.079

57

1897

4097!!

!

totalf 28.072.01 !!Ftotalf

18 9762

Eutectic mixture EF

Proeutectic orPrimary E

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Phase Transformations


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Lead Tin Phase Diagram


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Aluminum Silicon Phase Diagram


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Iron Carbon System


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Mild steel 0-0.3 wt% C

BicycleframeShiphullCarbody

Medium C steel 0.4-0.7 wt% C

Railwheelrailaxle

rails

High C steel 0.8-1.4 wt% C

Razorbladesscissors, knives

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Phases in Fe-Fe3Csystem


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Phase Symbol Description

Liquid L Liquid solution of Fe and C

H-Ferrite H Interstitial solid solution of C in H-Fe (hightemperature bcc phase)

Austenite K Interstitial solid solution of C in K-Fe (FCC phase

of Fe)

Ferrite E Interstitial solid solution of C in E-Fe (roomtemperature bcc phase) Soft and Ductile

Cementite Fe3C Interstitial compound of Fe and C (orthorhombic

system)H

ard and BrittlePearlite E +Fe3C Eutectoid mixture of ferrite & cementite

Ledeburite K +Fe3C Eutectic mixture of austenite & cementite

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Invariant Reactions in Fe-Fe3Csystem


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Peritectic Reaction

)%18.0()%5.0()%1.0( 1493 CwtCwtCwt Co

HE p

Eutectic Reaction

)67.6()1.2()3.4(3

1150 CwtCFeCwtCwtL Co

p K

Eutectoid Reaction

)%67.6

()%02.

0()%8.

0( 3725

Cwt

CFe

Cwt

Cwt

Co

p EK

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The Eutectoid Reaction


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Pearlite

CFeCo

3

725 p EK

0.8 0.02 6.67cool

117.065.6

78.0

02.067.6

02.08.03

!!

!pearlite

CFf

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Hypoeutectoid Steel


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Development of

Microstructure

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Hypereutectoid Steel


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Development ofMicrostructure

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Microstructureof Steels


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Eutectoidsteel

E+Fe3C

Pearlite

Hypoeutectoidsteel

E+Fe3CPearlite +proeutectoid ferrite

Hypereutectoidsteel

E+Fe3C

Pearlite +proeutectoid cementite

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Heat Treatment

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Torelieveinternalstressessetupduringcold-working,casting,weldingandhot-working operations.Toimprove machineabilityTochangegrainsize

Tosoften metalsforfurthertreatmentaswiredrawingandcoldrollingToimprove mechanicalpropertiesTo modifythestructuretoincreasewear,heatandcorrosionresistanceToremovetrappedgases

Toremovecoringandsegregation

WhyHeat Treatment?

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MetalsHandbookdefinesheattreatmentas

Acombination ofheatingandcooling operations,timedandappliedtoa

metal oralloyinthesolidstateinawaythatwillproducedesired*properties

*structureandhencethedesired mechanical

What isHeat Treatment?

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Aim To refine the grain To induce softness To improve machineability in some cases

Process Heat to 20-25 degree C above the UCT (A3) for

hypoeutectoid steel and LCT (A3,1) for hypereutectoidsteels

Hold Cool in thermally insulated vessel or the furnace

itself.

Annealing

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Annealing

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Prolonged holding at a temperature just belowthe LCT

Heating and cooling alternately between

temperatures that are just above and just belowthe LCT

Heating to a temperature above LCT and then

either cooling very slowly in the furnace orholding at a temperature just below LCT

Spheroidising

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Spheroidised Structure

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Aim

To produce a harder and stronger steel thanfull annealing

Process

Heat to 35-40 degree C above the UCT

Hold

Cool in still air

Normalising

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Effects offastercooling

Equilibrium diagram no more valid

Lesser amount of proeutectoid constituent is formed

Temperature of Eutectoid transformation is lowered

and Pearlite becomes finer.

Normalising

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Normalised Structure

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Process

Heat to 20-25 degree C above the UCTforhypoeutectoid steel and LCTforhypereutectoid steels

Hold

Cool rapidly in water / brine

Hardening

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Heating Rangefor variousProcesses

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TransformationisdiffusionlesswithnochangeinChemicalcomposition.

Transformationproceedsduringcoolingandceasesifcoolingisinterruptedi.e.itdepends only ondecreaseintemperatureandindependentoftime.

Transformation ofagivenalloycanneitherbesuppressednorthe Mstemperaturebechangedbyalteringcoolingrate.

Itisaproductintransitionbetweenunstableausteniteandstableferritecementite mixture.

Maximumhardnessfrom martensiticconditioninsteelisafunction ofcarboncontentonly.

Itcanbeformedfromaustenite only.

Alsofoundin Fe-Ni,Cu-Zn,Cu-Alsystems.

Martensitic Transformation

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Martensitic Transformation

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Martensitic Hardness

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Martensitic Structure

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Martensitic Structure

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I h l T f i Di

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Why? SinceausteniteisunstablebelowLCT,itisnecessaryto knowataparticulartemperaturea]whentransformationbegins,b]whenitendsandc]whatisthetransformationproduct.

Construction ofITDiagram

Step 1: Takea largeno. ofsamplesandaustenitisethem.Step2: Transfersome ofthemtoafurnaceatsub-criticaltemperatureStep3: Takethem outoneby oneatregularinterval oftime and

quenchinicedwater /brine.Step4: Studythemfor microstructureandconfirmby hardness

measurements,Step5: Repeatsteps2,3 & 4forvarioussub-criticaltemperatures.

Isothermal Transformation Diagrams

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Isothermal Transformation Diagram

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F t f A t lC li R t

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Thestructure,hardnessandstrengthresultingfromaheattreatmentprocessaredependentontheactualcoolingratebythequenching operation.

1.Type of Quenching Medium2.Temperature of Quenching Medium

3. SurfaceCondition ofthepart, &4. Sizeand Mass ofthepart

FactorsforActualCooling Rate

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S f C li C IT Di

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SurfaceCooling Curveson IT Diagram

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H d P t ti C

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HardnessPenetration Curves

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H d P t ti C

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HardnessPenetration Curves

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Animportantconclusion

Forasteel offixedcompositionandausteniticgrainsize,regardless ofsizeandshape oftheworkpieceandquenchingconditions,wherevertheactualcoolingrateisthesame,thehardness mustbethesame.

Converse ofthisstatementisnotnecessarily true.

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Tempering

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In asquenched martensitic condition, the steel is not only too

brittle but also contains lots of residual stresses and so suchsteel is not straight away applied in most of the application.

Quenched steel, in general, are followed by a process called

TEMPERING for relieving residual stress and to improve

ductility and toughness (which can be attained with thedecrease in strength and hardness).

The process consists of heating to a desired* temperature

below the LCT (Eutectoid temperature), held and cooledslowly.

*dependsupon the required microstructure and hence the properties.

Tempering

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Effect of Tempering

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Effect of Tempering

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Effect of Tempering Temperature

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Effect of Tempering Temperature

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Tempering Products

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Tempering Products

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Transformation Products of Austenite and Martensite

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Transformation ProductsofAusteniteand Martensite

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Conventional Quenching & Tempering

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AdvantageofQuenchingand Temperingprocess liesinthebestYieldStrength,bestDuctilityand Fatigue Strengthaswellashighest

Toughnesswhen medium Tensile Strengthisdesired.


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Austempering

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Austempering

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Austempering

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Austempering

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Austempering

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AustemperedsteelbesideshavinggreaterDuctilityandToughnessalongwithhighHardnesshas lessdistortionanddanger ofquenchingcracks,sincethequenchisnotthat

drasticasintheconventionalprocess.Limitation lies,however,inthesectionthickness.(

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Martempering

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Advantagesinclude

minimisation ofresidualstressesandgreatreductionindanger ofdistortionandcracking.

Heat Treatment Defects

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Types of defects and characteristics Causes Remedies

1

.Overheating

Causes coarse grained microstructureWidmanstatten structure in annealed

steel, coarse crystallic martensite in

hardened steel, reduced ductility and low

impact strength value

Heating for long periods

at temperatures

exceeding normal values

(a) Normal annealing and normalizing for slight overheating

(b) Repeated normalizing for about 6 times

2 BurningGrain boundaries having (a) regions

enriched in carbon in first stage of

burning; (b) non-oxidized cavities and

blow holes in second stage of burning and

(c) iron oxide inclusions in the third stage

of burning, resulting in stone-like fracture

and poor ductility.

Heating for long duration

at high temperature

under oxidizing

conditions or heating

hear to melting point of

steel

(a) Homogenizing followed by double annealing for first stage of

burning

(b) Forging followed be annealing for second stage

Not remediable if third stage has occurred

3 OxidationThick layer of scale is seen on the surface

of steel component

Oxidizing atmosphere in

heating furnace

(a) To use reducing, neutral or protective atmosphere in heating

furnace

(b) Heating in box with used carburizing agent

Heating in molten salt bath

4 DecarburizationCarbon content decreases in the surface

layer of steel component. Hardness and

fatigue limits are lower

Oxidizing atmosphere inheating furnace

(a) Heating in furnace under reducing neutral or protectiveatmosphere

(b) Heating in box with used carburizing agent or case iron chips

Heating in molten salt bath

(d) Removing decarburized layer by machining if machining

allowance is available


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5 Excessive hardness of Hotworked

Annealed steel

Excessive cooling rate for simple annealing or

insufficient soaking period for isothermal

annealing.

Repeated annealing with cooling at

specified rate

6 Black FractureFree carbon inclusions are seen in the steel

Excessive heating time and slow cooling after

annealing

Heating the steel to high temperature and

thorough forging.

7 Deformation andDimensional Changes

After HardeningThe Higher the hardenability of steel, more

sever is the deformation in hardening.

Increase in volume of steel due to martensitic

transformation

(a) Using steels which are slightly

deformed by quenching

(b) Cooling slowly in martensitic range

Apply surface hardening where

possible

8 WarpingAsymmetrical deformation of component

occurs during quenching

(a) Change in volume in heating or cooling(b) Non-uniform heating or cooling of

component

Internal stresses in the component before

heat treatment

(d) Lowering component into quenching bath

in inclined position

(a) Using alloy steels which are onlyslightly deformed by quenching

(b) Cooling slowly in martensitic range

Applying surface hardening wherever

possible

(d)Annealing, normalizing or tempering

at high temperature before hardening

(e) Heating uniformly for hardening

(f) Quenching as uniformly as possible

(g) Keeping component in proper position

in quenching bath(h) Using special quenching jigs

9 Low hardness after Quenching Low hardening, temperate cooling rate, andinsufficient soaking period at hardening

temperature

Normalizing or annealing, followed by

hardening with proper procedure


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10 SoftSpotsCertain portions on the surface of

component with lower hardness

(a) Presence of vapour blanket on the

surface of component

(b) Localized decarburization

Inhomogeneity of internal structure

after solidification

(a) Using more effective quenchant

(b) Annealing or normalizing before hardening for

more homogeneous structure

11 Excessive Hardness after Tempering Low temperature or insufficient soakingtime in tempering

A second tempering with proper temperature and

soaking time

12 Insufficient Hardness after Tempering Low temperature or insufficient soakingtime in tempering

Annealing, hardening and tempering at normal

temperature

13 ErosionReduction in size of component or in

respect of form due to loss of material

from its surface

Chemical reaction and oxidation of

components heated in molten salt baths

(a) Using deoxidizing salt bath with ferro-silicon

or borax

(b) Proper positioning of component in salt bath

14 CorrosionPitting

(a) High content of sulphuric salts (over

0.7-0.8%) in molten salt bath

(b) Bath having become rich in oxygen or

iron oxides

(a) Careful control of salt composition

(b) Deoxidizing the bath

15 Quench CrackExternal or internal and zig-zag in

appearance

(a) Internal stresses

(b) Non-uniform cooling

Cannot be remedied but may be prevented by

(a) avoiding sharp projections, sharp corners and

sudden change in size;

(b) heating to minimum before hardening

heating to minimum suitable temperature for

hardening

(d) cooling slowly in martensitic range by using oil

as the quenching medium and

(e) quenching, followed by tempering immediately

ACKNOWLEDGEMENT

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Thespeakerwould liketoacknowledgewithgratitudethediscussionswithhissenioraswellas juniorcolleaguesandtheir lecture-notesforpreparingthispresentation.

Thespeakerwouldalso liketoacknowledgethevariousbooksauthoredbynamely (a) YLakhtin,(b) SH Avner,(c)VRaghavan,(d)VanVlack,(e)Rajan, Sharma & Sharmaetc.,

whichhewentthroughduringhis morethan26 years oftenureas Teacher.

Thespeakeralsoacknowledgestheefforts ofhisstudentsandcolleaguesfordownloadingand / or makingvariousdrawings /picturesshowninthepresentation.

ACKNOWLEDGEMENT

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