Page 78 - Failure Analysis Case Studies II

Failure Analysis Case Studies II
D.R.H. Jones (Editor)
0 2001 Elsevier Science Ltd. All rights reserved 63

TYPE I11 CREEP CRACKING AT MAIN STEAM LINE
WELDS

K. G. SEDMAN and J. C. THORNLEY
RPC, 921 College Hill Road, Fredericton, NB, Canada E3B 629
and

R. M. GRIFFIN
NB Power, 515 King Street, Fredericton, NB, Canada E3B 4x4
(Receiwd 21 January 1997)

Abstract-After 204,000 h of operation, some welds in the main steam line of a thermal generating station
were examined ultrasonically. This examination found a major reflector close to a butt weld in the line. That
reflector proved to be cracking that was 90% through-wall. The cracking was not visible on the outside surface
of the pipe. The cracking was type 111 creep cracking, and had initiated in the mid-wall region within bands
of coarse-grained material. From that mid-wall position, cracking had propagated outwards towards the OD
surface, and inwards to reach the ID surface. There was no evidence of the involvement of any weld flaws in
the initiation or the growth of the cracking. Stress analysis of the piping system, using commercially available
software, indicated that the stress in the major area of cracking exceeded that allowed by the ASME B31.1
design code. 0 1997 Elsevier Science Ltd.

I. INTRODUCTION
The #8 unit of the Grand Lake generating station of New Brunswick Power has an output of
60 MW. The boiler is coal-fired, and its design pressure and temperature are 1475 psi (10.17 MPa)
and 1005 OF (541 "C). During normal operation, the boiler produces steam at a pressure of 145Opsi
(10.00MPa) and 1000°F (538 "C). The path of the main steam line is shown in Fig. 1. The steam
leaves the superheater outlet header in a single 12 in. (300 mm) diameter line, After making several
turns, this 12in. line enters a forged tee (Fig. 1). Here, the steam flow branches into two 8in. (200
mm) lines. A sketch of the tee is shown in Fig. 2. The branches from the tee are labeled X and Y.
The tee achieves the diameter reduction, not within the forged body of the tee itself, but via a pair
of eccentric reducers, one welded to each side of the tee. All of the materials are of the 1.25Cr-
0.5Mo type, conforming to ASME standards appropriate to piping, forgings or castings. In the fall
of 1994, cracking was discovered in the unit's main steam line. At this time, the unit had accumulated
204,000 h of operation.

2. INITIAL DISCOVERY OF CRACKING

The welds along this steam line had been previously inspected in some detail in 1987, 1990 and
199 1. These inspections had used fluorescent magnetic particle examination (MT), and met-
allographic (replica) examinations. Only shallow cracking had been found using these two inspection
methods, and, at many sites, the most that was found was a few creep cavities at grain boundaries
in the weld heat-affected zones (HAZs).
In 1994, the decision was made to augment the inspection with ultrasonics. A large reflector was
recognized almost immediately in the X-side 8 in. pipe to reducer weld. The inspector responsible
placed the reflector in the neighbourhood of the HAZ on the pipe side of the weld, and believed
that, at its extremities, the reflector might consist of a number of smaller reflectors. A substantial
boat sample was removed to help determine the nature of the reflector. The boat sample was centred
Reprinted from Engineering Failure Analysis 4 (2), 89-98 (1 997)