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

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 M 7 C 3 , M 23 C 6 , M 6 C, and M 2 C. 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.

Micromechanics of high temperature hydrogen attack

“…Thus, the calculation of the methane pressure is a formidable, coupled problem for which no solution is available at this moment [3]. A common approach is to assume that the chemical reaction and the dissolution are much faster than cavity growth and creep on the time scales of the lifetime of a component (around 20-30 years).…”

“…The most recent version of the thermodynamical model can be found in Reference [3]. In the remainder of the present paper, we focus on the mechanical part of the model.…”

“…The thermodynamic model mentioned above [3] can be used to calculate the methane pressure in dependence of the hydrogen pressure, and the compositions of the carbides and ferrite. The compositions of the carbides vary in the di erent regions of the weldment.…”

“…In Figure 4 we show contours of damage at the instant of ÿrst cavity coalescence. The damage variable plotted is the value of (a=b) 3 on facets normal to the circumferential direction. It is seen that at this stage most damage occurs in FGHAZ, somewhat less in CGHAZ, followed by WM while BM shows the least damage.…”

Computational model for carbon diffusion and methane formation in a ferritic steel during hydrogen attack

“…Several interacting processes are involved in building up the methane pressure, including diffusion of C and of the metal atoms, dissolution of carbides, as well as reaction of C with H to methane inside the growing cavities. Nevertheless, estimates of the lifetime under HA in the literature have decoupled some of these processes by assuming that the methane pressure in the cavity is at its equilibrium value [1][2][3]. This allows to develop models to predict the HA lifetime of components on the basis of direct material data (e.g.…”