We investigated the effect of hydrogen on the tensile properties of a quench tempered low alloy steel, SCM435, with the tensile strength of 930 MPa used for hydrogen storage cylinders. Tensile specimens were machined from a cylinder with the inside and outside diameters of 245 and 315 mm. The specimens were immersed in a 20 mass aqueous solution of ammonium thiocyanate (NH 4 SCN) at 313 K for 48 hours and then charged with hydrogen. Tensile tests were performed in the air at room temperature. The cross head speed was ranged from 0.01 to 100 mm/min. Hydrogen charged specimens were hold in the air for a period of 1 and 300 hours. The 0.2 proof stress and tensile strength for the hydrogen charged specimens were similar to those for the uncharged specimens, whereas the reduction of area was lower in the hydrogen charged specimens than in the uncharged specimens. Thermal desorption spectroscopy showed that the residual hydrogen contents in the hydrogen charged specimens fractured by tensile tests were between 0.14 and 0.93 mass ppm. The reduction of area of the hydrogen charged specimens decreased linearly with increasing residual hydrogen content. Scanning electron microscopy showed that the cup corn fracture occurred in the hydrogen charged and the uncharged specimens and that the fracture surfaces were covered with dimples. The normal stress fracture area in the center of the hydrogen charged and uncharged specimens was almost the same. The shear stress fracture area near the specimen surface was wider in the hydrogen charged specimens than in the uncharged specimens. This means that hydrogen enhances slip deformation near the specimen surface and resulted in the lower reduction of area in the hydrogen charged specimen. We therefore concluded that the hydrogen embrittlement behavior of the 900 MPa class SCM435 steel was explained by the hydrogen enhanced localized plasticity model rather than by the lattice decohesion model.
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