Optimization of Steels for use in Hydrogen Environment

Abstract

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.

All results and apparent contradictions are then explained in terms of newly developped concepts on hydrogen traps. The conclusions to be drawn are multiple. For one thing, it becomes important, besides heat treatment to explore the role of parameters related to the “past” of the steel, i.e. chemical composition, elaboration mode, mechanical treatments, etc... Then, it will become necessary to interpret the results in terms of interaction between hydrogen and microstructure (traps), in order to understand the phenomenon. This should eventually allow future design of hydrogen resisting steels.