Hydrogen sensitivity of different advanced high strength microstructures (HYDRAMICROS) : final report

Modern advanced and ultra-high strength steel grades are subject to issues of hydrogen embrittlement (HE), particularly for strength levels above 1000 MPa. The present project has been designed in order to investigate microstructural features and to obtain insights into causes and mechanisms of HE in such steels. Therefore, three material classes with strength levels of 1000, 1200 and 1400 MPa and differing microstructures have been generated through laboratory heat treatments from industrial cast and rolled steels. The samples have been charged with hydrogen and systematic mechanical testing has been performed, thus enabling a comparison of the various microstructures in terms of susceptibility to hydrogen. Accurate microstructure investigations of the uncharged and charged samples, both before and after the mechanical testing have been used as indicators for assessing the embrittlement behaviour. Particularly relevant features include the amount of martensite, non-metallic inclusions, carbides and nitrides within the microstructure. The investigations revealed that there is no direct correlation between the hydrogen uptake and the sensitivity to HE, but significant differences between the embrittlement in tempered and untempered samples. Extensive computer modelling has been performed to support the experimental work and has shed light on some specific mechanisms for H. Examples are indications of a substantial solution of H in the retained austenite of the investigated samples as well as suggesting that there are indications that hydrogen-induced fracture may occur within inclusions rather than at the boundaries to the matrix.