Microstructure and properties of low manganese API X70 pipeline steel for sour service application

Abstract

The microstructure and properties of a low manganese (0.9-1.0%Mn) X70 pipeline steel for sour service application was investigated. The microstructure of thermomechanically processed X70 pipeline steel primarily consisted of acicular ferrite (AF) together with small fraction of polygonal ferrite(PF), degenerated pearlite (DP) and martensite/austenite (M/A) constituent. The AF predominant microstructure is fine, homogeneous and free of band structure of hard phase. Precipitates were characterized in terms of morphology, size and chemistry. Higher yield strength levels up to 545MPa and tensile strength levels up to 640 MPa were achieved while excellent low-temperature toughness was obtained. The steel also exhibits high hydrogen induced cracking (HIC) resistance. The results indicates that low manganese X70 steel can be an alternative to conventional high manganese X70 steel applied in sour environment.