Section 18

Heat Treatment of Steel & Weld Joints

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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

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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

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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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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.

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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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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

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: - ,^ a

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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

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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

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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

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