Mechanical behaviour of 17-4 PH martensitic stainless steel in stress corrosion cracking and embrittlement in environmental hydrogen.

Age hardened martensitic stainless steels have high resistances to mechanic al stress, to friction corrosion and to stress-corrosion cracking (after ce rtain ageing heat treatments). These steels have also high mechanical tough ness levels. The mechanical strength of these steels increases when perform ing specific heat treatments in order to promote the precipitation of harde ning phases such as copper-rich epsilon and/or chromium-rich alpha'. Howeve r, it is well known that the susceptibility to stress-corrosion cracking an d to hydrogen embrittlement increases when the mechanical strength of the m artensitic steels is high. This paper is devoted to a study the mechanical behaviour of the 17-4 PH ma rtensitic 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 degrees C) and ageing times (1 and 4 ho urs). The two behaviours were studied by carrying out low strain rate tensi le tests (epsilon = 2.7 x 10(-6) s(-1)) in H2SO4 1N. By using differential scanning microcalorimetry, we have identified the precipitation mechanisms of epsilon and alpha' phases, and calculated their activation energies by a pplying KISSINGER's relationship. The results obtained show that the susceptibility to stress-corrosion crack ing and to hydrogen embrittlement is maximum at the optimum ageing temperat ure for which the strength level is maximum. The optimum ageing temperature increases when the epsilon phase, initially coherent, becomes non-coherent . The fracture mechanism changes from cleavage to intergranular mode, respe ctively. Finally, the fracture mechanism by stress-corrosion cracking is th e same as that provoked by hydrogen embrittlement.