The effect of hydrogen on the bainite transformation

The effect of hydrogen on the upper bainite transformation in two silicon c ontaining steels has been investigated. For comparison, isothermal transfor mation at the same temperature has also been performed in a helium atmosphe re. In both Fe-0.2C-3Mn-2Si and Fe-0.4C-4Ni-2Si (nominal wt %) alloys it wa s discovered that the bainite reaction proceeds further towards completion when the transformation is carried out in a hydrogen atmosphere. This can r esult in the reduction or elimination of the martensite phase which forms f rom residual austenite upon quenching to room temperature. The resultant mi crostructure of specimens heat treated in hydrogen was a fine aggregate of upper bainitic ferrite and interlath retained austenite. This effect is dis cussed in terms of hydrogen interactions in the lattice undergoing bainite transformation via a displacive mechanism. Additionally, it is found that t he stability of the retained austenite in the final bainitic microstructure is not markedly influenced by hydrogen. (C) 1999 Kluwer Academic Publisher s.