Effect of microstructure on hydrogen embrittlement sensitivity and failure mechanism of X52 pipeline steel

Hydrogen embrittlement (HE) susceptibility is intrinsically linked to the material's microstructural characteristics. This study systematically investigated API X52 pipeline steel through in-situ tensile testing under 6 MPa hydrogen pressure, complemented by microstructural characterization, fractographic analysis, and hydrogen microprint test (HMT). A comparative assessment of hydrogen embrittlement resistance was conducted between base metal and welded joints, particularly on microstructural influences on failure mechanisms. It was found that under a 6 MPa hydrogen environment, the hydrogen embrittlement sensitivity of the API X52 base metal selected in this article is higher than that of the welded joint. A gradient microstructural distribution was observed along the direction of the pipe wall thickness. The outer wall region predominantly consists of polygonal ferrite with a minor fraction of granular bainite. A progressive increase in bainitic phase fraction was detected toward the inner wall, ultimately transitioning to bainite structures. Secondary cracks appear on the tensile fracture surface of the base metal under a hydrogen pressure of 6 MPa. The cracks mainly originate from granular carbides and propagate along the grain boundaries. HMT analysis revealed preferential hydrogen segregation at the carbide or grain boundary. The precipitated carbides at grain boundaries play a significant role in hydrogen trapping, and the hydrogen accumulation at the interface between precipitated carbides and the matrix is the main reason for transgranular fracture. The tensile fracture of the welded joints in nitrogen and 6 MPa hydrogen environments was found in the softened zone of the heat-affected zone. The failure mode of the welded joint was mainly dominated by strength and had no hydrogen-induced failure characteristics.