Section 18

Heat Treatment of Steel & Weld Joints

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18.0 HEAT TREATMENT OF STEEL & WELD JOINTS

18.1 INTRODUGTION

The heat treatment given

to
on
shown
be
should
steelmaker/supplier
referred to as the'supply condition''

particular grade of steel bY the

the material telt certiflcate and may be

test certificates and it is

welding inspectors may need to refer to material is used and have some
that
terminology
appropriate tnat tnei te-famifiar with the
of the most commonly applied heat
understanding of the principles of some
treatments.
hea.t treatment after welding (postwelded joints may need to be subjected to
the thermal cycle and checking the
weld heat treatmeni;';;l th" irsxi oirnon'rtoring
heattreatmentrecor,dsareoftendelegatedtoweldinginspectors.

18.2 HEAT TREATMENT OF STEEL

are:
The main supply conditions for weldable steels

as rolled
hot rolled
hot finished

plateishotrolledtofinishedsizeandallowedtoaircool;

from ..
ln.-i.rp.tature at which rolling finishes may varyproperties
pirt. t" plate and so strength and toughness
varv and are not oPtimised,
relatively thin, lower strength C-steel

"pirGJto

steel plate given precisely,controlled thickness

controlled
reductioni iuiind hot rotting within carefully
also
is
temperature
final roiling
thermo-mechanically rolled temperature ranles;
carefullY controlled;
steels
applied t" i"frt'"ly thin, high streng.th low alloy

TM.P*
control-rolled

(HSLAsteels)andforSomesteelswithgoodtoughness
at low temperatures, e'g', cryogenic ?t"9lt, - *TMCP=Thermo-MechanicalControlledProcesang
normalised

is
after working the steel (rolling or forging) to .size' .it to
air
in
cool
to
1900'C
allowed
then
and
to
n.rt"O
and
amO,ent temperature; this optimises strength
item
torghn"tt and gives uniform properties from item to

particular grade of steel;

,pplitJ io C-nr"n steels and some low alloy steels

for

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Heat Treatment
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after working the steel (rolling or forging) to size, it is

heated to -900"C and then cooled as quickly as possible
by quenching in water or oil; after quenching, the steel
must be tempered (softened) to improve the ductility of
the'as-quenched' steel;
applied to some low alloy steels to give higher strength,
or toughness or wear resistance

quenched
&

tempred

solution annealed
solution heat treated

after hot or cold working to size, steel heated to -1100'C

and rapidly cooled by quenching into water to prevent
any carbides or other phases from forming
applied to austenitic stainless steels such as 304 & 316
grades

; ,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 to
3-s-'e that the properties of the weldment will be suitable for their intended
ai;E, :tjcns.

The temperature at which PWHT is carried out is usually well below the
r: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;-:

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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 cracking
-c enable welded joints to be machined to accurate dimensional tolerances

Seoause the main reason for (and benefit of) PWHT is

str"ses PWHT is often called'stress relief'.

to reduce

residual

f,mm f : rhere are circumsfances when a welded joint may need to be normalised to
r?,,: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 because
rtire. s,ng will significantly reduce weld metal strength
1C

4 PWHT THERMAL CYCLE

luhfi{f,rE ^scection

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must be
TheApplicationStandard/Code,willspecifywhenPWHTisrequiredtogive
giu.'grii'u;;;;;t'i the thermal cvcte that

benefits #1 or
used.

*z ,[1". ,ni

atso

the
lnordertoensurethataPWHTcycleiscalrleditinaccordancewitha
procedure-is prepared and that
;'pwfii
that
oarticurar code, it rs essentiar

fiiliil
o
o
.
.

specified: Parameters are

the maximum heating rate

the soak temPerature range

soak tempqrature (soak time)

the minimum time at the
the maximum cooling rate

18.4.1

HEATING RATE

gradients) will
Thismustbecontrolledtoavoidlargetemperaturedifferenceswithinthe
fabricated item. Large differenc-es ].'i.;G;'t"u'tr"o"-thermal
,6 cause distortion (or even
i!"rl'gh
these
and
streiEs
produce rarge
cracking)'

;;rrin

,;;

ApplicationStandardsY:y.,lllrequirecontrolofthemaximumheatingrate
init ii because steels start toto
l[H."-;6ij;i'
is
item.
the
of
when the temperature
and are more susceptible
t"*ptrature
auovJlni"
.ti.ngtn
r.r,
show significant
"t thermal gradients'
distortion if there aie large

int thickness

range of the item'

positions
Bymonitoringfurnace'and'.1"'''temperaturestherateofheatingcanbe
*ir,-cii"',Equi,uments 4 all
to
controtred
"nru,.!
item.

;il;ffi.;

MaximumheatingratesspecifiedforC-Mnsteeldependonthicknessofthe
range -60 to -200"C/h'
item but tend to n"ltihE

Welding lnsPection
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Heat Treatment
Ltd
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18.3

Thetemperatureofthefabricateditemmustbe.monitoredduringthethermal
tn" surface at a number
r"qr;i,n.i:^i",i.iipres attacneJto

cycle and this is otn.'nv

of locations I."p'"t""n"tini

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I8.4.2

SOAK TEMPERATURE

The soak temperature specified by the Code depends on the type of steel and
T-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 alloy
such as Cr-Mo steels used for elevated temperature service) require higher
Er-s'atures - typically in the range -700"C to -760'C.

3i3e

ipri*

Soak temperature is an essential variable for a WPQR. Thus, it is very

mportant that the it is controlled within the specified limits othenuise it
may be necessary to carry out a new WPQ lesf lo validate the properties of
ihe 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 specified
lts-c'a:ure throughout the full joint thickness.

'^e

temperature is monitored by surface-contact thermocouples and it is the

1rc.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
- to avoid distortion (or cracking)
gradients.
ch stresses from thermal

$&r 'eascn that heating rate needs to be controlled

rr*rs

i: -

rlE-

l:Ces usually specify controlled cooling to -300"C. Below this temperature the

:a' oe wrthdrawn from a furnace and allowed to cool in air because steel is

'rE*iil"r
tl*1a,-

-a.,

;'lqrlune

strong and is unlikely to suffer plastic strain by any temperature gradients

cevelop.

5 s a schematic of a typical PWHT thermal

cycle.

lC 5 HEAT TREATMENT FURNACES

xrllar

s rmportant that oil and gas fired furnaces used for PWHT do not allow flame
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

I -rr ,:
.ililf -r".a:^t
Lilr,nrEr-r r :::6.

18.4

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18.6 LOCAL PWHT

Forapipelineorpipes.ooo|jtisoftennecessarytoapptypostweldheat
tocat application of heat'

i'"r['I6y

treatment to indivioul

parameters
specify the previously described
procedure.must
For this, a PWHT
to specify the followtng:
ttt"tt'ry
.r.J
it
for controlting the rn'Ji*ri.r;iJ;riit

.thewidthoftheheatedband(thatmustbewithinthesoaktemperature
range)

.thewidthofthetemperature.decay,band(soaktemp.to-300.C)
other

T:ilil:::1il:',."rmocoupres

width and the

within in the heated band

decaY band

way to allow
s-upported in a particular
be
to
needs
item
if the
movemenVavoid distortion
is by means insulated
of. heating for local,P_wHT
method
The commonest
attached to the weld
etectrical ut"r"#
used'
elements can also be
Gas fired, radiant' heating

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i:i;;-'ili;;t''ii;;i"are

Figure6showstypicat,controlzones,forlocalisedPWHTofapipebuttweld.

Welding lnsPection
Rev 0 Jun 06
Heat Treatment
Ltd
Copyrignt o 2006, TWI

18.5

Wi

iffij'"'"!r.it

NORMALISING

.
o
.
-

.-

rapid heating to soak temperature (100% austenite)

short 'soak' time at temperature

cool in air to ambient temperature

900oC

Schematic of a typical normalising heat treatment applied to C-Mn

and some low alloy steels

QUENCHING & TEMPERING

.
o
.
.
-

rapid heating to soak temperature (100% austenite)

short 'soak' time at temperature

rapid cooling by quenching in water or oil
reheat to tempering temperature, soak & air cool

900oC

quenching
cvcle
Time
: -

,^ a

a -a^l

4 n& ;i'

Schematrc of a typical quenching & tempering heat treatment

applied fo some low alloy steels

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>
slab heating temperature

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105OoC

austenite

(il

austenite + ferrite

O
o

(Y+ct)

0)

(o

o)

F
ferrite + pearlite
(rl+ iron carbide)

(TMCP) and'as'rolled' conditions

Comparison of the'controbrotled'
(= hot rolling)

Figure 3

SOLUTION HEAT TREATMENT

(100% austenite)
rapid heating to soak temp'
short 'soak' time at temPerature
quenching into water or oil
rapid coot cooling by

.
.
o

?.

:.19.q9.:9...,...rm'

0)

quenching

0)

o
E

0)

Figure 4

-iSchematicofatypicatsolutionheattreatment(solutionannealing)
less s teels
a u sten' itic s tain

ii i i ii"
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Welding lnsPection
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18.7

o
=
(t

Tili ffiJ##:

PWHT (C-Mn steels)

.
o
o

controlled heating rate from 300.C to soak temp.

minimum soak time at temperature
controlled cooling to

300'C

o
o

o
L
f
(U
L

o_

q)

controlled
heating & cooling
rates

F
^,300oC
air cool
Time

Figure 5

Schematic of a typical PWHT applied to C-Mn steels

Figure 6

Schemafic of local PWHT of a pipe girth seam

Welding lnspection
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'{@@l TiJ,T$#jJ'',"INING
Questions

Heat treatment

OUl.

applying heat to a metal

List three methods of

QU2.Givethenamesoffourheattreatments,whichmaybeappliedto
steel

QU3.

treatments' and
the following heat
of
description
Give a brief
each'
state the PurPose of

a)

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Normalising'

b) Annealins
c)

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Quench harden'

d) TemPerins'

Qu4.Statethefactors,whichrequiremonitoringforheattreatments.l

OUs.

toughness
used to give maximum
is
treatment(s)
heat
Which
values?

Welding insPection
Rev 0 Jun 06

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QU 18