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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
4. Heat Treatment and its Function During Welding 33
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
4. Heat Treatment and its Function During Welding 34
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
4. Heat Treatment and its Function During Welding 35
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
4. Heat Treatment and its Function During Welding 36
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
4. Heat Treatment and its Function During Welding 37
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
4. Heat Treatment and its Function During Welding 38
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
4. Heat Treatment and its Function During Welding 39
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
4. Heat Treatment and its Function During Welding 40
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
4. Heat Treatment and its Function During Welding 41
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
4. Heat Treatment and its Function During Welding 42
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
4. Heat Treatment and its Function During Welding 43
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