|GAUDE-FUGAROLAS Daniel||Independent Research in Physical Metallurgy and Engineering|
Hydrogen trapping plays an important role in the embrittlement of some high performance alloys. A correct understanding of hydrogen redistribution from the onset of the manufacturing process would allow the prediction and prevention of severe material degradation. Hydrogen presents high solubility at high temperatures in many alloys, but not so at room temperature. As the solubility of hydrogen drops with lowering temperature or microstructure change, excess hydrogen becomes stored in various microstructural features (traps) including dislocations, grain boundaries or precipitate interfaces. Traps are often categorised in two wide groups: shallow or reversible traps and deep or irreversible ones. A physically more accurate model describes each trap as potential energy pits and considers the characteristic energy barrier to release an hydrogen atom from it. Using that description, a physical model accounting for the redistribution of interstitial atoms in ferrous alloys has been implemented. The model permits the study of the effect of different microstructure characteristics on the trapping, detrapping and general redistribution of hydrogen, taking into account the thermal cycle and the separate contribution of deformation level, dislocation distribution, grain size, carbide presence and distribution, et c. and their interaction. To illustrate the predictions of the model, a comparison between two idealised low alloy steels is finally presented. Both alloys are rapidly cooled and in each austenite transforms into a different microstructure. Steel A transforms at high temperature to become large grained, undeformed allotriomorphic ferrite while Steel B transforms at lower temperature becoming a stressed mixture of bainite and martensite.