Onto the role of copper precipitates and reverted austenite on hydrogen embrittlement in 17-4 PH stainless steel

High strength materials are of interest for hydrogen powered mobility where structural components can be exposed to high pressure (350–700 bars) of dihydrogen. However, high strength stainless steels such as 17-4 PH grade with a martensitic matrix are known to be more prone to hydrogen embrittlement (HE). The mechanisms affecting this type of grade have not been completely elucidated yet. In particular, in this work, the role of copper rich precipitates (Cu-pp) and reverted austenite (γrev) on mechanical behavior in presence of hydrogen was investigated. At first, advanced structural characterizations (Synchrotron-XRD, ACOM/TEM) were performed on different metallurgical conditions. Then, hydrogen trapping ability of Cu-pp and γrev was investigated using thermal desorption spectroscopy (TDS). Hydrogen embrittlement susceptibility was qualified through slow strain rate tensile testing. The results showed that the formation of tiny Cu-pp (< 10 nm) during ageing was deleterious for the HE resistance of the steel, since it increased the ultimate tensile strength. In addition to their role in the steel hardening, tiny Cu-pp actively trapped hydrogen at the Cu-pp/matrix interfaces. The nature of Cu-pp/matrix interfaces, which affects their surface energy and therefore the binding energy of hydrogen, impacted clearly the HE resistance. In particular, TDS experiments showed that tiny 'slightly coherent' Cu-pp acted as a low energy hydrogen trap contrary to tiny 'incoherent' fcc Cu-pp which strongly trapped hydrogen. The former was found to have a deleterious effect while the latter had a rather beneficial effect. Besides, a high fraction of reverted austenite was beneficial for the HE resistance since it softened the material, and trapped rather strongly hydrogen at the γrev/matrix. The fcc (Cu-pp or γrev)/martensitic matrix interfaces seemed to play a major role in hydrogen trapping in 17-4-PH stainless steel. When hydrogen is tightly bound to these interfaces, it happens to reduce the hydrogen embrittlement susceptibility of the material, 17-4 PH that is generally 'H fragile' because of its martensitic matrix.