Page 398 - Failure Analysis Case Studies II

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“rat holes”, and a suspected delay between the completion of welding and subsequent post-weld
heat treatment. Oxidation of the cracks leads to the conclusion that they were most probably present
before post-weld heat treatment was camed out [Z, 31.

4. POSTSCRIPT

In both of the above cases it is evident that there was considerable debate over whether the defects
were original fabrication defects, or whether they occurred in service. These are real, practical issues
which often have contractual as well as technical implications, and for the failure investigator it is
sometimes difficult to be dogmatic either way, as he is rarely in possession of all the facts at the time
the investigation is carried out. If the above cases had been entirely fabrication related then the
question arises as to why they were not detected during the final release inspection or during the
pre-commissioning inspection?
The metallographic evidence which was presented builds a strong case for the defects having
originated at the fabrication stage. The absolute truth of the matter, however, probably lies some-
where between the two extremes, in that the initial defects may not have been easily detectable
during routine inspection but probably propagated during service to a size that only subsequently
became readily detectable.
An interesting corollary to the saga of the external support leg cracking became apparent shortly
after the above investigation was undertaken. A design review of the reactor installation revealed
that radial thermal expansion of the support brackets had been restricted by grouting in the hold-
down bolts in the slotted holes, which would probably have exerted considerable stress on the
existing HAZ cracks, and caused crack propagation. The problem was rectified by removing the
grouting, and installing a stainless steel foot plate under the bracket, resting on a bronze support
plate to reduce friction.
’ In the final analysis, it must be recorded that weld repair of all of the defects was carried out
meticulously under close supervision during August/September 1992, and that numerous subsequent
inspections carried out on the vessel have failed to reveal any re-occurrence of the cracking.

REFERENCES
1. Laneaster, J. F., Merdfurgy of Welding, 1987, Allen & Unwin, pp. 206-209.
2. ASM Metals Hundbmk, Vol. I, 10th edn, “Properties & Selection: Irons, Steels & High-Pdomance Alloys”, 1990,
p. 617.
3. Smithens, Meids Reference Book, Butteworth, 1983, p. 31-13.
4. Coe, F. R., We/dhg Steels Withour Hydrogen Cracking, The Welding Institute, 1973.