4140 round steel (ASTM A29) has a composition of 0.38–0.43% carbon (C), 0.75–1.00% manganese (Mn), 0.15–0.30% silicon (Si), 0.80–1.10% chromium (Cr), 0.15–0.25% molybdenum (Mo), ≤0.035% phosphorus (P), and ≤0.040% sulfur (S).
42CrMo4 round steel (EN 10083-3) is nearly identical but has minor adjustments: 0.38–0.45% carbon (C), 0.60–0.90% manganese (Mn), 0.15–0.35% silicon (Si), 0.90–1.20% chromium (Cr), 0.15–0.30% molybdenum (Mo), ≤0.035% phosphorus (P), and ≤0.035% sulfur (S).
The similarities mean both steels have comparable chemical traits: chromium (0.80–1.20%) enhances hardenability and forms chromium carbides for wear resistance, while molybdenum (0.15–0.30%) improves high-temperature chemical stability by slowing carbide coarsening. The differences are subtle but impactful: 42CrMo4's higher chromium (0.90–1.20% vs. 0.80–1.10%) slightly boosts oxidation resistance, while its lower sulfur (≤0.035% vs. ≤0.040%) reduces brittle sulfide inclusions. 4140's higher manganese (0.75–1.00% vs. 0.60–0.90%) marginally improves deoxidation but has little effect on long-term chemical performance. Both grades rely on carbon (0.38–0.45%) for strengthening, with levels balanced to avoid excessive brittleness.