For the application of carbon and low alloy steels in sour environments the NACE standard MR-01-75 specifies, on an empirical basis, a maximum allowable hardness of 22 HRc (Hv 248). This criterion was essentially developed for OCTG and its introduction has been successful in ensuring a low incidence of SSCC failures. However, this limit may be over-conservative in mildly sour environments where higher hardnesses can be tolerated, or inappropriate where straining conditions could promote soft zone cracking. These two effects are particularly relevant in the case of welds, where local hardnesses can exceed the NACE limit of 22 HRc and soft zones may be subject to plastic strain.

In this paper the influence of local high hardness on the SSCC resistance of API 5L X60 grade pipeline steel in mildly sour environments has been investigated using:

  1. a weld simulator to generate different hardness profiles in tensile samples;

  2. bead on plate welds of different heat inputs to produce different HAZ hardnesses.

The tests on the tensile samples and bead on plate welds were conducted at 7 mbar and 20-25 mbar H2S, respectively. Both sets of tests indicted that only specimens with local hardness values above Hv 340 (at 7 mbar H2S) and Hv 322 (at 20-25 mbar H2S) were found to be susceptible to SSCC.

These results indicate that there may be scope for relaxation in the current NACE hardness limitation for local hard zones associated with welds exposed to mildly sour environments.

Subject
Constant load, Environmental zones, Materials, Modeling, Gauges, Sour environments, Crack initiation, Beads, Stress cracking, Maximum hardness, Steel, Hardness, Stress corrosion cracking
Keywords:
SSCC, sour service, C-Mn steel, hard zone cracking, soft zone cracking, welds
© 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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