Evaluation of hydrogen-induced cracking behavior in duplex stainless steel by numerical simulation of stress and diffusible hydrogen distribution at the microstructural scale

Abstract

Duplex stainless steels possess ferrite and austenite microstructures, which exhibit different mechanical properties. 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 consider 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 the hydrogen-charged specimen was performed and the cross-section of the specimen was observed following the test. Hydrogen-induced cracks were mainly observed in the ferrite phase. A numerical simulation was performed to determine the contribution of the stress and hydrogen concentration distribution to the initiation of hydrogen-induced cracks. A microstructure-based finite element model consisting of ferrite and austenite phases was designed based on the micrograph of the duplex stainless steel used. The stress–strain curves of the ferrite and austenite phases were used and macroscopic tension was applied to calculate the microscopic stress distribution. The microscopic distribution of hydrogen concentration was calculated by incorporating the stress distribution into the hydrogen diffusion simulation as one of the driving forces. From the simulation results, the stress concentration and hydrogen accumulation occurred at the ferrite phase or at the 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.