Corrosion and Toughness of Experimental and Commercial Super Ferritic Stainless Steels

Abstract

The effect of minor alloying in a super ferritic stainless steel 26% Cr-3% Mo matrix was investigated. The corrosion resistance of several experimental heats was examined in terms of their sensitization and intergranular corrosion (IGA) susceptibility following a low-temperature anneal (620°C). Constant potential etching, electrochemical (electropotentiokinetic reactivation [EPR]), and immersion (modified Strauss test) studies showed that the principal corrosion initiation site of heat-treated steels was intergranular even at low [C + N] (< 130 ppm) and relatively high [Nb + Ti] concentrations. Despite the significant contribution of Nb and Ti to the IGA resistance, these elements had a deleterious effect on the notch toughness. Charpy V testing demonstrated increases in the upper shelf energy and ductile-to-brittle transition temperature (DBTT), the latter shifting from ≈ –50°C in the absence of stabilizing elements to > 25°C for the stabilizing ratio [(Nb + Ti)/(C + N)] > 9. Corrosion resistance of the experimental heats was compared with that of several commercial alloys with intermediate stabilization ratios. The interaction of toughness and corrosion resistance in ferritic stainless steel was discussed with respect to the lack of consistency between published evaluation methods and the ideal stabilization ratio at low [C + N] values.