42CrMo round steel (GB/T 3077) has a composition of 0.38–0.45% carbon (C), 0.17–0.37% silicon (Si), 0.50–0.80% manganese (Mn), 0.90–1.20% chromium (Cr), 0.15–0.25% molybdenum (Mo), ≤0.035% P, ≤0.035% S.
35CrMo round steel (GB/T 3077) differs in two key ways: lower carbon (0.32–0.40% C) and lower chromium (0.80–1.10% Cr), with identical Mo (0.15–0.25%) and Mn (0.40–0.70%) ranges.
These differences drive distinct chemical behavior: 42CrMo's higher carbon increases the formation of chromium carbides (Cr₇C₃, Cr₂₃C₆) during heat treatment, enhancing hardness and wear resistance compared to 35CrMo. Its higher chromium content (0.90–1.20% vs. 0.80–1.10%) further boosts carbide formation and improves oxidation resistance by thickening the Cr₂O₃ passive layer. 35CrMo's lower C/Cr reduces carbide density, making it more chemically ductile and less prone to brittleness after tempering. Both grades rely on molybdenum to improve high-temperature chemical stability-Mo slows carbide coarsening, but 42CrMo's higher C/Cr means it retains strength at elevated temperatures better than 35CrMo.