900 MPa 級低合金鋼 SCM435 の引張特性に及ぼす水素の影響

Abstract

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 (NH4SCN) 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.