Hydrogen Embrittlement in TRIP and TWIP Steels

DoctorThe presence of hydrogen in steels often leads to embrittlement which is sensitive to the strength level of steel. High strength steels developed recently for automotive applications, such as the TRIP and TWIP steels, also suffer from such embrittlement even though they contain austenite which is relatively immune to brittle behaviour.One of the important factors for the embrittlement in TRIP steels is the mechanical stability of retained austenite. The stability is attributed usually to chemical composition, size and shape of austenite. The work presented here demonstrated that another factor, the partitioning of strain between phases with different mechanical properties, can dramatically influence the stability.The trapping of hydrogen at a variety of sites in TRIP steels with the different austenite mechanical stabilities has been characterised using thermal desorption spectroscopy, and the results have been modelled using diffusion theory to predict the desorption quantitatively as a function of phase transformation. The work shows that the hydrogen dissolved in austenite can be regarded as trapped given the large activation energy of diffusion, which is greater than that of generic traps found within the ferrite and martensite. As a consequence, the deformation-induced martensitic decomposition of the austenite leaves the inherited hydrogen in a more mobile state. The mechanical degradation of the steel by hydrogen is therefore more pronounced in TRIP steel containing austenite which is relatively less stable to martensitic transformation during deformation.It was discovered that an aluminium addition in TWIP steel improves the resistance to hydrogen embrittlement, using tensile tests with dynamic hydrogen charging. This is because the Al suppresses not only the hydrogen-induced ε martensite reported to be susceptible to embrittlement but also mechanical twins which act as diffusible-hydrogen trap sites based on their huge density of dislocations. The hydrogen can be accumulated on the twins and grain boundaries during the tensile test through hydrogen transport via dislocations, thereby increasing a crack initiation there. Consequently, fracture surfaces were observed with both intergranular and transgranular mode of failure. It was demonstrated that transgranular fracture surfaces were parallel to interfaces between the ε martensite and austenite, or to mechanical twin boundaries