8. Technical Heat Treatment - kau.ac.krmercury.kau.ac.kr/welding/Welding Technology II - Welding...8. Technical Heat Treatment 100 Figure 8.11 shows the quenching and tempering procedure.

8.

Technical Heat Treatment

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 critical

cooling 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-acethylene welding

manual metalarc welding

tem

pera

ture

723°C

distance from weld central line

heat affected zone duringoxy-acethylene welding

heat affected zone duringmanual metal arc welding

300°C

400°C500°C

600°C 700°C800°C

900°C300°C

400°C

500°C 600°C

700°C

1250

1000

20-20 0

© ISF 2002br-er04-01.cdr

Figure 8.1

Classification of Welding Process IntoIndividual Mechanisms

47

58 9 10

2

1

36

Heating and melting the weldingconsumable

1

Melting parts of base material2

Reaction of passing weldingconsumable with arc atmosphere

Reaction of passed welding consumablewith molten base material

Interaction between weld pool and solidbase material (possibly weld passes)

3

4

5

Reaction of metal and fluxwith atmosphere

6

Solidification of weld pool and slag7

Cooling of welded joint insolid condition

8

Post-weld heat treatmentif necessary

Sustainable alteration ofmaterial properties

Specific heat, melting temperature and interval, meltheat, boiling temperature (metal, coating)

Specific heat, melt temperature and interval, heatconductivity, heat expansion coefficient, homogeneity, time

Compositionof atmosphere, affinity, pressure,temperature, dissotiation, ionisation, reaction speed

Solubility relations, temperature and pressure underinfluence of heat source, specific weight,weld pool flux

Diffusion and position change processes, time,boundary formation, ordered - unordered structure

Affinity, temperature, pressure, time

Melt heat, cooling conditions, density andporosity of slag, solidification interval

Phase diagrams (time dependent), heat conductivity,heat coefficient, shear strength, ductility

Phase diagrams (time dependent), texture by warmdeformation, ductility, module of elasticity

Phase diagrams, operating temperature, mechanicaland chemical strain, time

9

10

© ISF 2002br-eI-04-02.cdr

Figure 8.2


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 the

level of annealing tempera-

Microstructure Zones of a Weld -Relation to Binary System

heat affected zone(visible in macro section)

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%

carbon content

Te

mp

era

ture

Ha

rdn

ess

ag

ein

gb

lue

britt

len

ess

weld bead

incomplete melt

coarse grain

standardtransformation

incompletecrystallisation

recrystallisation

hardness peak

hardness sink

0,8

2,0

6

0,2


Figure 8.3

Figure 8.4


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

gas metalarc welding

electron beam welding100

submerged arc weldingpass / capped pass4

0

12


Figure 8.5

Metallurgical Survey ofHeat Treatment Methods

1600

°C1536

metastable system iron-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

Carbon content in weight %

302520151050

Fe

A

H B

1493°C

d - solidsolution + austenite

d - solid solution

cbcatomic lattice

melt

melt + austenite

diffusion heat treatment

coarse grainheat treatment

E2,06

cfcatomic lattice

A4heat colors

yellow white

light yellow

yellow

yellow red

light red

cherry-red

dark red

brown red

dark brown

1147

A3

austenite

( - Mischkristalle)g

A cm

austenite + secondarycementite (Fe C)3

KS

austenite+ ferriteA2 M

A1 P 723°CO

recrystallisation heat treatment

recrystallisation heat treatmentQ

ferrite

( -solid solution)a

soft annealing

stress relieving

cbcatomic lattice

hard

enin

g

tem

pering

MS

eute

kto

idic

ste

el

Cementite content in weight %© ISF 2002br-er04-06.cdr

melt +

- solid solutiond

N

normalising +

hardening

G

769°C

hypoeutectoidic steel hypereutectoidic steel

Figure 8.6

Coarse Grain Heat Treatment

Tem

pera

ture

Time

900

700

500

300

°C austenite

A3

A1

austenite+ ferrite

ferrite +perliteT

em

pera

ture

C-Content

long timeseveral hours

intenseheating

0,4 0,8 %


Figure 8.7


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 30°C 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; formation

of ferrite and perlite, normalised micro-structure).

1: Normalizing2: Simple hardening3: Broken hardening4: Hot dip hardening5: Bainitic annealing6: Patenting (isothermal

annealing)

0,1

900

0

100

200

300

400

500

600

700

°Caustenite

ferr

ite

line

Te

mp

era

ture

MS

2 3 4 6 5 1

1 10 10² 10³s

A3

A1

ferr

ite

pe

rlite

ba

inite

ma

rte

nsite

Time

TTT-Diagram With Heat Treatment Processes


Figure 8.8


To harden a material, aus-

tenisation and homogeni-

sation is carried out also at

30°C 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

Tem

pera

ture

Time

900

700

500

300

°C austenite

A3

A1

austenite+ ferrite

ferrite +perliteT

em

pera

ture

C-Content

0,4 0,8 %

transformation and homogenizing

of -solid solution (30-60 min)at 30°C above A

g

3

quick heating

air cooling


Figure 8.9

Hardening

Tem

pera

ture

Time

900

700

500

300

°C austenite

A3

A1

austenite+ ferrite

ferrite +perliteT

em

pera

ture

C-Content

0,4 0,8 %

start of martensiteformation

quenchingin water

about 30°C above A3

start of martensiteformation


Figure 8.10


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.

Hardening and Tempering

Tem

pera

ture

Time

900

700

500

300

°C austenite

A3

A1

austenite+ ferrite

ferrite +perliteT

em

pera

ture

C-Content

0,4 0,8 %

quenching

about 30°C above A3

hardening and tempering

slowcooling


Figure 8.11

Soft Annealing

Te

mp

era

ture

Time

900

700

500

300

°C austenite

A3

A1

austenite+ ferrite

ferrite +perliteT

em

pe

ratu

re

C-Content

0,4 0,8 %

time dependent on workpiece

10 to 20°Cbelow A1

oscillation annealing+ / - 20 degrees around A1

or

cementite


Figure 8.12


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 20°C 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 tempering

temperature.

Stress Relieving

Te

mp

era

ture

Time

900

700

500

300

°C austenite

A3

A1

austenite+ ferrite

Te

mp

era

ture

C-Content0,4 0,8 %

time dependent on workpiece

between450 and650 °C

ferrite +perlite


Figure 8.13

Stress releaving

Heat treatment at a temperature below the lower transition point A1, mostly

between 600 and 650°C, 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 A1), followed by

cooling in tranquil atmosphere.

Hardening (quench

hardening)

Acooling from a temperature above the transition point A3 or A1 with such a

speed that an clear increase of hardness occurs at the surface or across

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 1000°C) 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.

Type and Purpose of Heat Treatment


Figure 8.14


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 Treatment in Connection With Welding

combinationpreheating

simplepreheating

increase ofworking

temperature

constantworking

temperature

localpreheating

preheating of thecomplete workpiece

isothermalwelding

postheating(”post weld heat

treatment”)

heat treatmentof the complete

workpiece

local heattreatment

annealing stressreleaving

stressreleaving

annealing hardening quenchingand

tempering

solutionheat

treatment

tempering

simplestep-hardening

welding

purestep hardening

welding

modifiedstep hardening

welding

Types of heat treatmentsrelated to welding

heattreatment

beforewelding

combi-nation

accompanyingheat treatment

combi-nation

heat treatmentafter welding

(”post-weld heattreatment”)


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

Tem

pera

ture

T

Time t

MS

TA

(1) (2) (3)

tH

(1): Welding without preheating,(2): Welding with preheating up to 380°C, without stoppage time(3): Welding with preheating up to 380°C and about 10 min. stoppage time

T : Stoppage temperature, t : Dwell timeA H


Figure 8.16


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, the

cooling rate can also be

reduced by covering with

heat insulating materials.

Another variant of welding

with preheating is welding

at constant working

temperature. This is

Temperature-Time-DistributionDuring Welding With Preheating

tV tS tA

start endseam

transformationrange

Time tTem

pera

ture

T

TS

A3

A1

TV

T : Preheat temperature,

T : Melting temperature of material,

t : Preheat time,

t : Welding time,

t : Cooling time (room temperature),

M : Martensite start temperature

A : Upper transformation temperature,

A : Lower

V

S

V

S

A

S

3

1 transformation temperature

Course of resultingtemperature in thearea of the heataffected zone ofthe base material.

Temperaturedistribution bypreheating,Course oftemperatureduring welding.


Figure 8.17

Welding With Simple Preheating

A3

A1

Tem

pera

ture

T

Time t

tV tS tA

TA

TV


T : Working temperature,

t : Preheat time,

t : Welding time,

t : Cooling time (room temperature)

V

A

V

S

A

Temperature of workpiece,Temperature of weld point


Figure 8.18


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 temperature

between 600 and 700°C, so that a formation

Welding With Preheating andStoppage at Working Temperature

Te

mp

era

ture

T

Time t

tS

tV tH tA

A3

A1

MS

TV

TA

: t = 0H T : Preheat temperature,


t : Preheat time,

V

A

V

t : Welding time,


t : Dwell time

S

A

H


Figure 8.19

Welding WithPre- and Post-Heating

Te

mp

era

ture

T

Time t

TN

tS

tN tA

A3

A1

A3

A1

Te

mp

era

ture

T

TN

TV TA

Time t

tV tS tNtRtA

2. Pre- and post-heating

1. Post-heating



T : Postheat temperature,

t : Preheating time,

V

A

N

V

t : Welding time,


t : Postheat time

t : Stoppage time

S

A

N

R


Figure 8.20


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 recrystallisation

temperature with the corresponding crystallisa-

Modified Step Weld Hardening

A3

A1

MS

TA

THa

TSt

TAnl

TAnl

tAtAnl

tAb

tHa

tS

tHtAtH

Time t

Tem

pera

ture

T


T : Tempering temperature,

T : Hardening temperature,

A

Anl

Hä

T : Step temperature,

t : Cooling time,

t : Quenching time,

St

A

Ab

t : Tempering time,

t : Dwell time,

t : Welding time

Anl

H

S

Temperature of workpiece,Temperature of weld point


Figure 8.21

Temperature-Time DistributionDuring Multi-Pass Welding


T : Melting temperature of material,

t : Preheat time,

t : Welding time


A : Upper transformation temperature,

M : Martensite start temperature

V

S

V

S

A

3

S

heat affected zone

1

432 } weld pass

observed point

1 2 3 4 weld pass

Te

mp

era

ture

T

A3

MS

TS

TV

Time t

tS

tV tA


Figure 8.22


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 160°C

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

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

8. Technical Heat Treatment - kau.ac.krmercury.kau.ac.kr/welding/Welding Technology II - Welding...8. Technical Heat Treatment 100 Figure 8.11 shows the quenching and tempering procedure.

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