Multiscale Investigations on the Mechanical Behaviour and Hydrogen Embrittlement of Advanced Medium Manganese Steel

Abstract

The current studied medium Mn steel has demonstrated remarkable mechanical performance among other 3rd-Gen AHSS, with a yield strength of about 1140 MPa, a tensile strength of around 1600 MPa, and a uniform elongation of ~ 30%. These extraordinary mechanical properties could be attributed to the high dislocation density resulting from warm rolling and quenching processes, while the large fraction of elongated retained austenite with a wide range of stability that will be gradually transformed into martensite during deformation can provide a sustained work-hardening effect to enable the excellent ductility. The stress-strain curves indicated the pronounced plastic instability featured by localised deformation that could be reflected by the Lüder band and Portevin-Le Chatelier (PLC) bands. It was found that the discontinuous yielding behaviours were significantly affected by the applied strain rates, in which more prominent stress serration events were observed with decreasing strain rates. In addition, extensive experimental studies were conducted on the effects of strain rates on the discontinuous yielding behaviours using a series of in-situ and ex-situ macroscale and microscale characterisation methods. The experimental results indicated that loading conditions in terms of strain rate and deformation temperature essentially influence the formation of shear bands during deformation, resulting in different kinetics of martensitic transformation and therefore affecting the mechanical properties remarkably.

The current studied medium Mn steel showed pronounced premature failure after hydrogen ingression, featuring a hydrogen embrittlement index (HEI) of 37%. Microstructural studies suggested that the deformation mechanism was not affected by the hydrogen, whereas the fracture mechanism was heavily altered by the release of mobile hydrogen atoms resulting from phase transformation.