Challenges in Avoiding Sigma Phase Embrittlement of Duplex Stainless Steel in Assets

Abstract

The use of duplex stainless steel has been increasingly common in the oil and gas industry since the material offers a unique combination of high strength and superior corrosion resistance. However, the material is not immune to failure. A process safety incident at Sarawak Shell Bhd. highlighted this fact, whereby a duplex stainless steel gas flowline flange had fractured and parted after around 11 hours upon being installed and put in service. Independent failure investigations revealed that the failure was due to the formation of harmful sigma intermetallic phase in the base material of the flowline flange, a phenomenon known as sigma phase embrittlement. This can occur due to incorrect heat treatment processes during the manufacturing stage of duplex stainless steel components or due to incorrect welding processes. For the specific example reported in this paper, it was concluded that the flange was subject to incorrect heat treatment during manufacturing which exposed it to slow cooling through the sigma phase formation temperature range. This resulted in a significant loss of toughness and ductility of the flange material, which caused it to fracture and part shortly after being placed in service. Actions were subsequently taken to identify the risks of other duplex stainless steel piping components in Sarawak Shell Bhd. having the similar in correct heat treatment and thus could likely be subject to the same failure mechanism. This paper presents how a systematic risk-based approach was implemented to verify existing installed piping components in terms of their risks to intermetallic phase embrittlement. Actions taken to prevent similar occurrences in the future are also detailed. Ferrite count verifications using portable ferrite scope instruments were found to provide a useful screening method for verifying the likelihood of the presence of sigma phase in duplex stainless steel material. During procurement of duplex stainless steel components in new projects, inspection Hold Points for ferrite scope inspections were specified and implemented in the Inspection and Test Plans. Additionally, 100% ferrite scope inspections were imposed on all duplex stainless steel piping components received at Sarawak Shell Bhd.'s supply base warehouse. The procurement process for small quantities of duplex stainless steel components was revised to ensure that proper review of the technical specifications is carried out prior to placing purchase orders especially with stockists.