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

Abstract

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 degrees C for 30 min. The annealed specimens had a dual-phase microstructure of retained austenite (gamma R) and ferrite (alpha) 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 inter granular cracking occurring along the boundaries of prior gamma grains by the H-enhanced decohesion (HEDE) mechanism. The intergranular cracking leaved both flat and rugged facets, which appeared at the prior gamma grain boundaries without and with gamma 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 gamma R grains by the HEDE mechanism. The empty dimples were made by intragranular cracking occurring inside the alpha 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 gamma R grains or into alpha grains.