AISI 4140 is a medium-carbon chromium-molybdenum (Cr-Mo) alloy steel known for its high strength, excellent toughness, good hardenability, and reliable fatigue resistance. It bridges the gap between plain carbon steels and higher-alloy grades such as 4340, offering a cost-effective high-performance solution.
4140 responds exceptionally well to quenching and tempering, making it one of the most widely used alloy steels in automotive, oil & gas, forging, heavy machinery, and power transmission industries.
Chemical Composition
| Element | % | Metallurgical Function |
|---|---|---|
| Carbon (C) | 0.38 – 0.43 | Strength & hardness |
| Chromium (Cr) | 0.80 – 1.10 | Hardenability, wear resistance |
| Molybdenum (Mo) | 0.15 – 0.25 | Toughness, high-temperature strength |
| Manganese (Mn) | 0.75 – 1.00 | Strength, impact resistance |
| Silicon (Si) | 0.15 – 0.35 | Deoxidation, strength |
| Phosphorus (P) | ≤ 0.035 | Controlled impurity |
| Sulfur (S) | ≤ 0.040 | Machinability |
| Iron (Fe) | Balance | Base metal |
Microstructure & Metallurgy
• Annealed: Ferrite + pearlite
• Normalized: Fine pearlitic structure
• Quenched: Martensitic structure
• Tempered: Tempered martensite for strength–toughness balance
• Cr and Mo delay pearlite formation enabling deep martensitic hardening
Mechanical Properties
| Condition | Tensile Strength | Yield Strength | Elongation | Hardness |
|---|---|---|---|---|
| Annealed | 655 – 760 MPa | 415 – 520 MPa | 22 – 25% | 200 – 217 HB |
| Normalized | 850 – 950 MPa | 650 – 720 MPa | 18 – 22% | ~240 HB |
| Quenched & Tempered | 900 – 1,080 MPa | 750 – 850 MPa | 12 – 18% | 28 – 45 HRC |
| Surface Hardened | — | — | — | Up to 60 HRC |
Key Characteristics
Excellent Hardenability
• Uniform hardness through medium-thick sections
• Superior to plain carbon steels such as C45 / 1045
High Strength & Fatigue Resistance
• Ideal for rotating and cyclically loaded components
• Long service life under mechanical stress
Good Wear & Abrasion Resistance
• Responds well to nitriding, carburizing, and induction hardening
Good Impact Toughness
• Resists shock loading without brittle failure
Elevated Temperature Performance
• Retains integrity up to ~400–450°C
Heat Treatment & Processing
Annealing: 830–860°C, furnace cooled
Normalizing: 870–900°C, air cooled
Quenching: Oil or polymer quench
Tempering: 200–650°C depending on application
Surface Hardening: Nitriding, induction, limited carburizing
Fabrication & Machining
• Good machinability in annealed/normalized condition
• Weldability is fair; preheating (200–300°C) required
• Hot forming preferred
Available Forms
✔ Hot rolled & forged round bars
✔ Bright bars (cold finished)
✔ Flat, square & hex bars
✔ Plates & cut blocks
✔ Hollow bars & seamless tubes
✔ Pre-hardened / Q&T components
Standards & Equivalents
| Standard | Equivalent |
|---|---|
| AISI / SAE | 4140 |
| ASTM | A29, A322 |
| DIN | 42CrMo4 |
| EN | 1.7225 |
| JIS | SCM440 |
| BS | EN19 |
Applications
Automotive: Crankshafts, axles, gears, steering components
Oil & Gas: Drill collars, studs, wellhead parts
Industrial: Hydraulic rods, spindles, press rams
Forging: Die holders, tool bases, forged parts
4140 vs 4340
| Property | 4140 | 4340 |
|---|---|---|
| Alloy System | Cr-Mo | Ni-Cr-Mo |
| Hardenability | High | Very High |
| Toughness | Good | Excellent |
| Cost | Lower | Higher |
| Typical Section Size | Medium | Very thick |
Advantages
✔ Excellent strength-to-cost ratio
✔ Deep and uniform hardening capability
✔ Good machinability before heat treatment
✔ Long service life
✔ Widely available global standard
Limitations
⚠ Lower toughness than 4340 in extreme shock
⚠ Controlled welding required
⚠ Limited corrosion resistance