chapter 8 - technical heat treatment

7/30/2019 Chapter 8 - Technical Heat Treatment

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8.

Technical Heat Treatment


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8. Technical Heat Treatment 95

When welding a workpiece, not only the weld

itself, but also the surrounding base material

(HAZ) is influenced by the supplied heat

quantity. The temperature-field, which ap-

pears around the weld when different welding

procedures are used, is shown in Figure 8.1.

Figure 8.2 shows the influence of the material

properties on the welding process. The de-

termining factors on the process presented in

this Figure, like melting temperature and -

interval, heat capacity, heat extension etc,

depend greatly on the chemical composition

of the material. Metallurgical properties are

here characterized by e.g. homogeneity,

structure and texture, physical properties like

heat extension, shear strength, ductility.

Structural changes, caused by the heat input

(process 1, 2, 7, and 8), influence directly the mechanical properties of the weld. In addition,

the chemical composition of the weld metal and adjacent base material are also influenced

by the processes 3 to 6.

Based on the binary system,

the formation of the different

structure zones is shown in

Figure 8.3. So the coarse

grain zone occurs in areas

of intensely elevated

austenitising temperature for

example. At the same time,

hardness peaks appear in

these areas because of

greatly reduced criticalcooling rate and the coarse

Temperature Distribution ofVarious Welding Methods

6

4

2

0

-2

-4

-6

-14 -12 -10 -8 -6 -4 -2 0 2 cm 6

cm cm6

4

2

0

-2

-4

-6

-8 -6 -4 -2 0 2cm

-60 -40 40 mmmm 60

250

500

750

C1750

oxy-acethylenewelding

manualmetalarcwelding

tempera

ture

723C

distancefromweldcentralline

heataffectedzoneduringoxy-acethylenewelding

heataffectedzoneduringmanualmetalarcwelding

300C

400C500C

600C 700C800C

900C300C

400C

500C 600C

700C

1250

1000

20-20 0

ISF2002br-er04-01.cdr

Figure 8.1

ClassificationofWeldingProcessInto

IndividualMechanisms

47

5

8 9 10

2

1

36

Heatingandmeltingtheweldingconsumable

1

Meltingpartsofbasematerial2

Reaction of passing weldingconsumablewitharcatmosphere

Reactionofpassedweldingconsumablewithmoltenbasematerial

Interactionbetweenweldpoolandsolidbasematerial(possiblyweldpasses)

3

4

5

Reactionofmetalandfluxwithatmosphere

6

Solidificationofweldpoolandslag7

Coolingofweldedjointinsolidcondition

8

Post-weldheattreatmentifnecessary

Sustainablealterationofmaterialproperties

Specificheat,meltingtemperatureandinterval,meltheat,boilingtemperature(metal,coating)

Specific heat, melt temperature and interval, heatconductivity,heatexpansioncoefficient,homogeneity,time

Compositionof atmosphere, affinity, pressure,temperature,dissotiation,ionisation,reactionspeed

Solubilityrelations,temperatureandpressureunderinf luence of heat source, specif ic weight,

weldpoolflux

Diffusionandpositionchangeprocesses,time,boundaryformation,ordered-unorderedstructure

Affinity,temperature,pressure,time

Meltheat,coolingconditions,densityandporosityofslag,solidificationinterval

Phasediagrams(timedependent),heatconductivity,heatcoefficient,shearstrength,ductility

Phasediagrams(timedependent), texturebywarmdeformation,ductility,moduleofelasticity

Phasediagrams,operatingtemperature,mechanicalandchemicalstrain,time

9

10

ISF2002br-eI-04-02.cdr

Figure 8.2


3/13


austenite grains. This zone of the weld is the area, where the worst toughness values are

found.

In Figure 8.4 you can see how much the forma-

tion of the individual structure zones and the

zones of unfavourable mechanical properties

can be influenced.

Applying an electroslag one pass weld of a 200

mm thick plate, a HAZ of approximately 30 mm

width is achieved. Using a three pass tech-

nique, the HAZ is reduced to only 8 mm.

With the use of different procedures, the differ-

ences in the formation of heat affected zones

become even clearer as shown in Figure 8.5.

These effects can actively be used to the ad-

vantage of the material, for example to adjust

calculated mechanical properties to one's

choice or to remove negative effects of a weld-

ing. Particularly with high-strength fine grained steels and high-alloyed materials, which are

specifically optimised to achieve special quality, e.g. corrosion resistance against a certain

attacking medium, this

post-weld heat treatment is

of great importance.

Figure 8.6 shows areas in

the Fe-C diagram of differ-

ent heat treatment meth-

ods. It is clearly visible that

the carbon content (and

also the content of other

alloying elements) has a

distinct influence on thelevel of annealing tempera-

Microstructure Zones of a Weld -Relation to Binary System

heataffectedzone(visibleinmacrosection)

4

1 2 3 4 5 6

5

6

3

2

1

100

1500

1300

C

1200

1000

G

800

P

600

400

300

S

723

1147

1 2 3%

carboncontent

Tempera

ture

Hardness

age

ing

blue

bri

ttleness

weldbead

incompletemelt

coarsegrain

standardtransformation

incompletecrystallisation

recrystallisation

hardnesspeak

hardnesssink

0,8

2,0

6

0,2


Figure 8.3

Figure 8.4


4/13


tures like e.g. coarse-grain heat treatment or normalising.

It can also be seen that the start of martensite formation (MS-line) is shifted to continuously

decreasing temperatures with increasing C-content. This is important e.g. for hardening

processes (to be explained later).

As this diagram does not

cover the time influence,

only constant stop-tempera-

tures can be read, predic-

tions about heating-up and

cooling-down rates are not

possible. Thus the individual

heat treatment methods will

be explained by their tem-

perature-time-behaviour in

the following.

Development of Heat Affected Zone ofEB, Sub-Arc, and MIG-MAG Welding

gasmetalarcwelding

electronbeamwelding100

submergedarcweldingpass/cappedpass4

0

12


Figure 8.5

Metallurgical Survey ofHeat Treatment Methods

1600

C1536

metastablesystemiron-carbon(partially)

1392

1300

1200

1100

1000

911

800

700

600

500

400

300

200

100

20

C

1400

1300

1200

1100

1000

900

800

700

600

500

400

300

200

100

20

1600

0 0,5 0,8 1 1,5 2

Carboncontentinweight%

302520151050

Fe

A

H B

1493C

d -solidsolution+austenite

d -solidsolution

cbcatomiclattice

melt

melt+austenite

diffusionheattreatment

coarsegrainheattreatment

E2,06

cfc

atomiclattice

A4heatcolors

yellowwhite

lightyellow

yellow

yellowred

lightred

cherry-red

darkred

brownred

darkbrown

1147

A3austenite

( -Mischkristalle)g

Acm

austenite+secondarycementite(Fe C)3

KS

austenite+ferriteA2 M

A1 P 723CO

recrystallisationheattreatment

recrystallisationheattreatmentQ

ferrite

( -solidsolution)a

softannealing

stressrelieving

cbcatomiclattice

hardening

tempering

MS

eutektoidic

steel

Cementitecontentinweight%ISF2002br-er04-06.cdr

melt+

-solidsolutiond

N

normalising+

hardening

G

769C

hypoeutectoidicsteel hypereutectoidicsteel

Figure 8.6

CoarseGrainHeat Treatment

Tempera

ture

Time

900

700

500

300

C austenite

A3

A1

austenite+ferrite

ferrite+perliteT

empera

ture

C-Content

longtimeseveralhours

intenseheating

0,4 0,8 %


Figure 8.7


5/13


Figure 8.7 shows in the detail to the right a T-t course of coarse grain heat treatment of an

alloy containing 0,4 % C. A coarse grain heat treatment is applied to create a grain size as

large as possible to improve machining properties. In the case of welding, a coarse grain is

unwelcome, although unavoidable as a consequence of the welding cycle. You can learn

from Figure 8.7 that there are two methods of coarse grain heat treatment. The first way is to

austenite at a temperature close above A3 for a couple of hours followed by a slow cooling

process. The second method is very important to the welding process. Here a coarse grain is

formed at a temperature far above A3 with relatively short periods.

Figure 8.8 shows schemati-

cally time-temperature be-

haviour in a TTT-diagram.

(Note: the curves explain

running structure mecha-

nisms, they must not be

used as reading off exam-

ples. To determine t8/5,

hardness values, or micro-

structure distribution, are

TTT-diagrams always read

continuously or isothermally.

Mixed types like curves 3 to

6 are not allowed for this purpose!).

The most important heat treatment methods can be divided into sections of annealing, hard-

ening and tempering, and these single processes can be used individually or combined. The

normalising process is shown in Figure 8.9. It is used to achieve a homogeneous ferrite-

perlite structure. For this purpose, the steel is heat treated approximately 30C above Ac3

until homogeneous austenite evolves. This condition is the starting point for the following

hardening and/or quenching and tempering treatment. In the case of hypereutectoid steels,

austenisation takes place above the A1 temperature. Heating-up should be fast to keep the

austenite grain as fine as possible (see TTA-diagram, chapter 2). Then air cooling follows,

leading normally to a transformation in the ferrite condition (see Figure 8.8, line 1; formationof ferrite and perlite, normalised micro-structure).

1:Normalizing2:Simplehardening3:Brokenhardening4:Hotdiphardening5:Bainiticannealing6:Patenting(isothermal

annealing)

0,1

900

0

100

200

300

400

500

600

700

Caustenite

ferr

ite

lin

e

Tempera

ture

MS

2 3 4 6 5 1

1 10 10 10s

A3

A1

ferr

ite

perl

ite

ba

inite

martens

ite

Time

TTT-DiagramWithHeat TreatmentProcesses


Figure 8.8


6/13


To harden a material, aus-

tenisation and homogeni-

sation is carried out also at

30C above AC3. Also in

this case one must watch

that the austenite grains

remain as small as possi-

ble. To ensure a complete

transformation to marten-

site, a subsequent quench-

ing follows until the

temperature is far below

the Ms-temperature, Figure

8.10. The cooling rate dur-

ing quenching must be high enough to cool down from the austenite zone directly into the

martensite zone without any further phase transitions (curve 2 in Figure 8.8). Such quenching

processes build-up very high thermal stresses which may destroy the workpiece during hard-

ening. Thus there are variations of this process, where perlite formation is suppressed, but

due to a smaller temperature gradient thermal stresses remain on an uncritical level (curves

3 and 4 in Figure 8.8). This

can be achieved in practice

for example- through stop-

ping a water quenching

process at a certain tem-

perature and continuing the

cooling with a milder cooling

medium (oil). With longer

holding on at elevated tem-

perature level, transforma-

tions can also be carried

through in the bainite area

(curves 5 and 6).

Normalizing

Tempera

ture

Time

900

700

500

300

C austenite

A3

A1

austenite+ferrite

ferrite+perliteT

empera

ture

C-Content

0,4 0,8 %

transformationandhomogenizing

of -solidsolution(30-60min)

at30Cabove A

g

3

quickheating

aircooling


Figure 8.9

Hardening

Tempera

ture

Time

900

700

500

300

C austenite

A3

A1

austenite+ferrite

ferrite+perliteT

empe

rature

C-Content

0,4 0,8 %

startofmartensiteformation

quenchinginwater

about30Cabove A3

startofmartensiteformation


Figure 8.10


7/13


Figure 8.11 shows the quenching and tempering procedure. A hardening is followed by an-

other heat treatment below Ac1. During this tempering process, a break down of martensite

takes place. Ferrite and cementite are formed. As this change causes a very fine micro-

structure, this heat treat-

ment leads to very good

mechanical properties like

e.g. strength and tough-

ness.

Figure 8.12 shows the pro-

cedure of soft-annealing.

Here we aim to adjust a

soft and suitable micro-

structure for machining.

Such a structure is charac-

terised by mostly globular

formed cementite particles, while the lamellar structure of the perlite is resolved (in Figure

8.12 marked by the circles, to the left: before, to the right: after soft-annealing). For hypoeu-

tectic steels, this spheroidizing of cementite is achieved by a heat treatment close below A1.

With these steels, a part of the cementite bonded carbon dissolves during heat treating close

below A1, the remaining cementite lamellas transform with time into balls, and the bigger

ones grow at the expense of

the smaller ones (a transfor-

mation is carried out because

the surface area is strongly

reduced thermodynami-

cally more favourable condi-

tion). Hypereutectic steels

have in addition to the lamel-

lar structure of the perlite a

cementite network on the

grain boundaries.

Hardeningand Tempering

Tempera

ture

Time

900

700

500

300

C austenite

A3

A1

austenite+ferrite

ferrite+perliteT

empera

ture

C-Content

0,4 0,8 %

quenching

about30Cabove A3

hardeningandtempering

slowcooling


Figure 8.11

Soft Annealing

Tempera

ture

Time

900

700

500

300

C austenite

A3

A1

austenite+ferrite

ferrite+perliteT

empera

ture

C-Content

0,4 0,8 %

timedependentonworkpiece

10to20Cbelow A1

oscillationannealing+/-20degreesaround A1

or

cementite


Figure 8.12


8/13


Spheroidizing of cementite is achieved by making use of the transformation processes during

oscillating around A1. When exceeding A1 a transformation of ferrite to austenite takes place

with a simultaneous solution of a certain amount of carbon according to the binary system Fe

C. When the temperature drops below A1 again and is kept about 20C below until the trans-

formation is completed, a

re-precipitation of cemen-

tite on existing nuclei takes

place. The repetition of this

process leads to a step-

wise spheroidizing of ce-

mentite and the frequent

transformation avoids a

grain coarsening. A soft-

annealed microstructure

represents frequently the

delivery condition of a ma-

terial.

Figure 8.13 shows the principle of a stress-relieve heat treatment. This heat treatment is

used to eliminate dislocations which were caused by welding, deforming, transformation etc.

to improve the toughness of a workpiece. Stress-relieving works only if present dislocations

are able to move, i.e. plastic structure deformations must be executable in the micro-range. A

temperature increase is the

commonly used method to

make such deformations

possible because the yield

strength limit decreases with

increasing temperature. A

stress-relieve heat treatment

should not cause any other

change to properties, so that

tempering steels are heat

treated below temperingtemperature.

StressRelieving

Tempera

ture

Time

900

700

500

300

C austenite

A3

A1

austenite+ferrite

Tempera

ture

C-Content0,4 0,8 %

timedependentonworkpiece

between450and

650C

ferrite+perlite


Figure 8.13

Stress releaving

Heat treatment at a temperature below the lower transition point A 1, mostly

between 600 and 650C, with subsequent slow cooling for relief of internal

stresses; there is no substantial change of present properties.

Normalising

Heating to a temperature slightly above the upper transition point A3

(hypereutectoidic steels above the lower transition point A 1), followed by

cooling in tranquil atmosphere.

Hardening (quench

hardening)

Aco olin g fro m a tem pera ture abo ve the tra nsi tio n poi nt A3 or A1 with such a

speed that an clear increase of hardness oc curs at the surface or ac ross

the complete cross-section, normally due to martensite development.

Quenching and

tempering

Heat treatment to achieve a high ductility with defined tensile stress by

hardening and subsequent tempering (mostly at a higher temperature.

Solution or

quenching heat

treatment

Fast cooling of a workpiece. Also fast cooling of austenitic steels from high

temperature (mostly above 1000C) to develop an almost homogenuous

micro-structure with high ductility is called 'quenching heat treatment'.

Tempering

Heating after previous hardening, cold working or welding to a temperature

between room temperature and the lower transformation point A1; stopping

at this temperature and subsequent purposeful cooling.

TypeandPurposeofHeat Treatment


Figure 8.14


9/13


Figure 8.14 shows a survey of heat treatments which are important to welding as well as their

purposes.

Figure 8.15 shows princi-

pally the heat treatments in

connection with welding.

Heat treatment processes

are divided into: before,

during, and after welding.

Normally a stress-relieving

or normalizing heat treat-

ment is applied before

welding to adjust a proper

material condition which for

welding. After welding, al-

most any possible heat

treatment can be carried

out. This is only limited by workpiece dimen-

sions/shapes or arising costs. The most impor-

tant section of the diagram is the kind of heat

treatment which accom-panies the welding.

The most important processes are explained in

the following.

Figure 8.16 represents the influence of differ-

ent accompanying heat treatments during

welding, given within a TTT-diagram. The fast-

est cooling is achieved with welding without

preheating, with addition of a small share of

bainite, mainly martensite is formed (curve 1,

analogous to Figure 8.8, hardening). A simple

heating before welding without additional stop-

ping time lowers the cooling rate according to

curve 2. The proportion of martensite is re-duced in the forming structure, as well as the

Heat TreatmentinConnectionWithWelding

combinationpreheating

simplepreheating

increaseofworking

temperature

constantworking

temperature

local

preheating

preheatingofthe

completeworkpiece

isothermal

welding

postheating(postweldheat

treatment)

heattreatmentofthecomplete

workpiece

localheattreatment

annealing stressreleaving

stressreleaving

annealing hardening quenchingand

tempering

solutionheat

treatment

tempering

simplestep-hardening

welding

purestephardening

welding

modifiedstephardening

welding

Typesofheattreatmentsrelatedtowelding

heattreatment

beforewelding

combi-nation

accompanyingheattreatment

combi-nation

heattreatmentafterwelding

(post-weldheattreatment)


Figure 8.15

TTT-Diagram forDifferent Welding Conditions

800

700

600

500

400

300

200

100

0

0 1 10 102

103

104

105

s

C

Tempera

ture

T

Timet

MS

TA

(1) (2) (3)

tH

(1):Weldingwithoutpreheating,(2):Weldingwithpreheatingupto380C,withoutstoppagetime(3):Weldingwithpreheatingupto380Candabout10min.stoppagetime

T :Stoppagetemperature,t :DwelltimeA H


Figure 8.16


10/13


level of hardening. If the material is hold at a temperature above MS during welding (curve 3),

then the martensite formation will be completely suppressed (see Figure 8.8, curve 4 and 5).

To explain the temperature-time-behaviours

used in the following, Figure 8.17 shows a su-

perposition of all individual influences on the

materials as well as the resulting T-T-course in

the HAZ. As an example, welding with simple

preheating is selected.

The plate is preheated in a period tV. After re-

moval of the heat source, the cooling of the

workpiece starts. When tS is reached, welding

starts, and its temperature peak overlays the

cooling curve of the base material. When the

welding is completed, cooling period tA starts.

The full line represents the resulting tempera-

ture-time-behaviour of the HAZ.

The temperature time course during welding

with simple preheating is shown in Figure 8.18.

During a welding time tS a

drop of the working tem-

perature TA occurs. A fur-

ther air cooling is usually

carried out, however, thecooling rate can also be

reduced by covering with

heat insulating materials.

Another variant of welding

with preheating is welding

at constant workingtemperature. This is

Temperature-Time-DistributionDuring Welding With Preheating

tV tS tA

start endseam

transformationrange

Timet

TemperatureT

TS

A3

A1

TV

T :Preheattemperature,

T :Meltingtemperatureofmaterial,

t :Preheattime,

t :Weldingtime,

t :Coolingtime(roomtemperature),M :Martensitestarttemperature

A :Uppertransformationtemperature,

A :Lower

V

S

V

S

A

S

3

1 transformationtemperature

Courseofresultingtemperatureintheareaoftheheataffectedzoneofthebasematerial.

Temperaturedistributionbypreheating,Courseoftemperatureduringwelding.


Figure 8.17

WeldingWithSimplePreheating

A3

A1

Tempera

ture

T

Timet

tV tS tA

TA

TV


T :Workingtemperature,

t :Preheattime,

t :Weldingtime,

t :Coolingtime(roomtemperature)

V

A

V

S

A

Temperatureofworkpiece,Temperatureofweldpoint


Figure 8.18


11/13


achieved through further

warming during welding to

avoid a drop of the working

temperature. In Figure 8.19

is this case (dashed line,

TA needs not to be above

MS) as well as the special

case of isothermal welding

illustrated. During isother-

mal welding, the workpiece

is heated up to a working

temperature above MS

(start of martensite forma-

tion) and is also held there

after welding until a transformation of the austenitised areas has been completed. The aim of

isothermal welding is to cool down in accordance with curve 3 in Figure 8.16 and in this way,

to suppress martensite formation.

Figure 8.20 shows the T-T course during

welding with post-warming (subsequent heat

treatment, see Figure 8.15). Such a treatment

can be carried out very easy, a gas welding

torch is normally used for a local preheating.

In this way, the toughness properties of some

steels can be greatly improved. The lower

sketch shows a combination of pre- and post-

heat treatment. Such a treatment is applied to

steels which have such a strong tendency to

hardening that a cracking in spite of a simple

preheating before welding cannot be avoided,

if they cool down directly from working tem-

perature. Such materials are heat treated

immediately after welding at a temperaturebetween 600 and 700C, so that a formation

WeldingWithPreheatingand

StoppageatWorking Temperature

Tem

pera

ture

T

Timet

tS

tV tH tA

A3

A1

MS

TV

TA

:t =0H T :Preheattemperature,T :Workingtemperature,

t :Preheattime,

V

A

V

t :Weldingtime,t :Coolingtime(roomtemperature),

t :Dwelltime

S

A

H


Figure 8.19

Welding WithPre- and Post-Heating

TemperatureT

Timet

TN

tS

tN tA

A3

A1

A3

A1

TemperatureT

TN

TV TA

Timet

tV tS tNtR tA

2.Pre-andpost-heating

1.Post-heating



T :Postheattemperature,

t :Preheatingtime,

V

A

N

V

t :Weldingtime,

t :Coolingtime(roomtemperature),

t :Postheattime

t :Stoppagetime

S

A

N

R


Figure 8.20


12/13


of martensite is avoided and welding residual stresses are eliminated simultaneously.

Aims of the modified step-

hardening welding should

not be discussed here, Fig-

ure 8.21. Such treatments

are used for transformation-

inert materials. The aim of

the figure is to show how

complicated a heat treatment

can become for a material in

combination with welding.

Figure 8.22 shows tempera-

ture distribution during multi-

pass welding. The solid line

represents the T-T course of a point in the HAZ

in the first pass. The root pass was welded

without preheating. Subsequent passes were

welded without cooling down to a certain tem-

perature. As a result, working temperature in-

creases with the number of passes. The

second pass is welded under a preheat tem-

perature which is already above martensite

start temperature. The heat which remains in

the workpiece preheats the upper layers of the

weld, the root pass is post-heat treated through

the same effect. During welding of the last

pass, the preheat temperature has reached

such a high level that the critical cooling rate

will not be surpassed. A favourable effect of

multi-pass welding is the warming of the HAZ

of each previous pass above recrystallisationtemperature with the corresponding crystallisa-

ModifiedStepWeldHardening

A3

A1

MS

TA

THa

TSt

TAnl

TAnl

tAtAnltAb

tHa

tS

tHtAtH

Timet

TemperatureT


T : Temperingtemperature,T :Hardeningtemperature,

A

Anl

H

T :Steptemperature,

t :Coolingtime,t :Quenchingtime,

St

A

Ab

t : Temperingtime,

t :Dwelltime,t :Weldingtime

Anl

H

S

Temperatureofworkpiece,

Temperatureofweldpoint


Figure 8.21

Temperature-Time DistributionDuring Multi-Pass Welding


T :Meltingtemperatureofmaterial,t :Preheattime,

t :Weldingtime

t :Coolingtime(roomtemperature),

A :Uppertransformationtemperature,

M :Martensitestarttemperature

V

S

V

S

A

3

S

heataffected zone

1

432 } weldpassobservedpoint

1 2 3 4 weldpass

Tempera

ture

TA3

MS

TS

TV

Timet

tStV tA


Figure 8.22


13/13


tion effects in the HAZ. The coarse grain zone with its unfavourable mechanical properties is

only present in the HAZ of the last layer. To achieve optimum mechanical values, welding is

not carried out to Figure 8.22. As a rule, the same welding conditions should be applied for all

passes and prescribed t8/5 times must be kept, welding of the next pass will not be carried

out before the previous pass has cooled down to a certain temperature (keeping the inter-

pass temperature). In addition, the workpiece will not heat up to excessively high tempera-

tures.

Figure 8.23 shows a nomogram where working temperature and minimum and maximum

heat input for some steels can be interpreted, depending on carbon equivalent and wall thick-

ness.

If e.g. the water quenched and tempered fine grain structural steel S690QL of 40 mm wall

thickness is welded, the following data can be found:

- minimum heat input between 5.5 and 6 kJ/cm

- maximum heat input about 22 kJ/cm

- preheating to about 160C

- after welding, residual stress relieving between 530 and 600C.

Steels which are placed in

the hatched area called

soaking area, must be

treated with a hydrogen

relieve annealing. Above

this area, a stress relieve

annealing must be carried

out. Below this area, a

post-weld heat treatment is

not required.

Figure 8.23

chapter 8 - technical heat treatment

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