440C stainless round steel (ASTM A276; GB 10Cr17Mo) has a composition: 0.95–1.20% carbon (C), 16.0–18.0% chromium (Cr), 0.75–1.25% molybdenum (Mo), ≤1.00% silicon (Si), ≤1.00% manganese (Mn), ≤0.040% P, ≤0.030% S.
440A differs in two key ways: lower carbon (0.60–0.75% C) and no intentional molybdenum (Mo ≤0.50%).
These differences make 440C significantly harder due to chemical mechanisms:
Higher carbon: 0.95–1.20% C increases the formation of chromium carbides (Cr₇C₃, Cr₂₃C₆) during heat treatment. Carbides are hard, brittle particles that embed in the martensitic matrix, increasing overall hardness-440C has ~50% more carbides than 440A.
Added molybdenum: 0.75–1.25% Mo forms molybdenum carbides (Mo₂C) that are harder than chromium carbides. Mo also enhances hardenability, ensuring carbides form uniformly throughout the steel (not just at the surface), which improves hardness consistency.
Chromium (16–18%) provides corrosion resistance but also contributes to hardness via carbide formation. Without Mo and with lower C, 440A forms fewer, softer carbides, resulting in lower hardness (56–58 HRC vs. 58–60 HRC for 440C).