Page 292 - Failure Analysis Case Studies II

277

FAILURE OF AUSTENITIC STAINLESS STEEL
COMPONENTS USED IN NITROGEN OXIDE PLANT

V. M. J. SHARMA, A. K. JHA, P. RAMESH NARAYANAN,
S. ARUMUGHAM and T. S. LAKSHMANAN*
Material Characterisation Division, Materials and Metallurgy Group, Vikram Sarabhai Space Centre,
Thiruvananthapurarn, 695 022, India
(Received 2 1 March 1997)

Abstract-Austenitic stainless steel components of a nitrogen oxide plant have been found to leak in service.
The failed components, namely pipe-to-pipe joints and pipe-to-flange joints, have been studied through
standard metallographic techniques to analyse the cause of the failure. In case study I, involving the failure of
pipe-to-flange joints, cracking was observed in the pipe wall next to the pipe-to-flange weld. In case study 11,
involving the failure of a sight port flange, cracking was observed in the flange adjacent to the pipe-to-flange
weld. In both cases, cracking was by an intergranular mechanism, and carbon contents were much higher than
permitted for "L" grades of austenitic stainless steel. Q 1997 Elsevier Science Ltd
Keywords: Metallography, residual stress, fractography

1. INTRODUCTION

Liquid propellants, with their higher specific impulse than solid propellants, have emerged as efficient
fuels for satellite hunch vehicles. For combustion during flight, various types of oxidizers have been
used. One such oxidizer is dinitrogen tetroxide (N,O,). In view of the nonusability of dinitrogen
tetroxide with some of the storage materials, mixed oxide of nitrogen (MON) has emerged as a
better choice. Its lower freezing point (- 14°C) makes this one of the best oxidizers. MON mainly
consists of a mixture of N204, NO and NO,. Further classification of MON has been done as MON-
3 (3% NO) and MON-10 (10% NO), depending on the content of NO. The plant, which produces
mixed oxides of nitrogen (MON-3), consists of reactors, absorption columns, collection tanks,
storage tanks, interconnecting piping, flanges and valves. One such plant, commissioned 7 years
back, is in operation. However, for the last 2 years, leaks were observed in some of the components,
mainly in pipe-to-pipe joints and pipe-to-flange joints. While attempting to rectify such a leak by
tightening the flange, the flange broke into pieces. Various components, which have cracked and
leaked at different locations of the process plant, have been grouped into two categories. In the first,
the failure is in the pipe portion, and, in the second, the failure is in flange portion. As such, two
failed components, (i) a pipe-to-flange joint and (ii) a sight port flange, were selected for detailed
studies.

2. CASE STUDY I: FAILURE OF PIPE-TO-FLANGE JOINT
The joint consists of a tube (25mm ID and 32mm OD) made of AIS1 304 L grade stainless steel
welded to a flange made of the same steel (Fig. 1) The joint is used to transfer HNO, from a storage
tank to an NO reactor. A leak was noticed in the pipe portion of the joint.

*Author to whom correspondence should be addressed.
Reprinted from Engineering Fuihre Analysis 4 (3), 17 1-1 78 (1 997)