20Cr2Ni4 round steel (GB/T 3077) has a composition of 0.17–0.23% carbon (C), 0.17–0.37% silicon (Si), 0.30–0.60% manganese (Mn), 1.80–2.20% chromium (Cr), 3.60–4.00% nickel (Ni), ≤0.035% P, ≤0.035% S.
Its high nickel content (3.60–4.00%) enhances chemical ductility via two atomic-level mechanisms:
Austenite stabilization: Nickel is a strong austenite stabilizer-it keeps the steel in the austenitic phase (face-centered cubic, FCC) at room temperature, even with low carbon content. The FCC structure has more slip systems (atomic planes along which deformation occurs) than the martensitic (body-centered cubic, BCC) structure of low-nickel steels. More slip systems mean the steel can deform plastically without cracking, increasing ductility.
Reduced carbide formation: Nickel increases the solubility of carbon in austenite, reducing the formation of hard, brittle chromium carbides (Cr₂₃C₆). Fewer carbides mean less disruption to the metal matrix, allowing atoms to move more freely during deformation. This is especially important for 20Cr2Ni4, as its high chromium content (1.80–2.20%) would otherwise promote excessive carbide formation in low-nickel grades.
Chromium provides corrosion resistance and strength, while low carbon (0.17–0.23%) further supports ductility by minimizing matrix hardening.