Comportement mécanique de l'acier inoxydable martensitique 17-4 PH en corrosion sous contrainte et à la fragilisation par l'hydrogéne environnemental

Abstract

Age hardened martensitic stainless steels have high resistances to mechanical stress, to friction corrosion and to stress-corrosion cracking (after certain ageing heat treatments). These steels have also high mechanical toughness levels. The mechanical strength of these steels increases when performing specific heat treatments in order to promote the precipitation of hardening phases such as copper-rich ε and/or chromiumrich α′. However, it is well known that the susceptibility to stress-corrosion cracking and to hydrogen embrittlement increases when the mechanical strength of the martensitic steels is high. This paper is devoted to a study the mechanical behaviour of the 17-4 PH martensitic stainless steel with respect to stress-corrosion cracking and to embrittlement by environnemental hydrogen in different ageing conditions, i.e. ageing temperatures (200 to 650 °C) and ageing times (1 and 4 hours). The two behaviours were studied by carrying out low strain rate tensile tests (ε′ = 2.7×10 −6 s −1) in H 2SO 4 1N. By using differential scanning microcalorimetry, we have identified the precipitation mechanisms of ε and α′ phases, and calculated their activation energies by applying KISSINGER'S relationship. The results obtained show that the susceptibility to stress-corrosion cracking and to hydrogen embrittlement is maximum at the optimum ageing temperature for which the strength level is maximum. The optimum ageing temperature increases when the ε phase, initially coherent, becomes non-coherent. The fracture mechanism changes from cleavage to intergranular mode, respectively. Finally, the fracture mechanism by stress-corrosion cracking is the same as that provoked by hydrogen embrittlement.