Designing steel to resist hydrogen embrittlement: Part 1 – trapping capacity

Abstract

A novel steel has been designed for use in the oil and gas industry, displaying properties comparable with the currently available F22 grade and possessing the additional quality of excellent hydrogen trapping capacity. Its high strength is derived from a martensitic microstructure containing a dispersion of fine vanadium-molybdenum carbides that evolve during thermal treatments. If the tempering cycle is controlled such that the precipitates maintain a degree of coherency with the matrix, then they act as hydrogen trapping sites, due to the associated strain fields, thus mitigating the problem of diffusible hydrogen. Using material modelling programmes and small-scale sample alloys this work describes the process of the new steel design and demonstrates its superior trapping capacity through thermal desorption analysis.

Analysis of the influence of microstructural traps on hydrogen assisted fatigue

“…The value obtained is listed in Table 1; the sensitivity to N (c) T will be explored, as increasing the carbide content is the main strategy in designing materials with beneficial traps and intrinsic resilience [31,32].…”

“…Thus, the critical value of C L at which embrittlement is observed must be within the plateau values of C Lmax,N predicted for f = 0.1 and f = 1 Hz. Results for the standard carbide trap density, N [31]. Increasing the trap density decreases the diffusion coefficient, see (5), and accordingly, the diffusion of lattice hydrogen within each cycle is reduced, leading to a lower value of C Lmax,N .…”

“…The ambition is to design hydrogen embrittlement-resistant alloys by engineering microstructures with a high density of beneficial traps, which will retain the hydrogen and hinder diffusion to the fracture process zone. One strategy involves incorporating finely dispersed nano-scale carbides [31,32]. Vanadium carbides have been successfully used to mitigate hydrogen embrittlement in refinery pressure vessels and other closed-systems, where hydrogen entry is essentially a one-off process.…”

“…7 also includes results obtained assuming that the density of carbides can be engineered. Namely, the density of carbide trapping sites is increased to N (c) T = 3.61 × 10 26 sites/m 3[31]. Increasing the trap density decreases the diffusion coefficient, see(5), and accordingly, the diffusion of lattice hydrogen…”

Cohesive zone modelling of hydrogen assisted fatigue crack growth: the role of trapping

“…Their findings imply that the hydrogen content can be decreased below the embrittlement threshold if D e is reduced, suggesting that materials can be engineered to bring down their hydrogen diffusivity and susceptibility to hydrogen-assisted fatigue crack growth. The effective diffusivity of materials can be reduced by increasing the density of beneficial traps -microstructural trapping sites that are not involved in the fracture process [27,28]. Fernández-Sousa et al [26] showed that increasing the density of carbides by 3 orders of magnitude in a CrMo steel enabled extending by an order of magnitude the regime of safe frequencies at which hydrogen has no effect.…”