Shafts & axles
Drive shafts, splined shafts, axles. The combination of strength, toughness, and machinability is hard to beat.
Cr-Mo alloy steel · UNS G41400
4140 Alloy Steel
The everyday alloy steel. Chromium-molybdenum chemistry gives it strength, toughness, and consistent response to heat treatment — at a fraction the cost of stainless or titanium. Standard for shafts, gears, axles, hydraulic cylinders, and machined components that need real strength.
UTS (prehard.)
950 MPa
Density
7.85 g/cc
Modulus
200 GPa
Mach.
65 /100
When to use
Pick 4140 when corrosion isn’t critical and strength is.
4140 is the bread-and-butter alloy steel. It comes in two main forms: prehardened (28–32 HRC, ready to machine) and annealed (soft for machining, then heat-treated to higher hardness afterward). The prehardened condition is most common for production work — it ships ready to cut and gives a 950 MPa UTS without further heat treatment.
For higher-strength applications, 4140 can be quenched and tempered to 50+ HRC (UTS around 1750 MPa at 50 HRC). It’s a workhorse — but it will rust without protection. Pair with paint, plating, or oil for any environment that sees moisture.
Strengths
Why it’s the default alloy steel.
- CostAbout 1/3 the price of stainless and roughly 1/15 the price of titanium per pound. Cheap stock, cheap chips.
- Predictable heat treatDecades of metallurgical data. Hardenability charts, tempering curves, response to oil quench — all very well-known.
- Wide hardness rangeFrom annealed (~197 HB, below the HRC scale) through prehardened (28–32 HRC) to fully hardened (50+ HRC). One alloy covers a lot of applications.
- Tough at strengthCr-Mo balance keeps it tough at high strength — won't shatter the way 1095 carbon steel might.
- Excellent machinability prehardenedMach 65/100 in prehardened condition. Better than 316L stainless.
Trade-offs
What it can’t do.
- Will rustNo corrosion resistance whatsoever. Bare 4140 in humid environments oxidizes within weeks.
- Heat-affected by weldingHAZ is brittle. Weldable with preheat (200 °C+) and post-weld stress relief, but not casual welding.
- Requires post-machining HT for max strengthFully hardened condition needs heat treat after rough machining — adds setup, distortion, and grinding cost.
- Hydrogen embrittlementSame as 17-4PH — bake-out required after acid pickling or plating, especially at higher hardness.
- Not for high-tempTempering temp limits service to ~370 °C (700 °F). For hotter, look at H13, A2, or step up to Inconel.
Specs
Mechanical, physical & chemical data.
Properties shown for prehardened (typically Q&T to 28–32 HRC) condition — the most common purchase form for machining bar.
Mechanical
Properties (prehardened)
| Ultimate tensile strength | 950 MPa (138 ksi) |
|---|---|
| Yield strength (0.2% offset) | 860 MPa (125 ksi) |
| Elongation at break | 18% |
| Reduction of area | 55% |
| Modulus of elasticity | 200 GPa (29.0 Msi) |
| Shear modulus | 76 GPa (11.0 Msi) |
| Poisson's ratio | 0.32 |
| Hardness | 28–32 HRC |
| Charpy V-notch impactat 21 °C | 55 J |
Source: ASM Handbook Vol. 1; Carpenter datasheet. Fully hardened (Q&T to 50 HRC): UTS 1750 MPa, YS 1500 MPa, elongation 12% (UTS tracks hardness; same as 4340 at 50 HRC).
Physical & thermal
Properties at room temp
| Density | 7.85 g/cc (0.284 lb/in³) |
|---|---|
| Melting range | 1416 – 1455 °C |
| Specific heat | 0.473 J/g·°C |
| Thermal conductivity | 42.6 W/m·K |
| CTE (20–100 °C) | 12.3 µm/m·°C |
| Electrical resistivity | 0.22 µΩ·m |
| Magnetic | Ferromagnetic |
| Critical temp (Ac1)Lower critical — softens above | 757 °C |
Standard ferrous-alloy thermal behavior — heat dissipates well during cutting, unlike titanium or Inconel.
Composition
Chemistry, weight %
| Carbon (C) | 0.38 – 0.43 |
|---|---|
| Manganese (Mn) | 0.75 – 1.00 |
| Chromium (Cr) | 0.80 – 1.10 |
| Molybdenum (Mo) | 0.15 – 0.25 |
| Silicon (Si) | 0.15 – 0.35 |
| Phosphorus (P) | 0.035 max |
| Sulfur (S) | 0.04 max |
| Iron (Fe) | balance |
The 0.40% carbon is what enables hardening. The Cr-Mo combination provides hardenability and tempering resistance.
Specifications
Common purchase specs.
AISI 4140
Standard SAE / AISI designation
AMS 6382
Bar, forgings, rings — annealed (aircraft quality)
AMS 6395
Sheet, strip, plate
ASTM A29
Hot-wrought carbon and alloy steel bars
ASTM A434
Steel bars — hot-finished, heat-treated
DIN 42CrMo4 / 1.7225
European equivalent
Machining
Machining considerations.
Prehardened 4140 cuts beautifully — chips break short, surface finish comes off clean, tools last well. Annealed 4140 is even easier but produces stringy chips that need management.
Cutting speed
200–300 SFM with carbide in prehardened. 350–450 SFM annealed.
Feed rate
Aggressive. Cr-Mo balance lets you push without work-hardening.
Tooling
Coated carbide (TiAlN) for production. HSS works fine for prototypes.
Coolant
Flood is standard. Chip control good enough for some shops to run with MQL.
Surface finish
32 µin Ra easily; 16 µin with sharp finishing tool. Cleaner finish than stainless.
Threading
Cuts taps, rolled threads, single-point all work. Cut taps have tendency to chip out — sharp tools.
Drilling
L:D up to 5:1 routine. Beyond that use peck cycles or coolant-fed drills.
After heat treat
If quenched and tempered above prehardened, finish-grind for tight tolerances. Distortion is real but predictable.
Applications
Where 4140 shows up.
Hydraulic & pneumatic
Cylinder rods, pistons, valve bodies. Hard chrome plating gives 4140 the surface durability for sealed dynamic service.
Gears & gear blanks
Standard low-cost gear material. Q&T to 28-35 HRC, then induction-harden teeth for surface durability.
Compare
Related materials.
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