Understanding Hydrogen Redistribution and Designing a New Hydrogen Extraction Method

Abstract

Hydrogen redistribution and trapping plays an important role in the embrittlement of some high performance alloys. A correct understanding of hydrogen redistribution would allow the prediction and prevention of this severe type of material degradation.A physical model of interstitial element diffusion is used to study the fluxes of hydrogen during manufacturing of metallic alloys. In particular, the present model contemplates diffusion in its most comprehensive description, i.e., atom diffusion is driven by a reduction of the Gibbs energy of the system (and not only related to composition gradients). The model presented incorporates physical description of thermal agitation and atom mobility of interstitial elements, the influence of temperature gradients, solubility and saturation of the interstitial elements as function of temperature and matrix phases, as well as the kinetics of degassing at high temperature. Finally, it incorporates the role of traps as potential energy pits presenting a characteristic energy barrier to the release of hydrogen atoms. Such a model permits the study of the effect of different microstructure characteristics on the trapping, de-trapping and general redistribution of hydrogen, taking into account the thermal cycle and the separate contribution of microstructure, deformation level, dislocation distribution, grain size, carbide presence and distribution, etc. and their interaction, to finally obtain the degree of saturation at the lattice and each trapping site type during and after a heat treatment.Two industry-relevant applications have been analysed: First, the study of hydrogen redistribution during casting and cooling; and second, a study on the effectiveness of baking. The first study has provided insight on redistribution patterns that lead to supersaturation related to processing parameters, and once that is understood, to the design of a new treatment to reduce hydrogen content in cast components, which has obtained several patents.The second, by studying the effect of storing a metal part in a heated oven for a long period of time, in a process known as baking, it is possible to explain why this standard method presents varying degrees of success. This variability is related to the alloy microstructure and cooling process prior to baking, which in turn influence where in the microstructure hydrogen is stored at the time of the treatment.