Investigation of Effect of Plastic Straining on C-Mn Steel Pipelines Prior to Sour Service

Sulphide stress cracking (SSC) of carbon manganese (C-Mn) steel is critically dependent upon the interaction of environment, microstructure, stress intensity and hydrogen concentration developed within the steel. Many years of industrial experience and laboratory studies have defined safe conditions for such steels, including weldments, via restriction on hardness being no greater than 250HV under elastic design conditions e.g. ISO15156/MR0175. It has been suggested that plastically straining a material prior to exposure to the sour environment may invalidate this, even where the total strain is within the ISO15156/MR0175 limits.

This study reports results from a Joint Industry Programme run by TWI, which examined the response of four welded typical pipeline C-Mn steels, (seamless, ERW and UOE product forms) to laboratory-based SSC testing after plastic strain. The strain regimes applied were designed to be representative of those experienced by pipelines during offshore laying procedures, including cyclic strains due to reeling, or due to cold field bends etc. Whilst there was scatter in the results, SSC cracking was found in specimens that fulfilled the ISO15156 / NACE MR0175 hardness requirements and hence DNV-OS-F101.  However, it is conjectured that these small-scale test results were over-conservative vs. specimens strained in full scale simulation, and work is continuing to clarify this for C-Mn steels.