03-ht processes.doc

8/14/2019 03-ht processes.doc

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Topic: Heat Treatment Processes

TailorMade Course For BHEL_J&KTailorMade Course For BHEL_J&KTrainees (TechTrainees (TechI) ANNEALING

Annealing consists of heating hypo-eutectoid steels 30 C to 50 C above the uppercritical range (Ac3) and 30 C above the lower critical range Ac1) for hyper-eutectoid steels

soa!ing the steel at this te"perature sufficiently enough ti"e and then cooling slowly along withthe furnace#

Carbon steels are cooled at the rate of 100C to 150C per hour and alloy steels arecooled "uch slower than carbon steels i#e# at the rate of 50C to 100 C per hour#

$ull annealing is used to obtain the following properties%

&o relieve the internal stresses and strains developed by various fabrication"ethods li!e forgings castings etc#

&o develop a definite unifor" "icro-structure &o refine the grains for i"proving properties of strength elasticity and ductility

&o alter the "agnetic or electrical properties

&o reduce hardness

&he first re'uire"ent in full annealing is to heat the steel unifor"ly to the prescribedte"perature#

II) NORMALISING

or"alising consists of heating the steel to about 0 C to 50 C above its upper criticalte"perature soa!ing for sufficiently and then cooling in roo" te"perature#

or"alising "ay be e"ployed to%

*efine the grain and provide ho"ogeneous "icro-structure to i"prove responseto hardening treat"ent#

+"prove "achining characteristics

,odify and refine cast dendritic structures

rovide desired "echanical properties#

&o reduce internal stresses and also to i"prove the structure at the welds#

.ypro-eutectoid steels on nor"ali/ing gives $errite and fine earlite#

.yper eutectoid steels on nor"ali/ing gives fine earlite and Ce"entite#

*apid cooling on nor"ali/ing results in austenite deco"position at low te"peratures#.igher cooling rate the for"ation of pro eutectoid constitutions and hence the eutectoidconstituent increases pearlite content and so higher hardness and tensile strength as co"pared

to annealed steels#

Module : 06 Area: Heat Treatment & NDT

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TailorMade Course For BHEL_J&KTailorMade Course For BHEL_J&KTrainees (TechTrainees (Tech

III) HARDENING:

.ardening is a heat treat"ent process in which steel is heated to an appropriatete"perature above the critical point and held at this te"perature for sufficient ti"e to allow the

steel to obtain a unifor" te"perature throughout the section# &hen the steel is rapidly cooledthrough a cooling "ediu"# ater oil "olten salt or air "ay be used as a cooling "ediu"depending upon the co"position of the steel and the hardness re'uired#

a P!"P#$E #F H%"E'')

&o develop high hardness

&o increase the tensile and yield point (elasticity)

&he high values of tensile strength and the yield point are obtained not in the hardeningcondition (when steel has a "icrostructure of 2#C#& "artensite) but in the te"pered conditions

when the steel structure beco"es two phase with a high degree of dispersion#* H%"E'') TEMPE"%T!"E

&he hardening te"perature for the .ypo-eutectoid steels is above AC 3te"perature#

2etween A1and A3the steel consists of austenite and ferrite# +n such steel is 'uenched inwater the "artensite transfor"ation ta!es such steel is 'uenched in water the "artensitetransfor"ation ta!es place in the austenitic grains but not with ferrite# As a result of this'uenched structure "ight contain "artensite with ferrite (un dissolved) which would give a patchyhardening and the hardness is not unifor" on the surface# &his is also conse'uently lower thestrength and yield point on 'uenching and corresponding changes in the other properties in

hardening and te"pering#&his is the reason why hypo-eutectoid steels are hardened fro" above the critical

te"perature and the inco"plete hardening is avoided# enerally the hardening te"perature forthe .ypo-eutectoid steel is 30 to 50 C above the A 3critical te"perature#

4uenching this type of steel considerably above the te"perature "ay lead to over heatingresulting in coarse "artensite (acicular structure) and has high brittleness#

&he te"perature of hardening for .yper-eutectoid steel is 30 to 50 C above the A 1te"perature#

At this te"perature as usual inco"plete austentisation results in s"all particles ofsecondary carbides# n 'uenching the steel consists of "artensite and secondary carbides (undissolved) 6ince the hardness of "artensite is around 750 8 950 : and that of carbides around;00 :# and the steel getting hardened through the inco"plete hardening is suitable forincreasing the hardness to "a


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c HE%T') F#" H%"E''):

&he heating of steel depends pri"arily upon four "a=or factor%

&he selected furnace &he fuel used for heating

&he "ediu" or at"osphere to surround the parts during heating and

&he ti"e interval and regulation in bringing the part upto the re'uired te"perature

d $#%K') TME

+t is the ti"e ta!en for the entire speci"en reach the austenising te"perature# 6oa!ingti"e depends on the thic! ness of the "aterial# A speci"en of >5 "" thic! is given a soa!ingti"e of one hour # +f the cross section of the speci"en varies? preheating is done to "ini"i/e theill effects of heating 'uenching "edia#

enerally after proper soa!ing at the preheating te"perature the tool is transferred to ahardening furnace which is "aintained at the hardening te"perature# &he furnace selectedshould be of such a si/e and heat capacity that the te"perature does not fall appreciably# &hepractically attainable hearting rate not only depends upon the ther"al capacity of the furnace butalso upon the volu"e of the charge and their arrange"ent in the furnace and other factors#here it is not possible to transfer the article due to their si/e and weight it is safer to heat in thesa"e furnace to the hardening te"perature#

.igh speed steels hot die steels high chro"e steel etc# which have a "uch higherhardening te"perature should always be pre-heated at ;50 C# +f tools "ade of above alloy steels

are of very large cross sections it is advisable to have "ore than one preheating#

&he preheated parts and tools when heated in the hardening furnace for hardening attainthe hardening te"perature in a "ini"u" ti"e shortening the soa!ing ti"e at the higherhardening te"perature# &his avoids the chance of heavy scaling and decarburi/ation resulting inlow surface hardness# +f the steel is directly heated without preheating undue stresses are setupin the steel which will lead to distortion and crac! also the surface of the steel will be sub=ectedto local over soa!ing with conse'uent grain growth and brittleness#

e +!E'CH'):

4uenching is the rapid cooling of "etal usually but not necessarily by i""ersion in a

li'uid its "a=or application is to steel which is the "ost widely used "etal effectively hardenableby 'uench along

ater 'uenching is properly applied to the carbon and low alloy steels co""onlydesignated as shallow hardening steels in which a thin hardened layer is produced by virtue ofthe e


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il 'uenching reduces (but does not always eli"inate) distortion because cooling is stillrelatively fast and te"perature gradients correspondingly large# Conse'uently co"ple< shapessuch as blan!ing dies "ade fro" highly alloyed steels should be given a still "ilder 'uench#

f late @interrupted 'uenching has found wide use because it reduces distortion#IV) TEMPERING:

&e"pering is a heat treat"ent operation consists of re-heating the hardened steels to ate"perature below lower critical te"perature at about 150 oC to 750 oC ( A1) soa!ing followedby cooling at any desired rate# enerally te"pering is carried out at a te"perature below 00oC( 950o$ ) while te"pering above this te"perature is called Brawing% Brawing is e"ployed toattain the best co"bination of strength and toughness in structural steels#

a P!"P#$E #F TEMPE"')

6teels in its hardened condition is often harder than necessary generally too brittle and

too severally strained in the 'uenching operation# &he ai"s of te"pering are%

&o relieve the steel fro" internal stresses and strains#

&o regulate the hardness and toughness

&o decrease the brittleness and to restore so"e ductility to induce shoc! resistance#

* TME 'TE",%L BET-EE' H%"E'') %' TEMPE"')

+n certain steels such as "ediu" and high carbon steels and other "anganese alloysteels the stresses produced during hardening is so great to produce crac!s on the articles#&e"pering i""ediately after 'uenching prevents develop"ent of such destructive crac!s

1# ,&.B6 $ &,*+

i) Dow &e"perature te"pering% &his operation is perfor"ed in the range of150oC to >50oC (300oto ;0o$) and its purpose is to reduce internal stresses and toincrease the toughness without any appreciable loss in hardness# &his type of te"peringis usually used for "easuring and cutting tools of carbon and low alloy steel as well as forparts that have been surface hardened#

&e"pering is still fre'uently carried out by =udging the colour# &he colourwhich appear on bright surfaces of the te"pered steels on te"pering between >00 o to3>0oC is !nown as &e"per colours# &he te"per colour is the colour of the thin layer of

e


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,ultiple te"pering will very often correct poor hardening practices# ,ultiplete"pering is "ore effective than a single te"pering operation for highly alloyed steels#.igh 6peed and other hot wor! of air hardening steels showing secondary hardening onte"pering are preferably given a double or triple te"pering i#e# te"pered and air cooled

and then the process repeated in the sa"e "anner#

Module : 06 Area: Heat Treatment & NDT

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03-ht processes.doc

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