321 stainless round steel (ASTM A276; GB 06Cr18Ni11Ti) has a composition of 17.0–19.0% chromium (Cr), 9.0–13.0% nickel (Ni), 0.04–0.10% titanium (Ti), ≤0.08% carbon (C), ≤1.00% silicon (Si), ≤2.00% manganese (Mn), ≤0.045% P, ≤0.030% S.
Compared to 304 (no intentional Ti), 321's key addition is titanium (0.04–0.10%), which plays a critical role in preventing intergranular corrosion (IGC). IGC occurs when carbon reacts with chromium at grain boundaries to form chromium carbides (Cr₂₃C₆), depleting chromium near boundaries and breaking down the passive Cr₂O₃ layer. Titanium in 321 has a higher affinity for carbon than chromium-it reacts with carbon to form stable titanium carbides (TiC) instead of Cr₂₃C₆. This "traps" carbon in the matrix, preventing it from migrating to grain boundaries and depleting chromium. The passive Cr₂O₃ layer remains intact, and IGC is avoided. Nickel stabilizes the austenitic microstructure, while chromium maintains general corrosion resistance-mirroring 304's traits but with IGC protection.