The chemical composition of AISI 4140 is tightly controlled to ensure consistent performance. The following table outlines the standard ranges as per AISI/SAE specifications and compares them with a key international equivalent.
| Element | AISI 4140 / SAE J404 (%) | EN 1.7225 / 42CrMo4 (%) | Typical Function |
|---|---|---|---|
| Carbon (C) | 0.38 – 0.43 | 0.38 – 0.45 | Primary hardening element; increases strength and hardness. |
| Manganese (Mn) | 0.75 – 1.00 | 0.60 – 0.90 | Increases hardenability and strength. |
| Silicon (Si) | 0.15 – 0.30 | ≤ 0.40 | Deoxidizer during steelmaking; increases strength. |
| Chromium (Cr) | 0.80 – 1.10 | 0.90 – 1.20 | Increases hardenability, wear resistance, and mild corrosion resistance. |
| Molybdenum (Mo) | 0.15 – 0.25 | 0.15 – 0.30 | Enhances hardenability, high-temperature strength, and reduces temper embrittlement. |
| Phosphorus (P) | ≤ 0.035 | ≤ 0.025 | Impurity; kept low to prevent embrittlement. |
| Sulfur (S) | ≤ 0.040 | ≤ 0.025 | Impurity; improves machinability if slightly elevated but generally minimized. |
| Iron (Fe) | Balance | Balance | Base metal. |
This composition is the blueprint for the steel's properties. The carbon content places it in the medium-carbon category, enabling significant strengthening via heat treatment. The synergistic effect of Cr and Mo allows for good through-hardening even in moderately large cross-sections, making it superior to plain carbon steels for larger diameter round bars.