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There are different methods in industrial use to apply the weld thermal cycle:
–
Gleeble HAZ Simulation,
–
induction heating and cooling in an oil bath and
–
heating in a hot salt bath and cooling in a moderate tempered salt bath.
The main objective of all these techniques is to apply the weld thermal cycle as
measurable in a distinct sub region of the HAZ to a sample with larger testing volume. In
the following these methods are described, /17, 18/.
4.1. GLEEBLE welding simulation
Gleeble simulation is a very useful tool for the determination of the microstructure in
the HAZ. By the controlled application of thermal and/or thermo mechanical cycles
according to particular weldments geometry and the welding parameters, nearly every
position in the weldments can be produced representatively and the microstructure can be
investigated using conventional or sophisticated skilful techniques.
Due to the high electric power available (maximum current is about 6000 A), round
(also rectangular) specimens up to diameter of 16 mm, about 140 mm long, can be used.
In order to determine a specific (creep) property of a particular region in the HAZ the
weld thermal cycle is fed in the control computer and samples is heated up very rapidly
(heating rates up to 10000 K/s) to the desired peak temperature by direct current flow
with up to 6000 A. The cooling is very well controlled; faster cooling rates can be
reached by reducing the free span between the jaws or by external gas cooling. The
resulting microstructure in the middle of the test specimen, where the thermocouple is
located, is then exactly the same as in real weldments at the considered position.
4.2. Induction heating and oil quenching
The HAZ simulation is performed by means of inductive overheating on 180 mm long
round specimens Ø 20 mm. After heating up in a coil (heating time up to 1300°C is 35 s)
and hold at peak temperature for several seconds, the specimens are dropped into an oil
bath. The typical cooling time after 1300°C peak temperature is 10 s for the range from
800 to 500°C (
Δ
t
8/5
). The homogenous structure in the specimens has a length of 70 mm.
4.3. Salt bath heating and quenching
For the simulation technique of this kind a rough machined specimen is heated up by a
hot salt bath corresponding to the desired peak temperature. After the specimen has
reached the peak temperature, it is transferred to another salt bath having a much lower
temperature (about 100°C). In order to get a specimen with uniform microstructure,
different cooling times can be realized by variation of size and temperature of the second
salt bath.
4.4. Comparison of the HAZ simulation techniques
The Gleeble HAZ simulation is most flexible (with the capacity for a larger range of
heat inputs) and controllable method giving the best agreement with the actual weld
thermal cycle. The length of constant microstructure is limited (about 10 mm), which has
consequences with the respect to the gauge length.
The other two methods provide a uniform microstructure along the whole specimen
length, the heating rate is much slower and the applicable cooling time is quite restricted.
They are mainly used for larger testing samples and series or for first screening tests.