Svetsaren 1/2007 - Esab

Figure 3. Martensite (light etching) in parent material/
weld overlay interface region of an as-welded SAW
overlay (courtesy TWI).
Figure 4. Interface region of SAW overlay weld after
PWHT with tempered martensite and carbide
precipitation (dark etching) [1].
Mechanisms
The exact mechanism of disbonding is under
discussion, but it is thought to occur, essentially,
as a result of hydrogen embrittlement. During
operation, atomic hydrogen diffuses into the
reactor wall and the hydrogen concentration can
build up to levels around 4 – 7 ppm in thick wall
reactors. Following shutdown, hydrogen tends to
accumulate at the transition zone between the
ferritic parent material and the austenitic weld
overlay. This occurs because hydrogen is about
one order of magnitude more soluble in the
austenitic weld overlay than in the base metal, but
its diffusivity in the overlay is much slower than in
the base material. Therefore, as the hydrogen
diffuses from the base metal it tends to
accumulate at the weld overlay interface [1 - 3].
As the risk of disbonding is connected to the
hydrogen concentration in the interface region,
the disbonding tendency increases with increased
hydrogen partial pressures and operating
temperatures, as well as faster cooling during
shutdown. Being essentially a result of hydrogen
embrittlement, it is also affected by the interface
region microstructure and, thereby, also dependent
on the applied PWHT [2]. Several studies
[2, 4] have shown, that the highest hardness is
measured in the weld metal in the interface region
near the parent metal, with maximum values of
>450 HV sometimes found after a single PWHT.
Disbonding test methods
Several test methodologies exist for evaluating the
susceptibility to hydrogen disbonding:
Autoclave testing
The most common method is by exposure of the
test coupon in an autoclave at high temperature
and high hydrogen pressure. Typical exposure
conditions are [5, 6]:
cooled to ambient temperature at a controlled rate.
A cooling rate of 150°C/h is commonly used for
qualification testing. The specimens are then held
at room temperature for a designated period to
allow for development of cracking between the
stainless overlay and the steel. Following the hold
period, the specimens are evaluated for
disbonding by ultrasonic methods often combined
with metallographic examinations. The size and
distribution of the disbonded region(s) are then
characterised (Figs. 7 and 8) e.g. according to
Table 3.
Table 3. Ranking of ultrasonic test results according to
ASTM G 146-01 [5].
Area ranking Area disbonded (%)
A 5
B 5 x 10
C 10 x 30
D 30 x 50
E 50
Distribution ranking
Distribution
1 isolated disbonded regions
2 interlinking disbonded regions
3 disbonding at weld pass overlaps
4 disbonding at joint with side overlay
5 other (to be described)
Various specimen geometries are used but recent
configurations utilise a round geometry, overlayed
on the top and side surfaces. The intention is to
better simulate in-service behaviour with respect
to hydrogen diffusion during cool down.
The cylindrical test specimen, according to
• Temperature: 300 – 500°C, usually 425°C
• Hydrogen partial pressure: 14 – 20 MPa
• Exposure time: 48±1 h
• Cooling rate: 150°C/h
• Holding time at 24±2,5 °C: 7 days
Figure 5. A comparatively narrow dark etching band of
tempered martensite and carbides seen in an electroslag
welded overlay after PWHT.
The test temperature and hydrogen pressures are
chosen with reference to the actual service
conditions. Following exposure, the specimens are
Figure 6. Schaeffler-diagram with line illustrating all
possible compositions for different mixtures between
parent material and welding strip.
12 - Svetsaren no. 1 - 2007