AISI 4140 is a chromium–molybdenum (Cr–Mo) hardenable low-alloy steel belonging to the 41xx series. It is engineered to provide high strength, excellent hardenability, good toughness, and superior wear resistance.
With a medium carbon content of approximately 0.38–0.43%, AISI 4140 achieves higher hardness and load-bearing capacity than 4130 while maintaining good ductility and machinability. Its ability to develop uniform hardness through thick sections makes it a workhorse alloy steel for demanding applications.
Chemical Composition
| Element | Content (%) | Function |
|---|---|---|
| Carbon (C) | 0.38 – 0.43 | Strength, hardness |
| Manganese (Mn) | 0.75 – 1.00 | Toughness, hardenability |
| Chromium (Cr) | 0.80 – 1.10 | Wear & oxidation resistance |
| Molybdenum (Mo) | 0.15 – 0.25 | High-temperature strength |
| Silicon (Si) | 0.15 – 0.30 | Strength, deoxidation |
| Phosphorus (P) | ≤ 0.035 | Controlled impurity |
| Sulfur (S) | ≤ 0.040 | Machinability |
| Iron (Fe) | Balance | Base metal |
Metallurgical Characteristics
• Ferrite–pearlite structure in annealed condition
• Converts to tempered martensite after quenching and tempering
• Chromium and molybdenum enable deep hardening
• Stable microstructure under cyclic stress and thermal loading
Mechanical Properties
| Condition | Tensile Strength | Yield Strength | Elongation | Hardness |
|---|---|---|---|---|
| Annealed | ~655 MPa | ~415 MPa | ~25% | 180 HB |
| Normalized | ~850 MPa | ~650 MPa | 20–22% | 220–240 HB |
| Quenched & Tempered | 950–1100 MPa | 800–950 MPa | 15–20% | 28–40 HRC |
Key Characteristics
High Hardenability
• Uniform hardness in thick sections
• Ideal for large shafts, gears, and forgings
Excellent Strength & Toughness
• Higher strength than 4130
• Maintains impact resistance under heavy loads
Superior Fatigue & Wear Resistance
• Excellent performance under cyclic stress
• Suitable for power transmission and rotating machinery
Good Machinability & Ductility
• Machines well in annealed or normalized condition
• Stable during machining and forming
Heat Treatment & Refining
Annealing: 830–860°C – stress relief and machinability
Normalizing: 870–900°C (air cooled) – grain refinement
Quenching: Oil or polymer – martensitic structure
Tempering: 200–650°C – toughness and fatigue resistance
Surface Hardening
• Induction hardening for localized wear resistance
• Nitriding up to ~60 HRC with minimal distortion
• Selective carburizing for special applications
Available Forms
✔ Hot rolled & cold drawn bars
✔ Forged shafts and bars
✔ Plates and flats
✔ Seamless and welded tubes
✔ Billets and rings
Standards & Equivalents
| Standard | Equivalent |
|---|---|
| AISI / SAE | 4140 |
| ASTM | A29 / A322 |
| EN | 42CrMo4 |
| DIN | 1.7225 |
| JIS | SCM440 |
| BS | EN19 |
Applications
Automotive: Axles, crankshafts, connecting rods, gear shafts
Industrial Machinery: Gears, couplings, shafts, bolts
Oil & Gas: Drill collars, tool joints, pressure components
Power & Mining: Turbine shafts, crusher parts, heavy structures
4140 vs 4130 Comparison
| Property | AISI 4130 | AISI 4140 |
|---|---|---|
| Carbon Content | Lower | Higher |
| Maximum Strength | Medium–High | High |
| Weldability | Better | Moderate |
| Wear Resistance | Good | Superior |
| Typical Use | Structural tubing | Shafts & gears |
Advantages
✔ High strength and toughness
✔ Excellent hardenability in thick sections
✔ Reliable fatigue and wear resistance
✔ Versatile heat treatment options
✔ Widely available and cost-effective
Limitations
⚠ Requires preheating for welding
⚠ Limited corrosion resistance without coating
⚠ Not suitable for extreme cryogenic conditions
AISI 4140 is a versatile Cr–Mo alloy steel delivering an outstanding balance of strength, toughness, fatigue resistance, and hardenability. Its reliability and adaptability make it one of the most widely used alloy steels in high-load, wear-critical, and mechanically demanding applications.