Alloy design for UOE linepipe material for standard and non-standard HIC conditions

The last several decades have seen a steady increase in the demand for high-strength linepipe steels. These offer the most economical option to transport large gas volumes at high pressures from remote areas to the market. Since the beginning of the 1980s, Salzgitter Mannesmann has been involved in the development of high strength heavy plates. Since then, these products have steadily been improved, for example in terms of toughness and weldability. As gas resources in increasingly hostile environments are developed, the requirements with regard to mechanical properties gain growing significance. A similar market trend towards higher strength levels is currently evident in the field of HIC-resistant pipe materials. The transport of natural gas, containing hydrogen sulphide, leads potentially to hydrogen uptake in the material which can cause damage via two distinct mechanisms. Hydrogen induced cracking (HIC) takes place without any external stress, while sulphide stress cracking (SSC) occurs in the presence of a critical tensile stress. Even though both mechanisms involve dissolved hydrogen, they are not directly connected. The present paper reviews the restrictions on the alloy design and the production processes that have to be taken into account in order to produce HIC-resistant large-diameter pipe material. Because of these restrictions, the production of HIC-resistant pipes has been largely limited to API Grades up to X65 for standard conditions, according to NACE TM0284. Investigations of the limits of HIC-resistance have been carried out at Salzgitter Mannesmann Forschung, using pipe material beyond the X65 strength level, in order to evaluate the potential of these materials for use under fit-for-purpose1 test conditions, ie. at higher pH-levels and lower H2S partial pressures. The test results have been compared to SSC severity regions according to ISO 15156-2 for the investigated materials. (AU) © CBMM