Relation between Intergranular Corrosion Susceptibility and Precipitation of Carbides and Nitrides in 17% Cr Ferritic Stainless Steels

Abstract

To clarify the cause of intergranular corrosion of 17% Cr fbrritic stainless steels, precipitation of chromium carbides and nitrides at grain boundaries was studied by electricalresistivity measurement and electron microscopic observation on the steels having various amounts of carbon and nitrogen.
On the quenched and isochronously annealed specimens, precipitation was found at two stages for chromium carbides, (Fe, Cr) ₇C₃ at 550°-650°C and Cr₂₃C₆ at above 700°C, and also at two stages for chromium nitrides, the one of not identified structure at 400°-550°C and Cr2N at 650°-750°C.
Intergranular corrosion susceptibility was remarkably raised by the precipitation of (Fe, Cr) ₇C₃ at 550°-600°C and “unknown” nitride at 400°-550°C.
Chromium depletion zone along grain boundaries formed by the very rapid precipitation of these special types of carbide and nitride at the intermediate temperatures around 550°C was indicated as the most probable reason for intergranular corrosion.