Page 388 - Failure Analysis Case Studies II

Failure Analysis Case Studies II
D.R.H. Jones (Editor)
0 200 1 Elsevier Science Ltd. All rights reserved 373

UNUSUAL CASES OF WELD-ASSOCIATED CRACKING
EXPERIENCED IN A HIGH TEMPERATURE CATALYST
REDUCTION REACTOR

M. L. HOLLAND
Metallurgical & Inspection Services, Mossgas, Private Bag X14, Mossel Bay 6500,
Republic of South Africa
(Received 3 February 1998)

Abstract-Two case studies are described which concern instances of weld-associated cracking discovered in
a high temperature Cr-Mo catalyst reduction reactor soon after commissioning. One of the defects was
diagnosed as re-heat cracking at a heavy section nozzle-to-shell weld, which was attributed largely to the high
stress concentration at the toe of the weld in conjunction with tri-axial stress, resulting from the thick section
geometry. Cracking was believed to have initiated during post-weld heat treatment which was only carried out
2 months after completion of welding.
The other defect described is a classic case of HA2 cracking at the external support legs of the reactor, again
attributed largely to the delay in conducting post-weld heat treatment after fabrication.
In situ replication metallography was instrumental in establishing the failure modes in both instances, and
was also able to demonstrate that the HAZ cracks were present before PWHT was carried out. 0 1998 Elsevier
Science Ltd. All rights reserved.
Keywords: Hydrogen-assisted cracking. metahrgical examination, process-plant failures, reheat cracking,
welded fabrications.

1. INTRODUCTION

The following case histories relate to two instances of weld-associated cracking encountered in a
catalyst reduction reactor soon after commissioning. The locations of the cracks and the mechanism
of cracking is different in each case, although there is a common linkage in terms of original
fabrication error.
The failure investigation undertaken is an interesting illustration of the value of in situ met-
allographic replication techniques as a non-destructive means of establishing vital microstructural
daCa needed to confirm the failure mode.
The catalyst reduction reactor operates in a hydrogen-rich environment at a temperature of 385°C
and a pressure of 1770 kPa and is therefore fabricated in ICr-l/2Mo steel. During routine inspection
of the internals of the reactor after approximately one month's operation, a crack was detected in
the hydrogen inlet nozzle C1. A more comprehensive inspection of all weld seams was therefore
undertaken and revealed the existence of numerous cracks in the external support ring of the vessel.
The failure investigations undertaken into the causes of these cracks are summarised in the
following sections.

2. CASE I-EXAMINATION OF CRACKED NOZZLE C1
2.1. Visual examination
The nozzle was a heavy wall set-in forging to SA182 gr F12, having a maximum section thickness
of 133 mm at the shell/nozzle weld where it was welded to SA 387 gr 12 cl 2 plate (see Fig. 1).
Magnetic particle testing revealed that the major crack was on the top of the nozzle and extended
for a distance of 240 mm along the toe of the weld, in the heat affected zone of the forging. The
defect was initially assumed to be a fatigue crack. It was noted that there was a relatively sharp
transition at the toe of the weld joining the nozzle to the shell wall.
Reprinted from Engineering Failure Analysis 5 (2), I7 I - 180 ( 1998)