2020 年 106 巻 4 号 p. 214-223
Duplex stainless steels have ferrite and austenite microstructures, which have material properties that are different. The strength level and hydrogen diffusion constant of the phases are different; therefore, it is expected that the microscopic stress and hydrogen concentration distribution are inhomogeneous. Assuming that hydrogen-induced cracking occurs at locally stress-concentrated and hydrogen accumulated locations, it is important to take into consideration the influence of the microstructure in the evaluation of hydrogen-induced cracking. In order to observe crack locations at the microstructural scale, a slow strain rate test of hydrogen-charged specimen was performed and the cross section of the specimen was observed after the test. Hydrogen-induced cracks were mainly found in the ferrite phase. In order to clarify the contribution of the stress and hydrogen concentration distribution to the initiation of hydrogen-induced cracks, a numerical simulation was performed. A microstructural-based finite element model consisting of ferrite and austenite phases was designed based on the micrograph of the duplex stainless steel used. Stress–strain curves of the ferrite and austenite phase were set and macroscopic tension was applied to calculate the microscopic stress distribution. Incorporating the stress distribution into hydrogen diffusion simulation as one of driving forces, the microscopic distribution of hydrogen concentration was calculated. From the simulation results, stress concentration and hydrogen accumulation occurred at ferrite phase or ferrite/austenite boundary. This tendency corresponds closely to the experimentally observed results; therefore, the above approach can be applied to the evaluation of hydrogen-induced cracking at the microstructural scale.
