Change in dislocation mobility of tempered martensitic steel through charged hydrogen

Abstract

To clarify the relationship between dislocation slip and hydrogen for elucidation of hydrogen embrittlement mechanism, the effect of hydrogen on dislocation slip was examined by comparing changes in the stress-strain curve and stress relaxation behavior with/without hydrogen charging. Tempered martensitic steel with the tensile stress of 1480 MPa was used. Hydrogen charging was conducted by the electrolysis charging. Tensile tests were conducted for hydrogen-charged specimens containing hydrogen from 0 to 5.6 mass ppm at strain rates from 1.83×10- 5 to 1.83×10- 2 s -1 . In the stress-strain curve, the elastic modulus did not change, but the upper and lower yield points increased through charged hydrogen. The tensile strength increased and the work hardening rate did not change through charged hydrogen. The yield points and tensile strengths increased with increasing the amount of hydrogen. It suggests that hydrogen suppressed dislocation slip, that is, hydrogen caused hardening. Stress relaxation tests were conducted for hydrogen-charged specimens containing hydrogen from 0 to 4.7 mass ppm. Even though the amount of hydrogen increased, the amount of stress relaxation remained constant. Therefore, hydrogen probably did not promote the increase in dislocation density. These results reveal that hydrogen causes not softening but hardening.