CS · carbon steel
Carbon Steel 4340 (40NiCrMo)
Tougher and harder than 4140 at the same strength, and it machines accordingly - expect lower speeds, shorter tool life and a real grinding allowance after heat treatment.
Quoting this material
- Typical lead time
- 12–18 working days
- Condition
- Annealed, or quenched and tempered to the specified hardness
- Achievable finish
- Ra 1.6 µm annealed typical · Ra 0.4 µm best
Machined in-house to your drawing: see the precision CNC machining service and the machine and inspection capability it runs on.
Machinability
Our own 1–10 shop ranking, not a published index: 10 is free-cutting brass C360, 1 is Inconel 718. It ranks how a material behaves on the floor - cycle time, chip control and tool life together. Where a published index exists it is quoted in the note.
Tougher and harder than 4140 at the same strength, and it machines accordingly - expect lower speeds, shorter tool life and a real grinding allowance after heat treatment.
Properties as supplied
| Parameter | Value | Unit |
|---|---|---|
| Tensile strength (Q&T) | 1,150 | MPa |
| Yield strength (Q&T) | 1,000 | MPa |
| Elongation (Q&T) | 12 | % |
| Hardness (annealed) | 230 | HB |
| Thermal conductivity | 38 | W/m·K |
| Modulus of elasticity | 205 | GPa |
Hardness, tensile and thermal figures are typical published values for the condition stated. Actual mill certificates govern; send the specification with the drawing and we confirm against the certificate before the material is cut.
Condition
Annealed, or quenched and tempered to the specified hardness
Heat treatment
Deep-hardening nickel-chromium-molybdenum steel. Oil quench from 830–860 °C, temper to the required strength and toughness. The nickel gives it the notch toughness that 4140 lacks at high strength - that is why it is used for the parts where a crack means a failure, not a repair.
Achievable surface finish
Ra 1.6 µm annealed
Ra 0.4 µm
At the delivered hardness the finishing pass is a grind; plan for it and put the allowance on the drawing.
Tooling and cutting parameters
Cutting speed
- 100–180 m/min annealed (carbide) · 60–90 m/min at 38 HRC
Feed
- 0.12–0.25 mm/rev turning · 0.06–0.15 mm/tooth milling
Tooling
- Tough coated carbide; CBN for hard turning
Coolant
- High-pressure flood
Starting points for carbide on a rigid setup, not a guarantee. The right numbers depend on the machine, the toolholder and the fixturing - ask and we will say what we would actually run.
Typical applications
- Aircraft landing gear and structural fittings
- High-load shafts and axles
- Heavy-duty gear and pinion blanks
- Pressure vessel and tooling components
- Marine propeller shafting
DFM notes for this grade
- The notch toughness that justifies the price is destroyed by a sharp corner or a poor surface finish - radii and finish are functional on this material
- Nitriding or hard chrome after heat treatment is common; both need the pre-treatment geometry to allow for the case
- Cost is roughly 1.5–2× 4140 plus a longer cycle - specify it because the load demands it, not as a safety margin
Questions buyers ask
- When is 4340 worth the premium over 4140?
- When the part is fatigue- or impact-critical at high strength. 4340 keeps its toughness at 1,100 MPa where 4140 becomes notch sensitive - for a landing-gear component, a high-load axle or a part where a crack is unacceptable, that difference is the whole justification.
- Can 4340 be welded?
- Technically with a controlled procedure: preheat, an appropriate filler, and post-weld heat treatment. In practice it is avoided on the parts that need 4340 in the first place. Welding a 4340 part usually means the joint should have been a forging or a bolted connection.