Hydrogen embrittlement and delayed fracture of advanced multiphase high-strength steels (Charge & load) : final report

This project was initiated with the objective of obtaining a thorough understanding of the H-embrittlement mechanisms in multiphase carbon and stainless steels, in particular the influence of the steel's strength level, the influence of second phases and the influence of strain ageing. Seven carbon steels and seven stainless steels with varying microstructure and mechanical properties were investigated. All steels were ranked in terms of their H-management, i.e. their capacity to trap hydrogen in their microstructure. The major influencing parameter turned out to be the Ti + Nb precipitation. A dual phase ferrite-martensite microstructure seemed to induce rather 'enhanced' H-mobility. For C-steels, as well as for stainless steels, mechanical characterisation via various characterisation techniques allowed ranking of the grades in three groups depending on their sensitivity to H-embrittlement: steels with an excellent resistance to H-embrittlement in all tests; steels which seemed to be rather vulnerable to embrittlement in all tests; and steels with a dual behaviour, i.e. whose sensitivity seemed very dependent on the conditions of laboratory tests. Detailed characterisation of the tested materials revealed various microstructural features that might influence the hydrogen induced cracking. Ti,Nb-precipitates, centreline segregation, inclusions, etc. were found to influence crack initiation and propagation in carbon steels. In austenitic stainless steels the martensite content seemed to have a major influence. Guidelines were given how to control this martensite content in metastable austenitic grades. This project can be a strong reference to future research on H-embrittlement, in terms of material evaluation and in terms of the development of experimental procedures.