On methane generation and decarburization in low-alloy Cr-Mo steels duringhydrogen attack

Low-carbon, low-alloy Cr-Mo steels may fail by hydrogen attack when they ar e exposed to high hydrogen pressures at elevated temperatures. During,this process, the dissolved hydrogen reacts with the carbides of the steel to fo rm methane in grain:boundary cavities. The methane pressure inside these ca vities depends on the microstructure of the used steel, which consists of a ferritic matrix and alloy carbides such as M7C3, M23C6, M6C, and M2C. The different phases in the multicomponent system Fe-Cr-Mo-V-C are modeled with the sublattice model. Their Gibbs energies are then used to calculate the equilibrium methane pressure as a function of the microstructure. Driven by the methane pressure, the cavities grow due to grain boundary diffusion an d dislocation creep, which is described by analytical relations. This leads to progressive development of damage inside the material but, at the same time, to a decrease of the carbon content in the steel. This reduction depe nds on, among other factors, the methane pressure and the damage state. As the carbon content also affects the creep parameters, this process of decar burization may accelerate the cavity growth. Model calculations are used to obtain insight into the influence of this decarburization process on damag e evolution and the final lifetime.