Page 14 - SSAB Welding Handbook Edition 2

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3.0 The thermal cycle of a welding performance Welding handbook

As an example, the thermal cycles during a welding There is no need to calculate the t value but it
performance, at different locations in a joint, are facilitates a good understanding of the thermal
8/5
©SSAB
illustrated in fig. 3.2.
treatment in the joint due to the welding process.
Another name for t is t and the definition is the
8/5 8/5
same for both symbols.
Temperature
T
1 When the t value is determined, it is most often
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calculated but it can also be measured. However,
studies have shown that measuring t can be difficult
T 8/5
2 due to sources of errors in the measurement techniques.
SSAB recommends that the t value is only calculated.
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T
3 Evaluations of the t value show that it is mostly
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T constant throughout different parts of a joint as long
0 x
as the peak temperature in the joint reaches above
Initial plate temperature
900°C during the welding process.
Welding The mechanical properties of toughness, static strength
direction
and hardness in the HAZ are affected by the thermal
x
cycle of the welding process. In many cases, when
Position welding Strenx grades, the impact toughness in the
of arc
T
1 HAZ is an important parameter to consider. The
T
2 minimum impact toughness value is normally reached
T
3
in the part of the HAZ located closest to the weld
Fig. 3.2: The heating / cooling rate and the peak temperature metal where the peak temperature has reached levels
decrease with increased distance to the arc.
between ~1500°C down to ~1100°C. This zone is
called the coarse grained HAZ (CGHAZ).
3.3 Description of the thermal cycle If the t value for a certain welding situation is not
8/5
calculated, appropriate t values are automatically
The thermal cycle of welding can be defined by the 8/5
cooling time in the HAZ between 800°C and 500°C. attained if all other welding recommendations are
This parameter is called the t value, and it is followed for the Strenx grades. Recommended t
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8/5 values for these steel grades are presented in chapter 5.
illustrated in fig. 3.3:
For the Hardox grades the recommended t
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values are not as emphasized as for Strenx. The
Temperature [°C]
reason is that the mechanical properties in the
joints are typically not as high as for Strenx.
However recommended intervals t values exist.
1400 8/5
These intervals depends both of the Hardox grade
and its plate thickness range. The exact values can
800 be found in the software WeldCalc developed by
SSAB.
500
t
8/5
The following example illustrates the effect on the
Time [s]
thermal cycle when welding a joint with different
Fig. 3.3: Schematic diagram for a thermal heat treatment in the part parameters. In this case, a single-V butt weld with a
of the HAZ situated closest to the weld metal. plate thickness of 5 mm is welded using two different
heat inputs, according to fig. 3.4.
14 This handbook contains general suggestions and information without any expressed or implied warranty of any kind. SSAB hereby expressly disclaims
all liability of any kind, including any damages, in connection with the use of the information and for their suitability for individual applications.
It is the responsibility of the user of this brochure to adapt the recommendations contained therein to the requirements of individual applications.