AISI 4140 Chromoly Steel: Heat Treatment, Hardness & Mechanical Properties (UNS G41400)
ASTM A29 / A322 / A331 / SAE J404 · Published: 2026-05-31 · Updated: 2026-05-31
AISI 4140 is the most widely used medium-carbon chromium-molybdenum alloy steel, offering an exceptional combination of strength, toughness, and fatigue resistance after heat treatment. Containing 0.38-0.43% carbon with chromium (0.80-1.10%) and...
AISI 4140 is the most widely used medium-carbon chromium-molybdenum alloy steel, offering an exceptional combination of strength, toughness, and fatigue resistance after heat treatment. Containing 0.38-0.43% carbon with chromium (0.80-1.10%) and molybdenum (0.15-0.25%), 4140 achieves deep hardenability in sections up to 50 mm, making it the default choice for high-stress machine components: shafts, gears, bolts, drill collars, and aircraft landing gear parts. It is through-hardening (not case-hardening) and responds well to induction hardening for surface wear resistance while maintaining a tough core.
Quick Facts
| Category | Alloy Steel |
| Standard | ASTM A29 / A322 / A331 / SAE J404 |
| Density | 7.85 g/cm³ |
| Yield Strength | 415-655 MPa (60-95 ksi) depending on heat treatment |
| Tensile Strength | 655-1075 MPa (95-156 ksi) depending on heat treatment |
Global Equivalents & Cross-Reference
| Alternative Standard / Grade | Action |
|---|---|
| EN 42CrMo4 | Compare |
| DIN 1.7225 | Compare |
| JIS SCM440 | Compare |
| GB 42CrMo | Compare |
| BS 708M40 | Compare |
Related Materials
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Frequently Asked Questions
What is the recommended heat treatment for AISI 4140?
Austenitize at 845°C (1550°F), oil quench for sections up to 25 mm (water quench for larger sections with risk of cracking), temper immediately at 205-650°C (400-1200°F) depending on desired hardness. Tempering at 205°C yields ~55 HRC (high strength, low toughness); tempering at 540°C yields ~35 HRC (balanced strength and toughness for shafting). Always temper immediately after quenching — untempered martensite is brittle and can crack spontaneously.
Can AISI 4140 be welded?
Yes, but with significant precautions due to its 0.38-0.43% carbon content (high carbon equivalent). Preheat to 200-300°C (400-570°F), maintain interpass temperature, use low-hydrogen electrodes (E7018 or E10018-D2 for high strength), and perform post-weld stress relief at 595-650°C (1100-1200°F) for 1 hour per 25 mm thickness. Without proper preheat and PWHT, the heat-affected zone will form brittle martensite and crack — especially in restrained joints.
References & International Standards
- ASTM International. Standard Specifications for Steel & Metal Alloys. astm.org
- International Organization for Standardization (ISO). Metallic Materials — Cross-Reference Database. iso.org
- American Iron and Steel Institute (AISI). Steel Grade Designations & Equivalents. steel.org
- European Committee for Standardization (CEN). EN Steel Standards & Numbering System. cencenelec.eu