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

Author： Schlogl Sabine Giessen Erik Leeuwen Yvonne

ISSN： 1543-1940

Source： Metallurgical and Materials Transactions A, Vol.31, Iss.1, 2000-01, pp. : 125-137

Disclaimer: Any content in publications that violate the sovereignty, the constitution or regulations of the PRC is not accepted or approved by CNPIEC.

Previous Menu Next

Abstract

Low-carbon, low-alloy Cr-Mo steels may fail by hydrogen attack when they are exposed to high hydrogen pressures at elevated temperatures. During this process, the dissolved hydrogen reacts with the carbides of the steel to form methane in grain boundary cavities. The methane pressure inside these cavities 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 and 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 depends on, among other factors, the methane pressure and the damage state. As the carbon content also affects the creep parameters, this process of decarburization may accelerate the cavity growth. Model calculations are used to obtain insight into the influence of this decarburization process on damage evolution and the final lifetime.