Evolution of the methane pressure in a standard 2.25Cr-1Mo steel during hydrogen attack

The material degradation process hydrogen attack has its origin in the diss olved hydrogen which reacts with the carbon of the steel to form methane in side grain boundary cavities. Hydrogen attack involves several interacting processes such as diffusion of carbon and of the metal atoms: dissolution o f carbides; reaction of C with H to methane; dislocation creep and grain bo undary diffusion. In this paper, a microstructural model is presented which takes into account the above-mentioned processes within the framework of a multi-component, multi-phase continuum description. The numerical model is developed for microstructures build up by a fenitic matrix and carbides su ch as M,C; and M,IC,. The model is applied to predict the microstructural e volution, the growth of cavities: and the resulting methane pressure in sta ndard 2.25Cr-1Mo steel during hydrogen exposure at 500 degreesC. They show that cavity growth and methane generation are strongly coupled, thus: falsi fying previous decoupled approaches to hydrogen attack. (C) 2001 Acta Mater ialia Inc. Published by Elsevier Science Ltd. All rights reserved.