431 stainless round steel (ASTM A276; GB 14Cr17Ni2) has a martensitic composition of 0.11–0.17% carbon (C), 15.0–17.0% chromium (Cr), 1.50–2.50% nickel (Ni), ≤0.80% silicon (Si), ≤0.80% manganese (Mn), ≤0.035% P, ≤0.030% S.
Nickel (1.50–2.50%) is the key to balancing hardness and corrosion resistance:
Hardness support: Nickel increases the solubility of carbon in austenite, allowing more carbon to be retained during quenching. This carbon reacts with chromium to form chromium carbides (Cr₇C₃) that enhance hardness-431 achieves 38–42 HRC after heat treatment, comparable to martensitic grades like 410 but with better ductility.
Corrosion resistance enhancement: Unlike low-nickel martensitic steels (e.g., 410, 0% Ni), nickel in 431 stabilizes the passive Cr₂O₃ layer. It reduces the formation of chromium carbides at grain boundaries by competing with carbon for chromium, maintaining a continuous Cr₂O₃ layer that resists general corrosion. Nickel also improves resistance to stress corrosion cracking by reducing the steel's susceptibility to martensitic transformation under stress.
This balance is unique to 431: higher-nickel austenitic grades (e.g., 304) have better corrosion resistance but lower hardness, while low-nickel martensitic grades have higher hardness but poorer corrosion resistance.