Effect of microstructure on charpy impact properties and LBB assessment of SCM435 steel for storage cylinders of 35MPa hydrogen stations

Abstract

Influence of hydrogen on tensile and Charpy impact properties of quench-tempered low-alloy steels, SCM435s for four types of storage cylinders in 35MPa hydrogen stations, was investigated. After tensile and Charpy impact specimens were exposed to hydrogen gas at 100MPa and 85°C, tensile tests were conducted at room temperature in air under a cross-head speed of 1mm/min, and Charpy impact tests were conducted at temperatures from -100 and 100°C in air. Residual hydrogen contents of these specimens were also measured by TDA (Thermal Desorption Analysis). The tensile strength of hydrogen-exposed specimens, whose residual hydrogen contents ranged from 0.29 to 0.44 mass ppm, were nearly equal to that of unexposed specimens, while the reduction of area of hydrogen-exposed specimens were about 10% lower than that of unexposed specimens. The Charpy impact properties of hydrogen-exposed specimens were consistent with those of unexposed specimens. Microstructures of these materials were observed by EBSD (Electron Back Scatter Diffraction). EBSD analysis clarified that the fracture appearance transition temperature became lower, when the block grain size of martensite became smaller. The upper-shelf absorbed energy increased with decreasing the block grain size. Fracture thoughness values of the materials were estimated from the upper-shelf absorbed energy ; then, evaluation of LBB (Leak-before-break) of 35MPa hydrogen storage cylinders was conducted. © 2010 The Society of Materials Science, Japan.