Optimization of Steels for use in Hydrogen Environment

Four different types of steels have been studied : a Cr Mo Ni 20 CND 10 steel and a Cr Mo 15 CD 12 steel for high temperature service, a 3.5 Ni steel for low temperature applications, and a C Mn API X 70 steel for piping. Various techniques were used to characterize each material : tensile tests on hydrogen charged specimens, disk pressure tests, delayed failure tests, permeation tests, and crack simulation tests. Results have shown that : a) appropriate heat treatments may ameliorate the resistance of the steel to hydrogen embrittlement — b) In some cases, at a given optimum heat treatment, there exists a large dispersion of the results ; this was the case for the 3.5 Ni steel, where the oxide content was proven as deleterious if too large — c) Test procedure may also influence the extent of embrittlement, as shown in the case of API X 70 — d) Oxygen pollution decreases hydrogen embrittlement of all steels — e) Increasing strain rate or temperature also improves resistance to hydrogen — f) On the other hand a polish surface finish is deleterious, compared to the as machined surface.