Sensitization of High-Nitrogen Austenitic Stainless Steels by Dichromium Nitride Precipitation

Abstract

High-nitrogen (N) stainless steels (SS) are receiving increased attention because of their strength advantages over carbon (C)-alloyed materials, but they have been found susceptible to dichromium nitride (Cr2N) precipitation during thermal exposure between ~ 600°C and 1,050°C. Sensitization susceptibility of a high-N, low-C austenitic SS by Cr2N precipitation at 700°C and 900°C was determined using the single-loop electrochemical potentiokinetic reactivation (EPR) test. High-N SS was found susceptible to sensitization caused by grain boundary (GB) precipitation of Cr2N, with the degree of sensitization increasing systematically with aging time at 700°C. Sensitization of high-N materials did not require the concomitant precipitation of chromium (Cr)-rich metal carbide (M23C6). Materials aged at 900°C were not sensitized, although the rate of precipitation was greater than at 700°C. This indicated the minimum Cr level in the Cr-depleted zone of the matrix associated with nitride precipitation at 900°C was higher than required to produce attack in the EPR test. Prior deformation accelerated Cr2N precipitation and sensitization kinetics at 700°C, in accordance with previous research on C-based alloys. Sensitization development and deformation effects in high-N austenitic SS as a result of Cr-rich Cr2N precipitation were qualitatively analogous to sensitization phenomena ascribed to Cr-rich M23C6 precipitation in C-based alloys. Results indicated high-N, low-C SS was susceptible to intergranular corrosion (IGC) and/or intergranular stress corrosion cracking (IGSCC) as a result of sensitization after thermal exposure to a temperature regime at which Cr2N precipitation occurs.