Influence of PWHT on the sulfide stress cracking susceptibility of 9%Ni low carbon steel

Abstract

The 9Ni steel low carbon steel is usually applied to pipes, pressure vessels and forged parts working at cryogenic temperatures, due to its high toughness at −196 °C and mechanical strength. Recently, this material was selected to facilities for storage and re-injection of CO2 gas in modern off shore oil and gas platforms. In this new application the steel may be subjected to high pressures in as environment with condensed water, CO2 and traces of H2S. The effect of such a harsh environment on the simulated heat affected zone (HAZ) was investigated by slow strain rate tensile (SSRT) tests. Two post weld heat treatments (PWHT) were reproduced, single tempering (ST) at 575 °C and double tempering (DT) at 670 °C (2 h) and 600 °C (2 h). Specimens without PWHT were more susceptible to sulfide stress cracking (SSC), while the single tempering at 575 °C decreased both the hardness and the SSC susceptibility. Double tempering also increased the resistance to SSC, but was not more effective than the single tempering. As general conclusion, PWHT is strongly recommended to 9Ni low carbon steels subjected to H2S containing environments.