THE EFFECT OF LOW-TEMPERATURE ISOTHERMAL HEAT-TREATMENTS ON INTERGRANULAR CORROSION OF TYPE-316 STAINLESS-STEEL SIMULATED WELD HEAT-AFFECTED ZONES

Type 316 (UNS(1) S31600) stainless steel (SS) is a candidate alloy for containment of high-level nuclear waste in deep underground storage. The heat-affected zones (HAZs) of welded containers could become sensitized under the appropriate thermal conditions. Prior to the present study, it was suspected that carbides that nucleated during welding could grow during the containment life of the nuclear waste. In this investigation, type 316 stainless steels containing variations in carbon and nitrogen have been exposed to thermal cycles simulating a single-pass weld HAZ, followed by long-term (2000 to 2800 hours) isothermal heat treatments in the range of 300 to 400-degrees-C. The degree of sensitization (DOS) was determined using two corrosion methods; the double-loop electrochemical potentiokinetic reactivation (DL-EPR) test and the ferric sulfate (Streicher) test. Optical metallography was used to reveal the presence of grain boundary carbides and analytical electron microscopy (AEM) was employed to determine the local composition of grain boundaries. The corrosion tests indicated that significant chromium depletion did not occur regardless of the heat treatment or carbon and nitrogen content. The Streicher tests showed that grain boundaries were selectively attacked after the HAZ simulations; however, the ensuing low-temperature exposure did not increase the susceptibility to intergranular attack (IGA). Discrete grain boundary carbides were observed on 15 to 25% of the boundaries in the high-carbon alloys (0.08 wt% C). The AEM results showed that when carbides were present, the chromium level at the grain boundaries was slightly depressed with respect to the matrix level of 16.7 wt%; however, the minimum level was never less than approximately 16.0 wt%.