4140 is the workhorse alloy steel for shafts, pins, gears, spindles and tooling. It is strong and tough, it can be heat treated across a wide range, and in pre-hard condition it can be machined into a finished part without a trip to heat treat. Machine shops, oilfield fabricators and equipment repair shops use it wherever mild steel wears out or bends. We source 4140 round and flat bar, and plate on request, and arrange saw cutting to length.
What 4140 steel is
4140 is a chromium-molybdenum steel, often called chromoly. It carries roughly 0.38–0.43% carbon, 0.8–1.1% chromium and 0.15–0.25% molybdenum. The chromium and moly let it harden deeper and more evenly than a plain carbon steel like 1045, and add toughness at high strength.
Bar is commonly supplied to ASTM A29 or A322 chemistry. 4142 is a near twin with slightly higher carbon, and the two are often interchangeable in practice.
4140 properties
| Property | Value | What it means in the shop |
|---|---|---|
| Tensile, annealed | About 95 ksi typical | Soft enough to machine easily, then heat treat |
| Yield, annealed | About 60 ksi typical | Already well above A36 |
| Hardness, annealed | About 197 Brinell | Machines like a tougher medium-carbon steel |
| Hardness, pre-hard (Q&T) | 28–32 HRC typical | Tough and wear resistant; machinable with carbide |
| Tensile, pre-hard | About 140 ksi typical | Roughly three times the yield of mild steel |
| Density | 0.284 lb/in³ (7.85 g/cm³) | Same as carbon steel |
| Weldability | Difficult | Needs preheat and slow cooling to avoid cracking |
| Corrosion resistance | Low | Rusts; oil, plate or coat it |
Typical values. Pre-hard properties vary by bar size and supplier; the cert shows hardness.
Typical uses
- Shafts, axles, spindles and arbors
- Pins, bushings, couplings and gears
- Oilfield subs, collars, adapters and tool bodies
- Die holders, fixtures and tooling parts
- Hydraulic cylinder rods and high-strength fasteners
Common conditions
Annealed 4140 is soft and easy to machine; you rough and finish it, then heat treat to the hardness the part needs. It quenches to high hardness and is usually tempered back, and it can be flame or induction hardened on the surface.
Pre-hard, also called quenched and tempered or HT, is heat treated at the mill to about 28–32 HRC. You machine the finished part and skip heat treat, which saves time and avoids distortion. Turned, ground and polished (TGP) bar holds tight diameter tolerance for shafts, and chrome-plated 4140 is common for cylinder rods.
4140 vs 4340
4340 adds about 1.65–2% nickel and a bit more chromium and moly. That gives it deeper hardenability, so large diameters harden through to the center, and better toughness at the same hardness. It is specified for heavy shafts, landing gear, high-impact tooling and critical oilfield parts. 4340 typically costs more because of the nickel. For most shop parts under a few inches in diameter, 4140 does the job.
| 4140 | 4340 | |
|---|---|---|
| Main alloying | Chromium, molybdenum | Nickel, chromium, molybdenum |
| Hardenability | Good | Deeper; better in large sections |
| Toughness at high hardness | Good | Better |
| Machinability, annealed | Good | Slightly harder to cut |
| Typical use | General shafts, pins, tooling | Heavy-duty and critical parts |
Welding and machining 4140
4140 can be welded, but it has to be done right. Its carbon and alloy content make the heat-affected zone harden and crack if it cools fast. Preheat, commonly in the 400–600 °F range depending on section size, use low-hydrogen filler, hold interpass temperature, cool slowly and stress relieve when the part allows. Pre-hard 4140 loses hardness near the weld. If a shaft needs a welded-on feature, ask whether 1045 or a bolted design would do.
Annealed 4140 machines well with HSS or carbide. Pre-hard at 28–32 HRC is best cut with carbide, with a rigid setup and steady feed. Plan to finish pre-hard parts in the same condition you bought them; a heat treat after machining will move them.