The High-Strength Workhorse Steel
4140 is a chromium-molybdenum "chromoly" alloy steel — the default when a part needs to be much stronger and tougher than mild steel but doesn't justify an exotic alloy. Its great advantage is that it's heat-treatable: we can machine it in the soft (annealed) or pre-hardened condition, then quench-and-temper it to the strength the job needs. That makes it the backbone material for shafts, axles, gears, couplings, spindles and tooling.
4140 Properties — Annealed vs Quenched & Tempered
| Property | Annealed | Q&T (~28–32 HRC) |
|---|---|---|
| Ultimate tensile strength | ~655 MPa | ~950–1020 MPa |
| Yield strength | ~415 MPa | ~830–900 MPa |
| Elongation | ~25% | ~16–18% |
| Hardness | ~197 HB | ~28–32 HRC |
| Density | 7.85 g/cm³ | |
| Elastic modulus | ~205 GPa | |
| Machinability | ~55–65% (annealed) — good for an alloy steel | |
4140 can be tempered higher (≈40–50 HRC) for wear applications, depending on section and temper. Values are representative; we work to the certified heat-treat and mill spec and provide certs on request.
4140 Chemical Composition
4140 is a chromium-molybdenum (Cr-Mo) low-alloy steel — chromium gives hardenability and wear resistance, molybdenum adds strength and fatigue resistance. Nominal composition (balance iron):
| Element | Content | Element | Content |
|---|---|---|---|
| Carbon (C) | 0.38–0.43% | Chromium (Cr) | 0.80–1.10% |
| Manganese (Mn) | 0.75–1.00% | Molybdenum (Mo) | 0.15–0.25% |
| Silicon (Si) | 0.15–0.35% | Phosphorus (P) | ≤0.035% |
| Sulphur (S) | ≤0.040% | Iron (Fe) | Balance |
International equivalents: 42CrMo4 / 1.7225 (EN), SCM440 (JIS), 708M40 / EN19 (BS) — useful when cross-referencing a discontinued part's material.
4140 Heat Treatment & Tempering Chart
The reason 4140 is so widely used is its heat-treat range: the same steel covers everything from easily-machined soft bar to hard, wear-resistant gears, just by choosing the temper. Typical reference values (vary with section size):
| Process / temper | Temperature | Resulting hardness |
|---|---|---|
| Annealing | 840–870 °C, slow cool | ≤197 HB (softest, easiest to machine) |
| Normalising | 870–900 °C, air cool | ~250–300 HB |
| Harden (austenitise + quench) | 845–870 °C, oil quench | ~55–58 HRC (as-quenched) |
| Temper @ 205 °C | 205 °C | ~52–57 HRC (max strength) |
| Temper @ 425 °C | 425 °C | ~38–42 HRC (tough, high-strength) |
| Temper @ 540 °C | 540 °C | ~32–38 HRC (general parts) |
| Temper @ 620 °C | 595–650 °C | ~28–32 HRC (impact-loaded, "T" condition) |
For most parts the sweet spot is 28–34 HRC — high strength with good toughness and still machinable. Above ~45 HRC we hard-turn or grind rather than mill.
Machining & Heat-Treat Sequence
The order of operations is the craft with 4140:
- Machine soft, then harden: we rough and semi-finish in the annealed/pre-hardened state, leaving grinding stock on critical features.
- Heat treat to the target hardness (quench & temper, or nitride for a hard wear case).
- Finish-grind bearing journals, bores and critical diameters after hardening for final tolerance and finish — see tolerance & inspection.
- Pre-hardened (Q&T) bar is often the practical choice when ~30 HRC is enough and post-machining heat treat isn't wanted.
Cutting Parameters by Condition
Machinability drops as hardness rises, so the cut is matched to the condition:
| Condition | Machining approach |
|---|---|
| Annealed (~197 HB) | Cuts well — carbide tooling, moderate speeds, the right state for heavy metal removal before hardening |
| Pre-hardened / Q&T (28–34 HRC) | Rigid setup, coated carbide, reduced speed & positive feed; our usual finish-machining state |
| Hard (40–45 HRC) | Hard-turning / milling with CBN or hard-grade carbide, light cuts |
| Very hard (>45 HRC) | Grind or EDM critical features rather than mill |
Most "4140" pages give the heat-treat chart but skip this — knowing how each hardness actually cuts is what lets us plan the soft-machine → harden → finish-grind sequence correctly.
Finishing & Corrosion Protection
4140 is not stainless, so unprotected it will rust. Common protective finishes:
- Black oxide — mild corrosion resistance, dark finish
- Zinc or manganese phosphate — corrosion protection, lube retention
- Hard chrome plating — wear and corrosion on shafts/rods
- Nitriding — hard, wear-resistant case without quenching distortion
4140 vs 4340 — Quick Comparison
| 4140 | 4340 | |
|---|---|---|
| Key addition | Cr-Mo | Cr-Mo + nickel |
| Strength in heavy sections | High | Higher (better hardenability) |
| Toughness | Very good | Excellent |
| Cost | Lower | Higher |
| Best for | Most high-strength parts | Heaviest-duty / aerospace shafts |
For most high-strength shaft, gear and tooling work, 4140 is the economical and fully capable choice; step up to 4340 for the heaviest sections and highest loads.
Typical 4140 Parts
- Drive shafts, axles and spindles — e.g. our deep-hole precision shafts and reverse-engineered drive shaft
- Gears, pinions and couplings — see steel spur gear batch
- Hydraulic bodies, pistons and rods
- Jigs, fixtures and tooling
- High-strength fasteners and studs
Frequently Asked Questions
High-strength, high-toughness parts: shafts, axles, gears, couplings, spindles, hydraulic bodies and tooling. It's heat-treatable, so it's machined soft then hardened to the needed strength.
Usually machine in the annealed or pre-hardened (~28–32 HRC) state, then heat treat if higher hardness is needed, leaving grinding stock on critical features that are finished after hardening.
Roughly 28–34 HRC for general high-strength use, and ~40–50 HRC for wear applications depending on section and temper. We can also nitride for a hard wear case.
4340 adds nickel for higher strength and toughness in heavy sections — best for the most demanding shafts. 4140 is more economical and adequate for most high-strength parts.
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