• • I' , .•, I ~. U '":. " •.~.' . \ . ,~"

11.13 Surface Heat Treatments .t .•' • ...;J~. ~ .. ( 'O, ','

In many applications," the .surface of the' components should have high. '.. ~.- ' " "~ ~. . {\ ~r (.' .' _J1

hardness, while 'the inside or cor'e should be' soft: e.g. axle, gears etc. Thetreatments given to steels to achieve tills are 'called surface heat traatments .

. The surfaceheat-treatments tan bebroadly' classified as: . ," ~,

". '~'(i)'!':.Diffusion metnods:(n ..,e'it.'rvi.o dt\e.~ .fV'e~d.s\,;,:. ;' ~'" ,i.: I. ~;.-:.~~ ~_j = t': f \'t ·~1·~ ~'. r4-.,.I· '·!"~,.t:,· .' ~ ,P '.!~:1_·,,;:(' ,'I .) .4 ":"'( t .. ',d':'\~":'; ~~)<,~~pn~,;:~ep~?q:s: (' ';'." "". .: t\,. ~.'. ", ...." :',:,;-~ '. _.,

IlJ'13.:1 Dlffugibn'metb(jd8~2h~ciL" ~, "-. , n. .;, _.'\'[;':''''-';~''\hi'§ ~meth'6d~·7;the-'h~db~s~-:'br the) ~fut~~e:"is' '~pfo~(ed'- by' '~ffusing

• "i'~ ; r . :' \". .interstitial _elements like Carbon, Nitrogen or both into the surface of steelcomponents.; Depending 'upon the diffuSiori.elements'chosen, 'diffusion' treatment,

. can.be classified 'as: .r: . " ' . 'i, < 'l 'I'" . ':~,>. <,' .:», \'.

">~)":datH~iini .:\.~.< "-. '1.:-:,-" ,',:.",,', ';,' ;,

: ',~ .:';::'.~• ~,.:~, ~"~'. ;~I!··· •. .:. \ )."

i ..\

. b) Nitriding..' ,

c). Cyaniding ,•• .:.•. : : ." : t J •••.:, •• ). _ .l!,t '~:.' ~. .!

..... " ('-

. -, 1 •.I t I...,'. _ ~ '-

. d) Carbonitriding, .' ,. :;. I ;.. ~:).t.'~ """1"'·' .r ."\

,.'

Carburizing: This is a process in which carbon atoms are introduced ontothe surfa"Ce-of--to'Wicarbolcste'"els·to-pr6quce'a-harn caseof'surface, while theinteriorlor core remains soft.~Thisis accomplishedlby the following methods.

L.: _..,__ 1 ... -+ -r-r- i... - -. ...- J.,.•.••.••• -. t ..J, :'.j

(i) . Pack carburizing. :.i '(:-'-~ _.. - ..-i ---:.---..-"1'- ·~-r·-1.. G.: . ..! J_.:":~ . ....(11) as-e-arbunz-lJ;lg;--"~··"--·'''··r--'''--"-- ',' - ..-.- ..-: --. r'~''''''~""':--~I! '}

.(...) L' !'d buri I· .'111 iqui car uqzlng I ' ., .~.' :. ')'1 ' _~~._;. '~'_d __ ....• _ ....._ .. _. '",!"~_ .-_0·_' •.•... _ .•.-v-r- .••_._••.. _ .... _ .• ';'--'r"'r'-~ ",--. --~ ~~. ';"',

(i) ..Pack c~rb;;rizin~: Here st,eel compo4ents ar~_p~ckedinto ~ box.along withcarburi~ing.llledi$ Thecarpurizing.Il\edj9IIl:consists.. ofcha!cQal~ a mixtureof sodium carbon t~.and.barium-carbonate. The~ the box i~ kept in a heat

.: ..;- t:! \ itreatment.furnace iand is heated- to a-tetpperature?f.900~G tel 950°C and thensoaked at "this temperaturefor'rabout ~5hours. The parts are then slowlycooled to-room-temperature, -.--...-.---.. - .:..--.----. - ;...~ --- .

a)

r , . ~r.;, oJ(.

The following things happen during heat treatment .•\'\.~.••~) 'lY~n~U\~l\)\\ ~\\Q'\\ :)')~v.)l";.'~Ui'.~'. t .';':"\'

a) During heat treatment CO is formed

b) CO dissociates into atomic carbon.

,:sd cy' f· This~~~1b-<)hr;:t2~;gJe~i~~bllie ~{irl~degr-~t~e(~o;hp~hk'ht~:~";J: ". .)frt ." ..'. ~.:~~,'-g .. q;~~([~.~.b . )~i.:~~:·q1 bL.!"()08 ~j-I~'rJ ~i;J 9t'!!~.~·~( 'Jrt~ ~}~i;.1~'l ~''''.~~~~:\ld:''f\

,. J ".-. 'C•... -e» 'r', ·'··:'·:~.'l- ')CO4if.3Fe. ~Fe3C +';J.(;,02- '1'" i"'~ .' o : j ','-\',',,: ze t ·'i' I"~.• -r'~ •· .•••'1· .)'"',1-1. I • JO.) ;.1. ,"J J')", f'1. r: ••. ~.I ..• "F .,1.' '_ .•...1..". ~ I.... .• __ "" -~' "- __ ' .•••.•• .....,. 4 ..••• ' .•.••••••••.

,-. ,~ t J ••

Formation of F.~3.q}mftl1£; ~uf.f~f~;gi~e,sJkY~:pf·l;lj.gn...:.l].jll:gp~s~;tq')~!\~l!l;, .

(iii) Gas carburizing: This involves heating steel COI~)}30n~t~i~tJ!gW,.t-;90Q~Cforthree to' four hours in an atmosphere containing gases which will depositcarbon atoms' at the surface of the compon~nts. TIi~'~~9rr~~ai~Zri.~~tsi~f20%

.CO, 40% lJ2 and 40% N2. Carbon .moncxide dis~9JCiiat~~"tQill~Dfl$itl ¢afbo.b5

·,.illi..>:,j\~~~~gni~~~li~~~C~,O~i~~~f~~eetfPIItR.OIJ~Ri~~~'D!es4r.Pt~.}~tl?l;f3,ex;~re~lfrom" 0 ~5 to 1 mm. .r , '. I ' . 'I - ••.. "." f" t 1 ..•..i."'~:'. r!·~~ ..• .'1.''"i.;· (~'jn.r i,'tf1.r nrc.. "iLj •• .••:.t~rJ(!' ',,~~r":") ~;X~~ ~'fT::::)fr')L;\ .' :!,:?1""} •.r:.

av) ;lki,q",id. c;grb~.. ·",g1.lIe.],'~.;,.a mixtureo' ~i odium carbonate 5Q-%''sdium''cyanide an 10.'0 sodium chlorides heate 0 a temperature SuptO:,e.50qGand~the work piec contained in a wire basket are immersed into the liquid bath

~ ~'l"i ' l"'f ( rf"~"tf·. • s ,for about 5 minutes. The cyanide dissociates' into ~.,.. ~ . " -.'" ,

•• '(1' , '1...;': *~ .r .#.:':ttA. .•. ~'~ ..• 1. '_.0'

NaCN + 2C02 ~Na2C03 + 2N + CO. , .. . . ·','ift #:. >.!,.~L ..r,_ . I #i

Dissociation of CO at the steel surface then takes place, Hi the Isiml'e manneras pack carburizing. .;g!1!;.:.!tJ~ '1ilC'j;::;) IY

(b) ,Nitriding: Nitriding 'is a process' of introducirig. Nitrogen; atoms o'tito the'surface of steel components. This is done by heating the components in ametal container in an atmosphere of ammonia upto a temperature of 500°C.Ammonia di~sociates into -. ,

.' ,

,NH3~"~H +t.~i,'.'~ ,

This atomic fo~ of Nitr6gen(N)is absorbed on the surface of steelcomponent. Nitriding gives very high surface hardness to steels and this process issimple and economical. '

.I '

(c) Cyaniding: This is a:process of introducing bothnitrogen and carbon on thesurface of low carbon ~teel components. Here ,the,steels are heated upto about

• ·I"":;'.·~ ,'-;" ',f';',>'- .'~;.·f ';~.".~ ;:'~.~~.'':. ....~' \. • .

800°C in a molten sodium cyanide bath for a penod of 30 mm to 3 hoursdepending 'upon tIle thickness required: ·llie 'cy~ide' bath .~corisists'of 30%

s, N~CN~40~ NaCO~arid-30%NaCt' ,:,. ',:, ,.:,',':z. " . " ' "i'

At this temperature sodium cyanide dissociates to give carbon and Nitrogenin the at~~lc' form, which goes i~to the SUrla:C~of t~e!steel component.'A cas~depth of upto 0~25 mm is obtained. ',; , "

1 ';.o} ~1.· , II .-I •• ,.~ '. ,'~... ~".~'.:~' ,.~ ~t .. ;". !~~.

d), ," C.arbpn~,t~di",g:"Thi~ is' also .a..process of introducing both nitrogen andcarbon onthe surface of low carbon steel components. Here the steel parts areheated 4l'~1furnace, in the. atmosphere of'ammonia and carrier gas (mixture

'I ••. .' . • J J" - ,., ,". -'., •• . ~' • • .., •.

ofN2, Hz and CO) upto 900°C for few hours.depending on the depthrequired.Both carb()p .and nitrogen are ..introduced on t~. theeurface of :t~~, steel.

••••• • , • • '~ 0" ." • ~' • • , .' '.1 • 4' , 1 " . ~ ",' .'S., ~ ''1>' ,'-', '. .."" . ~

components:'. " ,'. "'" ~ .

1l~13.2"Thermalmethods /S.WtJa£;.c2,EtahJeN..~', '.',,-In: .this method, 'only' the 'surface; of the steel', components ar~'::heated to

temperatures above the upper' critical' ~temperahrre '~d' is su~ld~nly'quenched toget 'fuariensife ~f6rnlation"o:n 'th;e~~surface'~iii~h'gives < higher'hardness:' it the

...•.. ',.... ~ :' 'v t, ,. ~ " "; ... '. <:.>, ".~ ,"; ~ . ,: I.: ~•. ":,,'!. ~ ,' .•."(' .~,~,:.:....•.~

surface. The different methods are: . , , ,. ~. : I ~ . .~" '. •• ;" ,J , "

, ,- .a) '. Flam~,hardening .. ,

, , bj <' Induction hardeIri~g. '

. .~ ::', .

•..•• t ',:' . ~.; ~' ." ~.. .,

.>,, "

a) Flame _harde~ing: 'J;hisprocess of surface hardening involves.

Work Piece

Heatedsurface

Hardenedsurface

(ii)

Fig. 11.14 Flame hardening

Rapid heating of the _surface, of the steel by means .of a flame to "-temperatures' of above austenite range (while .the interior core will be at alower temperature).

Thisis followed by quenching of the 'heated surface by spraying cold water onthe surface as shown in the figure.

Heating the surface of steel can be achieved by means of all oxy-acetyleneflame. Depth of hardness zonedepends on the flame intensity and heatingtime.

(i)

b)

, '

This is a very simple andeconomicalmethod of surface hardening. The basicdisadvantage lies in overheating which can cause damage to the component,- - ' , , ,-_', J

Induction hardening: Here instead of direct heating of the surface 'using.flame, heating is done using induction heating principle.

High frequency current is passed through the inductor which' is kept near theworkpiece as shown in fig. 1.1.15. This sets:up a magnetic field Whichis cut by

'~orkpiec~ .kept. near _the inductor. As a, result, an _alternating current is I

'~nduced in .the workpiece which heats 'up the workpiece .due to I2R losses.Using this prindple,' the surface of the st~ei' component can be he~ted to 'a'

! ' ';: .

temperature just above the critical temperature to anydesired depth. This isfollowed by immediate quenching by means of water-spray. As a resultmartensite formation occurs at the surface only. Thus, only the surface of thesteel gets hardened while the interior core' remains normaL' Depth of.hardening depends on the extent of heating which is a function of frequencyofthe current.

I, Ia...c(-, f l D k1t~ S-DD ¥---1t0, 7Sv - §oDC-

-Here a very, close eontro] C~Il bE.!exercised .OJ}; the q~p,~hof harq,I1e,l?,sX~Ruired~, I. '.' ~

which is one of the major advantages of this process ov:erfl~f3;.,haz;9:el'llp.g.'

- ..• ----"'- - - ~I.~ j~ _; - - .,

.... - --

:,\.'

,;.~....

----~--- .'- - -~~:-:~:~. ,.. ••.. ~ ...•. ,

Work ;Piece

Gapsfowater to pass

Inductor'

: " I. ~., ...',' :, ..., ....

,...

.,: "("<'.

, \ ~

\ ,~

, Hign.frequency- generator

'l! .••.•,,;:

,. ,.. ; (

Capacitance" ~> ~ ."~ ~--f'

. ....

Fig~-11;15Induction hardening.