Duplex stainless steels are increasingly widely used in the oil and gas production industry for a variety of applications. The stress corrosion cracking (SCC) behaviour of wrought material is reasonably well understood, and limits of use are placed upon these alloys in NACE MR0175, for sour service. However, the SCC behaviour of weldments is less well understood, and this has limited the use of welded material in H2S-containing conditions.

The SCC resistance of duplex stainless steels is influenced by their microstructure as well as their chemical composition and the objective of the research reported in this paper is to investigate the SCC behaviour of welded 22%Cr and 25%Cr alloys in a simulated oilfield environment. Mechanised orbital TIG was used to butt weld 168mm outside diameter tubes. The shielding gas contained nitrogen additions of up to 10% (in the case of UNS S32760) and 7% (in the case of UNS S31803). Slow strain rate testing (SSRT) was conducted on cross-weld specimens in sodium chloride solutions overpressured with varying partial pressures of H2S and CO2. The SSRT results, in terms of ductility parameters and secondary cracking, are correlated with fractography and metallurgical examination of crack morphology in order to establish the effects of the welding process and the nitrogen content of the shielding gas.

It was found that the nitrogen uptake from the shielding gas has a detrimental effect on SCC resistance of duplex stainless steel weldments. Whilst this effect is only modest, it is in direct contrast to the beneficial effect it has on pitting corrosion resistance.

Subject
Test methods, Materials, Parent metals, Welding, Mechanical failure, Nitrogen, Shielding, Corrosion resistance, Duplex stainless steel, Ferrite, Stress corrosion cracking, Ductility, Austenite
Keywords:
duplex stainless steel, welds, nitrogen, stress corrosion cracking, sour
© 1995 Association for Materials Protection and Performance (AMPP). All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means (electronic, mechanical, photocopying, recording, or otherwise) without the prior written permission of AMPP. Positions and opinions advanced in this work are those of the author(s) and not necessarily those of AMPP. Responsibility for the content of the work lies solely with the author(s).
1995
Association for Materials Protection and Performance (AMPP)
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