Type 321 stainless steel is specified for 800–900°C service primarily because of its titanium stabilization, which prevents sensitization. In the 427–816°C range, chromium in ordinary austenitic grades such as 304 combines with carbon to form chromium carbides at grain boundaries. This depletes chromium locally and destroys intergranular corrosion resistance. Type 321 contains titanium at a minimum of five times the carbon content. Titanium has a stronger affinity for carbon than chromium does, so it forms stable titanium carbides instead, keeping chromium in solid solution and preserving corrosion resistance.
Beyond sensitization control, 321 also offers good creep strength and oxidation resistance at elevated temperature. The titanium additions stabilize the microstructure and improve high-temperature mechanical performance, allowing the alloy to remain structurally sound in continuous or intermittent service above 800°C. Compared with 304L, whose allowable stress drops sharply above 427°C, 321 can be used up to roughly 816–900°C. For welded components that cannot be solution-annealed, or for equipment exposed long-term to sensitizing temperatures, 321 is therefore the preferred choice, balancing corrosion resistance with high-temperature strength.
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