The mechanism of hydrogen embrittlement in intercritically annealed medium Mn TRIP steel

The objective of this study was to investigate the mechanisms of hydrogen embrittlement (HE) in intercritically annealed medium Mn steel. For this purpose, both hot-rolled and cold-rolled Fe-7Mn-0.1C-0.5Si (wt.%) steels were annealed at 640 °C for 30 min. The annealed specimens had a dual-phase microstructure of retained austenite (γ_R) and ferrite (α) with different morphologies; a lath shape for the hot-rolled and annealed (HRA) specimen and a globular shape for the cold-rolled and annealed (CRA) specimen. Although the difference in microstructural morphology did not influence the H permeation, it significantly affected the HE behavior. The H-charged HRA (HRA_H) specimen was fractured by intergranular cracking occurring along the boundaries of prior γ grains by the H-enhanced decohesion (HEDE) mechanism. The intergranular cracking leaved both flat and rugged facets, which appeared at the prior γ grain boundaries without and with γ_R, respectively. The H-charged CRA (CRA_H) specimen was fractured to leave both dimples filled with grains and empty dimples at the fractured surface. The dimples filled with grains were generated by intergranular cracking occurring along the boundaries of γ_R grains by the HEDE mechanism. The empty dimples were made by intragranular cracking occurring inside the α grains by the H-enhanced local plasticity (HELP) mechanism. The CRA_H specimen exhibited a smaller elongation loss than the HRA_H specimen because cracks were propagated by frequently changing their direction along the boundaries of nano-sized γ_R grains or into α grains.