Effect of Loading Rate and Tempering Temperature on Fracture Toughness of Hydrogen-Charged Low Alloy Steel SCM440

J - R curves under continuous hydrogen charge were obtained to evaluate the effect of absorbed hydrogen on fracture toughness of SCM440 low alloy steel. The effect of absorbed hydrogen, loading rate and tempering temperature were investigated. Five materials which were tempered at different temperature to change Vickers hardness were used. Continuous hydrogen charge, which isolated the crack tip from the electrolyte and kept the crack in dry condition, was conducted during the test. Material hardness and loading rate significantly affected J - R curves and JIC. Low loading rate and high hardness resulted in drastically decreased JIC. The materials which were tempered at 803K showed 90% reduction of JIC compared with that of uncharged specimen. The significant reduction was experienced in materials with Vickers hardness higher than 310. Quasi-cleavage fracture surface was predominantly observed in the tests which resulted in low JIC. It is indicated that the hardness of low alloy steel used as a structural material should be limited below 300 on the viewpoint of fracture toughness. This study showed that low loading test that lasts for eighty hours at least is necessary to assess the JIC of hydrogen charged material.