Formation and effects of α'-martensite on the stress crack corrosion in Cr-Ni steels

Martensite formation depends on the alloying constituents, deformation, and undercooling. Unlike the engineering material X 10 CrNiTi 18 9, a pure Fe-19Cr-10Ni alloy becomes partly martensitic upon deformation and annealing to -196 C. During undercooling and subsequent deformation, the deformation-induced martensite adds up to the annealing-induced martensite. Martensite formation is suppressed to a large extent by addition of 2% of Mo to the allloy. The ultimate stress crack corrosion strength in aqueous MgCl2 solutions strongly rises with the content of α'-martensite. Cracks develop and grow for the most part along the edges of the martensite plates. Addition of molybdenum reduces the stress crack corrosion resistance, as it largely prevents the formation of α'-martensite. Crack growth in this alloy predominantly proceeds in the intercrystalline space. (MM)