1Cr18Ni9Ti stainless round steel (ASTM A276; GB 06Cr18Ni9Ti) has a composition of 17.0–19.0% chromium (Cr), 8.0–11.0% nickel (Ni), 0.04–0.10% titanium (Ti), ≤0.12% carbon (C), ≤1.00% silicon (Si), ≤2.00% manganese (Mn), ≤0.045% P, ≤0.030% S.
Titanium (0.04–0.10%) prevents intergranular corrosion (IGC) via a chemical "carbon trapping" mechanism:
IGC occurs when carbon reacts with chromium at grain boundaries to form Cr₂₃C₆ carbides. This depletes chromium near boundaries, creating a "chromium-depleted zone" where the passive Cr₂O₃ layer breaks down, leading to corrosion along boundaries.
Titanium has a higher affinity for carbon than chromium-it reacts with carbon in the steel to form stable titanium carbides (TiC) in the matrix, not at grain boundaries. This traps carbon before it can migrate to boundaries and react with chromium, eliminating the chromium-depleted zone.
The Ti content is carefully controlled to be 5–10 times the carbon content (by weight), ensuring all carbon is trapped as TiC. Nickel stabilizes the austenitic microstructure, while chromium forms the passive layer-titanium ensures these protective traits are not undermined by IGC.