4. heat treatment and its function during weldingdantn/wt/wt2-c4.pdf · heat treatment and its...

4.

Heat Treatment and

its Function During Welding

4. Heat Treatment and its Function During Welding 32

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 4.1.

Figure 4.2 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 4.3. 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 austenite

Figure 4.1

Figure 4.2


grains. This zone of the weld is the area,

where the worst toughness values are found.

In Figure 4.4 you can see how much the for-

mation of the individual structure zones and

the zones of unfavourable mechanical prop-

erties 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

technique, the HAZ is reduced to only 8 mm.

With the use of different procedures, the

differences in the formation of heat affected

zones become even clearer as shown in

Figure 4.5.

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 welding.

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

cifically optimised to achieve special quality, e.g. corrosion resistance against a certain at-

tacking medium, this post-weld heat treatment is of great importance.

Figure 4.6 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-

tures like e.g. coarse-grain

Figure 4.3

Figure 4.4


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. fo r hardening

processes (to be explained later).

As this diagram does not

cover the time influence,

only constant stop-

temperatures can be read,

predictions about heating-up

and cooling-down rates are

not possible. Thus the indi-

vidual heat treatment meth-

ods will be explained by

their temperature-time-

behaviour in the following.

Figure 4.5 Figure 4.6

Figure 4.7


Figure 4.7 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 4.7 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 4.8 shows sche-

matically time-temperature

behaviour in a TTT-

diagram. (Note: the curves

explain running structure

mechanisms, they must not

be used as reading off ex-

amples. To determine t8/5,

hardness values, or micro-

structure distribution, are

TTT-diagrams always read

continuously or isother-

mally. Mixed types like

curves 3 to 6 are not a llowed 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 4.9. 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 4.8, line 1; formation

of ferrite and perlite, normalised micro-structure).

Figure 4.8


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

4.10. 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 4.8). 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 4.8). 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).

Figure 4.9

Figure 4.10


Figure 4.11 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 4.12 shows the procedure of soft-annealing. Here we aim to adjust a soft and suitable

micro-structure for machining. Such a structure is characterised by mostly globular formed

cementite particles, while the lamellar structure of the perlite is resolved (in Figure 4.12

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

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 be-

low 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 be-

cause the surface area is

strongly reduced → ther-

modynamically more fa-

vourable condition).

Hypereutectic steels have

in addition to the lamellar

structure of the perlite a

cementite network on the

grain boundaries.

Hardening and Tempering

Tem

pe

ratu

re

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

br-eI-04-11.cdr

Figure 4.11

Soft Annealing

Tem

pe

ratu

re

Time

900

700

500

300

°C austenite

A3

A1

austenite+ ferrite

ferrite +perliteT

em

pera

ture

C-Content

0,4 0,8 %

time dependent on workpiece

10 to 20°Cbelow A1

oscillation annealing+ / - 20 degrees around A1

or

cementite

br-eI-04-12.cdr

Figure 4.12


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

cementite and the frequent

transformation avoids a

grain coarsening. A soft-

annealed microstructure

represents frequently the

delivery condition of a ma-

terial.

Figure 4.13 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 de-

formations possible be-

cause the yield strength

limit decreases with in-

creasing temperature. A

stress-relieve heat treat-

ment should not cause any

other change to properties,

so that tempering steels

Figure 4.13

Figure 4.14


are heat treated below tempering temperature.

Figure 4.14 shows a survey of heat treatments which are important to welding as well as their

purposes.

Figure 4.15 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 important

section of the diagram is the kind of heat treatment

which accom-panies the welding. The most impor-

tant processes are explained in the following.

Figure 4.16 represents the influence of different

accompanying heat treatments during welding,

given within a TTT-diagram. The fastest cooling is

achieved with welding without preheating, with

addition of a small share of bainite, mainly mart-

ensite is formed (curve 1, analogous to Figure 4.8,

hardening). A simple heating before welding with-

out additional stopping time lowers the cooling rate

according to curve 2. The proportion of martensite

is reduced in the forming structure, as well as the

Figure 4.15

Figure 4.16


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 4.8, curve 4 and 5).

To explain the temperature-time-behaviours

used in the following, Figure 4.17 shows a

superposition 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

removal 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

4.18. During a welding time

tS a drop of the working

temperature TA occurs. A

further 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

Figure 4.17

Figure 4.18


achieved through further

warming during welding to

avoid a drop of the working

temperature. In Figure 4.19

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 4.16 and in this way,

to suppress martensite formation.

Figure 4.20 shows the T-T course during

welding with post-warming (subsequent heat

treatment, see Figure 4.15). 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

Figure 4.19

Figure 4.20


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 4.21. 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 4.22 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-

Figure 4.21

Figure 4.22


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 4.22. 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 4.23 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 re-

lieve annealing. Above this

area, a stress relieve anneal-

ing must be carried out. Be-

low this area, a post-weld

heat treatment is not re-

quired.

Figure 4.23

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