Sensitization of 12 Wt Pct chromium, titanium-stabilized ferritic stainless steel

Abstract

The sensitization behaviors of 19 heats of titanium stabilized, 12 Wt Pct chromium, ferritic stainless steel were evaluated by measuring the intergranular corrosion resistance of samples given a two-step heat treatment. The latter consisted of a one-hour high temperature exposure followed by a water quench and one of 19 different low temperature treatments. The results indicate that heat treating at temperatures ≳ 1000 °C produces a microstructure which can be sensitized by subsequent aging at temperatures ≲ 600 °C. The amount of carbon and not the amount of nitrogen nor the amount of carbon plus nitrogen dictated the sensitization resistance of the titanium-stabilized, 12 Wt Pct chromium alloy. The proposed mechanism of sensitization in these alloys suggests that during the high temperature exposure, the titanium carbonitrides decompose, freeing carbon or carbon and nitrogen into interstitial solid solution. During subsequent heat treating at temperatures ≲600 °C, chromium-containing carbides precipitate intergranularly, and a chromium-depleted zone is formed along the grain boundaries. The presence of the chromium-depleted zone results in susceptibility to intergranular corrosion, α′ precipitates form after the intergranular precipitates during heat treating at 600 °C. The presence of α′ enhances the intergranular corrosion rate.