Material Design for Grade X65 UOE Sour Linepipe Steels with SSC Resistant Property

摘要

Linepipe steels for sour service need to have a sufficient resistance tosulfide stress cracking (SSC) under sour environment. Thermo-Mechanical Controlled Processing (TMCP) has been applied as a keyprocess for producing high strength steel plate with fine bainiticmicrostructure which also exhibits superior toughness. Application ofsour linepipes has been expanded toward several severe sourenvironment regions. Hence, the requirements for four point bending(4PB) SSC test has been becoming stringent in order for ensuring thesafety of pipeline. Therefore, precise and strict material design shouldbe needed in the production of Grade X65 UOE sour linepipe steels forpreventing SSC. This paper firstly summarizes the material designconcepts for controlling SSC resistant property. For ensuring SSCresistant property, surface hardness control near the inner surface areaof pipe is required. The effect of TMCP conditions on surface hardnessand its microstructure were investigated in this study. The advancecontrolled cooling process (Super-OLAC®-A) was applied to GradeX65 UOE sour linepipe steels in order to achieve homogeneousmaterial properties and lower surface hardness in the plate. In X65 sourlinepipe steels, surface microstructure is usually bainite microstructure.Bainite microstructure is categorized into two types including lathbainite (LB) and granular bainite (GB), depending on thetransformation temperature. From actual pipe investigations, the innersurface hardness of pipe increased with increasing the volume fractionof LB microstructure instead of GB microstructure. In addition, in orderto clarify whether SSC occurs or not, 4PB SSC tests were conductedunder several H2S partial pressure conditions. It was revealed that SSCoccurred when the inner surface hardness of pipe increased withincreasing the volume fraction of LB microstructure under 1 bar orhigher H2S partial pressure condition. In the case of 1 bar or higher H2Spartial pressure condition, lowering the surface hardness by suppressingLB microstructure lead to good SSC resistant property.