Page 80 - Failure Analysis Case Studies II

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Fig. 3. Section through the cracking whose discovery prompted this investigation. It is in the 8 in. line on the
X side of the tee. The cracking extended from the bore to a spot 90% through-wall. The cracking ran through
the inner part of the HAZ. (Nital etch, bright field.)

grain boundary cavitation suggested that, if the crack had reached the outer surface, it would have
done so along the outer parts of the HAZ, Le. as type IV cracking.
The cracking did not consist of a single crack from the root to near the crown. There were many
short overlapping cracks, in a band in the inner HAZ, and these had joined together. The widest
cracks were in the mid-wall region. For this reason, it was believed that the cracking had probably
initiated there, and had grown inwards to reach the pipe bore, and outwards towards the pipe’s OD
surface. The larger crack segments had faces coated with about 60 pm of oxide.
This crack site was one of those where the MT examination had found no indications whatever
in the years from 1987 to 1994. The replica metallographic examination had found the creep damage
to be more advanced here than at any other site sampled along the steam line, but, even here, on
the outside surface, at its maximum, the damage was only to the stage of aligned voids being present
(Fig. 4). These external examinations had not indicated the extent of the damage that lay underneath.

3. EXAMINATIONS AT OTHER SITES
When this major cracking was recognized, several other steam line welds and their HAZs were
examined. These examinations used ultrasonics and in situ metallography, and two further boat
samples were taken. The examination of one of these, removed at an ultrasonic reflector, revealed
subsurface cracking in the pipe side HAZ of the weld joining the Y branch to its steam chest. This
cracking was not as severe as it was at the X-side reducer. However, like that cracking, it was always
seen to be in the HAZ close to, but never on, the fusion line (Fig. 5). Some minor lack-of-fusion
and slag entrapment flaws were also found. These weld flaws had often been extended by grain
boundary cavitation, and by microcracks a few grain boundaries in length. They were sometimes
adjacent to the creep cracking in the HAZ. However, no interaction was seen between the cracking
or creep damage in the HAZ, and the creep damage associated with the weld flaws.
Creep damage in the 8 in. line, was recognized, to some degree, at every site examined. This creep
damage was either seen directly by metallographic examination or inferred by finding ultrasonic
reflectors in the appropriate HAZ sites. In contrast, very little damage was found in the 12 in. line.
One reflector located in the weld metal in the 12in. line was interpreted as coming from a creep
crack, and a boat sample was taken there. However, when the boat sample was sectioned, only
relatively small weld defects were found. At another spot in the 12 in. line, there was a 2.5mm long
intergranular crack in the weld metal. There were cavities at the tip of this crack, so that it was