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identifiable number of sites. If the cracking is dependent only upon material composition, service
and microstructure, that cracking can be general throughout those parts of a system utilizing
common materials, design and methods of fabrication. In the present case, the evidence was that
weld flaws were not involved in the initiation or growth of the cracks. The observation that weld
defects tend not to be involved in creep cracking in high-temperature piping has been made
elsewhere, based on a much larger database [6].

5.5. Signijicance with respect to life extension inspections
The incident emphasizes the need for a volumetric inspection of weld zones of high-temperature
pipework as part of either periodic inspection or life extension studies. Creep cracking can be
initiated at any position within a weld HAZ, or within the weld metal itself [6-9]. Hence, inspections
of piping systems, for which there is not an established history of cracking, must take into con-
sideration the possibility of cracks growing from any position through the pipe wall. It also needs
to be emphasized that the volumetric inspection needs to be accompanied by a metallographic
examination, probably by replication, of the pipe’s outer surface.
The probable location of final failure can be predicted with a reasonable degree of reliability for
some particular combinations of materials, fabrication procedures, and design. Thus, in main steam
lines longitudinally welded using the submerged arc process, the critical zone appears to be the weld
metal itself [8], and the greatest attention has to be paid to flaws found within the weld deposit. For
butt welds, in most cases [6, 81, it seems to be the outer part of the HAZ, Le. the type IV region,
that seems to be where creep cracks are most likely to be found. In the present case, based on the
experience in many other boilers, the outer part of the HAZ would also have been expected to have
been the most critical area. The experience at the Grand Lake station shows, however, that, until
cracks have been found for some specific combination of circumstances, or even in some particular
boiler, all weld zones must be considered candidates for creep cracking.

6. CONCLUSIONS

(1) The main steam piping at this generating station had cracked 90% through-wall with only the
outer 10% of the pipe wall remaining uncracked.
(2) The cracking had grown in the inner part of a weld HAZ (type 111 cracking).
(3) Fluorescent magnetic particle inspection on the OD surface at the site of the 90% through-wall
crack did not reveal any indications.
(4) Replica metallographic examination at the same OD surface site revealed creep rupture cavities,
but did not reveal cracking even to the stage of a single microcrack one grain boundary long.
(5) The cracking was discovered by using an ultrasonic examination.
(6) The cracking had started in coarse-grained weld metal close to the fusion line, and within the
body of the pipe.
(7) The piping which cracked was overstressed with respect to current allowable stresses for P11
material under ASME rules.

Acknowledgements-The authors would like to thank New Brunswick Power for their permission to publish this article.
They would also like to thank Chris Steeves and Maribeth MacNutt for specimen preparation, and John Capar for ultrasonic
examinations.

REFERENCES

1. Schiller, H. J.., Hagn, L., Woitscheck, A,, Der Maschinen Schaden, 1974,47, 1-13.
2. Autopipe (Pipe Stress Analysis and Design Program) Version 4.60.03, Rebis, Walnut Creek, CA, 1995.
3. American Society of Mechanical Engineering, B31.1 1993 edition.
4. Price, A. T. and Williams, J. A., in Recenz Advances in Creep and Fracture of Engineering Materials and Structures, ed. B.
Wilshire and D. R. J. Owen. Pineridge Press, Swansea, 1982, pp. 265-353.
5. Alberry, J. T. and Jones, W. K. C., Metals Technology, 1977, 14,45-51.