Page 397 - Failure Analysis Case Studies II

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I,

Fig. 10. Detail of oxide filled crack in martensite area of HAZ, support G1 (magnification x 100).

close to ambient. It is therefore possible to avoid cracking in a hard, i.e. susceptible, microstructure
by maintaining it at a sufficiently high temperature, either until hydrogen has diffused away or until
the microstructure is softened by tempering, to render it less susceptible. This principle is employed
in multipass welding and in post-weld heat treatments.
An increase in temperature increases the rate of diffusion of hydrogen and thus accelerates its
removal from the weld. Any measure which slows down the weld cooling rate is therefore helpful
in reducing the hydrogen level. Preheat, for example, by slowing the cooling rate, not only softens
the microstructure but also helps hydrogen to escape. As a result, higher hardness levels can be
tolerated without cracking than if preheat had not been used.
For welds in those steels with hardenability so high that soft microstructures cannot be produced
at all, and where preheat cannot remove sufficient hydrogen, (such as the Cr-Mo steels) a weld
interpass temperature, or a post-weld heating temperature, high enough to avoid cracking must be
held for a sufficiently long time to allow hydrogen to diffuse away before the weld cools.

3.4. Fabrication considerations
During fabrication of the supports of the catalyst reduction reactor, details of possible hydrogen
sources and measures taken to effectively diffuse out the hydrogen are not known. Factors which
would exacerbate the tendency to heat affected zone cracking, however would be the complex tri-
axial stresses acting on the gusset/shell supporting welds due to the absence of the specified “rat
holes”, and the suspected delay between welding and subsequent post-weld heat treatment as
reported in Section 2.4.l(d). If this delay was also applicable to the fabrication of the support ring
it would have been essential to carry out an intermediate heat treatment to diffuse out hydrogen, as
discussed above.
The oxidation of the cracks as shown in Fig. 10 can only have occurred in the Cr-Mo steel by
exposure to an oxidising environment at a temperature of at least 550°C [2, 31. This is well above
the vessel operating temperature of 385°C and therefore indicates that oxidation of the cracks
most probably occurred during post-weld heat treatment, or possibly during some subsequent
undocumented welding operation.

3.5. Conclusions
It is concluded that the cracking found in the catalyst reduction reactor supports is due to heat
affected zone cracking, otherwise known as “hydrogen-induced cracking”. Significant contributory
factors are believed to be the complex tri-axial stresses imposed due to the absence of the specified