CS · carbon steel
Carbon Steel 4140 (42CrMo4)
The workhorse alloy steel. Published index around 66% annealed against B1112 = 100%, dropping as the delivered hardness rises. Quenched and tempered to 28–32 HRC it is a grinding job for the finish passes.
Quoting this material
- Typical lead time
- 7–14 working days
- Condition
- Annealed, normalised, or quenched and tempered to the required 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.
The workhorse alloy steel. Published index around 66% annealed against B1112 = 100%, dropping as the delivered hardness rises. Quenched and tempered to 28–32 HRC it is a grinding job for the finish passes.
Properties as supplied
| Parameter | Value | Unit |
|---|---|---|
| Tensile strength (Q&T) | 1,020 | MPa |
| Yield strength (Q&T) | 655 | MPa |
| Elongation (Q&T) | 15 | % |
| Hardness (annealed) | 197 | HB |
| Thermal conductivity | 42.6 | 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, normalised, or quenched and tempered to the required hardness
Heat treatment
The standard heat-treatable steel: austenitise around 840–870 °C, oil quench, temper to the required strength. Chromium and molybdenum give it deep hardenability, so a 100 mm section hardens through rather than only on the skin - that is the difference from 1045.
Achievable surface finish
Ra 1.6 µm annealed
Ra 0.4 µm
In the annealed state it machines to a fine finish. Supplied pre-hardened, the final pass is a grind or a hard turn with CBN.
Tooling and cutting parameters
Cutting speed
- 120–250 m/min annealed (carbide) · 80–120 m/min at 30 HRC (CBN or coated carbide)
Feed
- 0.15–0.30 mm/rev turning · 0.08–0.20 mm/tooth milling
Tooling
- Coated carbide annealed; CBN or ceramic for the hardened condition
Coolant
- Flood, and more of it as the hardness rises
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
- Drive shafts and axles
- Gears, pinions and spline shafts
- High-tensile studs and bolts
- Jigs and fixture bodies
- Hydraulic cylinder rods and glands
DFM notes for this grade
- Order the delivered hardness deliberately: 4140 at 28–32 HRC machines very differently from the annealed condition, and the machining allowance for the final grind follows from that choice
- It can be welded with preheat and post-weld heat treatment, but a welded 4140 joint is a controlled process, not a shop convenience
- A stress concentration in a heat-treated 4140 part is more dangerous than in a mild steel: fillet radii matter more once the material is strong
Questions buyers ask
- Should 4140 be machined before or after heat treatment?
- Both, in sequence. Rough in the annealed or pre-hardened state, heat treat, then finish the critical features. Trying to rough at 32 HRC wastes tool life; trying to finish in the annealed state means the part moves when it is later hardened.
- Where does 4140 stop being the right answer?
- When the section is very large and the core properties matter, or when the load is high enough to need 4340. 4140 is the default for shafts and gears up to a point; beyond about 150 mm diameter with through-hardening required, or where notch toughness matters, the answer moves to 4340.
Grades we would quote instead
4340
5/10Tougher 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.
1045
6/10Medium carbon: the first grade on this list that actually hardens.
8620
7/10Low carbon and soft in the delivered state, so it machines like a mild steel - then it is carburised and hardened to give a hard case over a tough core.