AISI 4140 Chromoly Steel (UNS G41400): Heat Treatment Guide
ASTM A29/A322 · Updated: 2026-05-25
AISI 4140 is a medium-carbon chromium-molybdenum alloy steel (0.38-0.43% C, 0.8-1.1% Cr, 0.15-0.25% Mo) offering an exceptional combination of strength, toughness, and wear resistance after heat treatment. In the quenched and tempered condition at 28-32 HRC, it delivers up to 655 MPa yield strength with good ductility—making it the workhorse alloy for automotive axles, crankshafts, drilling tools, bolts, and heavy-duty machinery. It can be through-hardened in sections up to 60 mm. Welds with preheat (200-300°C) and post-weld stress relief using low-hydrogen electrodes.
Quick Facts
| Category | Alloy Steel |
| Standard | ASTM A29/A322 |
| Density | 7.85 g/cm³ |
| Yield Strength | 655 MPa (95 ksi) Q&T |
| Tensile Strength | 930 MPa (135 ksi) Q&T |
Global Equivalents & Cross-Reference
| Alternative Standard / Grade | Action |
|---|---|
| EN 42CrMo4 | Compare |
| DIN 1.7225 | Compare |
| SCM440 | Compare |
| 42CrMo4 | Compare |
| BS 708M40 | Compare |
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Frequently Asked Questions
What is the difference between 4140 and 4340?
4340 contains 1.65-2.0% nickel in addition to Cr and Mo, dramatically improving low-temperature toughness and through-hardening depth (100+ mm sections). 4340 is used for aircraft landing gear, heavy shafting, and critical fasteners. 4140 is more economical (~30% lower cost) and sufficient for most industrial applications.