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Section 18
Heat Treatment of Steel & Weld Joints
;'r9 I -g rspectiOnI: - ^ naVU
i:c "- 3-: 3 2006, TWt Ltd
TWI woRLDCENTREFoR
voc, MATERTALsrorNrNG
18.0 HEAT TREATMENT OF STEEL & WELD JOINTS
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18.1 INTRODUGTION
The heat treatment given to
steelmaker/supplier should be shown on
referred to as the'supply condition''
a particular grade of steel bY the
the material telt certiflcate and may be
welding inspectors may need to refer to material test certificates and it is
appropriate tnat tnei te-famifiar with the terminology that is used and have some
understanding of the principles of some of the - most commonly applied heat
treatments.
welded joints may need to be subjected to hea.t treatment after welding (post-
weld heat treatmeni;';;l th" irsxi oirnon'rtoring the thermal cycle and checking the
heattreatmentrecor,dsareoftendelegatedtoweldinginspectors.
18.2 HEAT TREATMENT OF STEEL
The main supply conditions for weldable steels are:
plateishotrolledtofinishedsizeandallowedtoaircool;ln.-i.rp.tature at which rolling finishes may vary from
..
pirt. t" plate and so strength and toughness properties
varv and are not oPtimised,
"pirGJto relatively thin, lower strength C-steel
TM.P* steel plate given precisely,controlled thickness
control-rolled reductioni iuiind hot rotting within carefully controlled
thermo-mechanically rolled temperature ranles; final roiling temperature is also
carefullY controlled;applied t" i"frt'"ly thin, high streng.th low alloy steels
(HSLAsteels)andforSomesteelswithgoodtoughnessat low temperatures, e'g', cryogenic ?t"9lt, - -*TMCP=Thermo-MechanicalControlledProcesang
after working the steel (rolling or forging) to .size' .it isn.rt"O to 1900'C and then allowed to cool in air to
amO,ent temperature; this optimises strength and
torghn"tt and gives uniform properties from item to item
for i particular grade of steel;
,pplitJ io C-nr"n steels and some low alloy steels
as rolledhot rolledhot finished
normalised
Welding lnsPectionRev 0 Jun 06Heat TreatmentCopyright o 2006' TWI Ltd
WORLD CENTRE FOR
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18.1 TWIvol,
quenched&
temp€red
solution annealedsolution heat treated
after working the steel (rolling or forging) to size, it isheated to -900"C and then cooled as quickly as possibleby quenching in water or oil; after quenching, the steelmust be tempered (softened) to improve the ductility ofthe'as-quenched' steel;applied to some low alloy steels to give higher strength,or toughness or wear resistance
after hot or cold working to size, steel heated to -1100'Cand rapidly cooled by quenching into water to preventany carbides or other phases from formingapplied to austenitic stainless steels such as 304 & 316grades
; ,gures 1,2, 3 & 4 are schematics of thermal cycles for the main supply conditions.
1E.3 POST WELD HEAT TREATMENT (PWHT)
Post weld heat treatment has to be applied to some welded steels in order to3-s-'e that the properties of the weldment will be suitable for their intendedai;E, :€tjcns.
The temperature at which PWHT is carried out is usually well below ther:r-:€.ature where phase changes can occur (note 1), but high enough to allow-:{ :-: st'esses to be relieved quickly and to soften (temper) any hard regions in the
TFere are major benefits of reducing residual stress and ensuring that the HAZ-a:-€ss is not too high for particular steels for particular service applications.
1n;-: es of these benefits are:
' Tc improve the resistance of the joint to brittle fracture
- To improve the resistance of the joint to stress corrosion crackingI -c enable welded joints to be machined to accurate dimensional tolerances
Seoause the main reason for (and benefit of) PWHT is to reduce residualstr"€ses PWHT is often called'stress relief'.
f,mm f : rhere are circumsfances when a welded joint may need to be normalised tor?,,:r:,'E UAZ toughness. However, these are relatively rare circums{ances and it is'rp;Es"ser'/ to ensure that welding consumables are carefully selected becausertire. s,ng will significantly reduce weld metal strength
1C 4 PWHT THERMAL CYCLE
luhfi{f,rE ^scection'-F* - --'- -tr*,r;fr --,e:-entl",m*-rr-: : 2C06. TWI Ltd
TWIv[c,
WORLD CENTRE FOR
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18.2
T
TheApplicationStandard/Code,willspecifywhenPWHTisrequiredtogivebenefits #1 or *z ,[1". ,ni atso giu.'grii'u;;;;;t'i the thermal cvcte that must be
used.
lnordertoensurethataPWHTcycleiscalrleditinaccordancewithaoarticurar code, it rs essentiar that ;'pwfii procedure-is prepared and that the
fiiliil Parameters are specified: -
o the maximum heating rate
o the soak temPerature range
. the minimum time at the soak tempqrature (soak time)
. the maximum cooling rate
18.4.1 HEATING RATE
Thismustbecontrolledtoavoidlargetemperaturedifferenceswithinthefabricated item. Large differenc-es ].'i.;G;'t"u'tr"o"-thermal
gradients) will
produce rarge streiEs and these ,;; i!"rl'gh ;;rrin ,6 cause distortion (or even
cracking)'
ApplicationStandardsY:y.,lllrequirecontrolofthemaximumheatingratewhen the temperature of the item. is l[H."-;6ij;i' init ii because steels start to
show significant r.r, "t .ti.ngtn auovJlni" t"*ptrature and are more susceptible to
distortion if there aie large thermal gradients'
Thetemperatureofthefabricateditemmustbe.monitoredduringthethermalcycle and this is otn.'nv r"qr;i,n.i:^i",i.iipres attacneJto tn" surface at a number
of locations I."p'"t""n"tini int thickness range of the item'
Bymonitoringfurnace'and'.1"'''temperaturestherateofheatingcanbecontrotred to
"nru,.! ;il;ffi.; *ir,-cii"',Equi,uments 4 all positions
item.
MaximumheatingratesspecifiedforC-Mnsteeldependonthicknessoftheitem but tend to n"ltihE range -60 to -200"C/h'
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TWIvocr
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TECHNOLOGYWelding lnsPectionRev 0 Jun 06
Heat TreatmentCoPYright o 2006, TWI Ltd
18.3
I8.4.2 SOAK TEMPERATURE
The soak temperature specified by the Code depends on the type of steel andT-us l^€ temperature range required to reduce residual stresses to a low level.
C & C-Mn steels require a soak temperature of -600"C whereas some low alloy3i3e s such as Cr-Mo steels used for elevated temperature service) require higherEr-s€'atures - typically in the range -700"C to -760'C.
ipri* Soak temperature is an essential variable for a WPQR. Thus, it is verymportant that the it is controlled within the specified limits othenuise itmay be necessary to carry out a new WPQ lesf lo validate the properties ofihe item and at worst it may not be fit for purpose.
18.4.3 SOAK TIME
: 's necessary to allow time for all the welded joints to experience the specifiedlts-c€'a:ure throughout the full joint thickness.
'^e temperature is monitored by surface-contact thermocouples and it is the1rc.es: ,oint of the fabrication that governs the minimum time for temperature*:-E Sa: Cn.
ilt,:E scecified soak times are th per 25mm thickness.
18.'t.4 COOLING RATE
: s necessary to control the rate of cooling from the PWHT temperature for the$&r€ 'eascn that heating rate needs to be controlled - to avoid distortion (or cracking)rr*rs i: - ch stresses from thermal gradients.
l:Ces usually specify controlled cooling to -300"C. Below this temperature therlE- :a' oe wrthdrawn from a furnace and allowed to cool in air because steel is'rE*iil"r€ , strong and is unlikely to suffer plastic strain by any temperature gradientstl*1a,- -a., cevelop.
;'lqrlune 5 s a schematic of a typical PWHT thermal cycle.
lC 5 HEAT TREATMENT FURNACES
: s rmportant that oil and gas fired furnaces used for PWHT do not allow flamexrllar u, ir the fabrication as this may induce large thermal gradients.
: s also important to ensure that the fuel (particularly for oil fired furnaces)me* -':: :ontain high levels of potentially harmful impurities - such as sulphur.
filfiSilffir6 -:steCion{Gu I -rr ,:€.ililf -r".a:€^tLilr,nrEr-r r :::6. TWI Ltd
TWI y?:::,:::Tt?tvccl Y,T,'ff"*:',;^^"
18.4
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18.6 LOCAL PWHT
Forapipelineorpipes.ooo|jtisoftennecessarytoapptypostweldheattreatment to indivioul i'"r['I6y tocat application of heat'
For this, a PWHT procedure.must specify the previously described parameters
for controlting the rn'Ji*ri.r;iJ;riit it .r.J ttt"tt'ry to specify the followtng: -
.thewidthoftheheatedband(thatmustbewithinthesoaktemperaturerange)
.thewidthofthetemperature.decay,band(soaktemp.to-300.C)
other
T:ilil:::1il:',."rmocoupres within in the heated band width and the
decaY band
. if the item needs to be s-upported in a particular way to allow
movemenVavoid distortion
The commonest method of. heating for local,P_wHT is by means insulated
etectrical ut"r"# i:i;;-'ili;;t''ii;;i"are attached to the weld
Gas fired, radiant' heating elements can also be used'
Figure6showstypicat,controlzones,forlocalisedPWHTofapipebuttweld.
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Welding lnsPectionRev 0 Jun 06
Heat TreatmentCopyrignt o 2006, TWI Ltd
18.5 Wi iffij'"'"!r.it
NORMALISING
. rapid heating to soak temperature (100% austenite)o short 'soak' time at temperature
. cool in air to ambient temperature
- 900oC
Schematic of a typical normalising heat treatment applied to C-Mnand some low alloy steels
QUENCHING & TEMPERING
. rapid heating to soak temperature (100% austenite)
o short 'soak' time at temperature
. rapid cooling by quenching in water or oil
. reheat to tempering temperature, soak & air cool
- 900oC
quenchingcvcle
Time
Schematrc of a typical quenching & tempering heat treatmentapplied fo some low alloy steels
: - .- I
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4 n& ;i' '::.+a^r
i ;-" , 2i36 TWt Ltd
TWI WORLD CENTRE FOR
MATERIALS JOININCTECHNOLOGY
18.6
voo,
slab heating temperature > - 105OoC
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austenite(il
Oo
0)!)(o
ooEo)F
austenite + ferrite(Y+ct)
ferrite + pearlite(rl+ iron carbide)
Figure 3
Figure 4
Comparison of the'controbrotled' (TMCP) and'as'rolled' conditions
(= hot rolling)
SOLUTION HEAT TREATMENT
. rapid heating to soak temp' (100% austenite)
. short 'soak' time at temPerature
o rapid coot cooling by quenching into water or oil
. ?. :.19.q9.:9...,...rm'
quenching
oo
0)
=(t0)oE0)F
Schematicofatypicatsolutionheattreatment(solutionannealing)-i ii t i i ii" a u ste n' iti c s tain less s teels
Welding lnsPectionRev 0 Jun 06
Heat TreatmentCoPYright O 2006, TWI Ltd
18.7 Tili ffiJ##:
oo
oLf(ULoo_Eq)F
PWHT (C-Mn steels)
. controlled heating rate from 300.C to soak temp.o minimum soak time at temperatureo controlled cooling to - 300'C
^,300oC
air cool
Time
Schematic of a typical PWHT applied to C-Mn steels
controlledheating & cooling
rates
Figure 5
Figure 6 Schemafic of local PWHT of a pipe girth seam
18.8Welding lnspectionRev 0 Jun 06Heat TreatmentCopyright o 2006, TWI Ltd
TWIvcc,
WORLD CENTRE FOR
MATERIALS JOINING
TECHNOLOGY
TWI \ryoRLD cENTRE FoR
'{@@l TiJ,T$#jJ'',"INING
Questions
Heat treatment
OUl. List three methods of applying heat to a metal
QU2.Givethenamesoffourheattreatments,whichmaybeappliedtosteel
IQU3. Give a brief description of the following heat treatments' and
state the PurPose of each' Ia) Normalising'
b) Annealins Ic) Quench harden'
d) TemPerins' I
Qu4.Statethefactors,whichrequiremonitoringforheattreatments.lI
OUs. Which heat treatment(s) is used to give maximum toughness
values?
Welding insPection
Rev 0 Jun 06
CopYright @ 2006, TWI Ltd
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