CS · carbon steel
Carbon Steel 8620 (case hardening)
Low 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.
Quoting this material
- Typical lead time
- 12–18 working days
- Condition
- Annealed or normalised for machining; carburised and hardened after
- Achievable finish
- Ra 1.6 µm (pre-hardening) typical · Ra 0.4 µm (ground after hardening) 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.
Low 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.
Properties as supplied
| Parameter | Value | Unit |
|---|---|---|
| Tensile strength (core, Q&T) | 650 | MPa |
| Yield strength (core) | 490 | MPa |
| Elongation | 20 | % |
| Case hardness | 58–62 | HRC |
| Thermal conductivity | 46.6 | 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 or normalised for machining; carburised and hardened after
Heat treatment
The carburising steel: hold in a carbon-rich atmosphere at around 900–930 °C until the case reaches the specified depth, then quench and temper. Case depth is a design decision - 0.5–1.0 mm for a gear tooth, 2–4 mm for a heavily loaded surface.
Achievable surface finish
Ra 1.6 µm (pre-hardening)
Ra 0.4 µm (ground after hardening)
The case is ground after hardening; a carburised surface is not a finished surface until it has been.
Tooling and cutting parameters
Cutting speed
- 150–280 m/min (carbide)
Feed
- 0.15–0.30 mm/rev turning · 0.08–0.20 mm/tooth milling
Tooling
- Coated carbide - it cuts like 1018 before hardening
Coolant
- Flood or mist
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
- Gears and pinions
- Camshafts and rocker components
- Wear pins and bearing surfaces
- Automotive driveline parts
- Heavy-duty fasteners
DFM notes for this grade
- Case depth is a specification, not a shop default: a tooth that is carburised too deep becomes brittle through its full section, and one too shallow wears through quickly
- Do not grind through the case: the part needs a defined allowance and the grinding has to be planned against the case depth
- Where the case is unwanted - a bore that must stay soft - masking or leaving stock that is later bored is required
Questions buyers ask
- Why not just harden 4140 through instead of carburising 8620?
- Because a gear tooth wants the opposite of uniform hardness: a hard wear surface and a tough core that absorbs impact without cracking. Through-hardening 4140 gives uniform hardness, and at 55 HRC that is brittle through the section.
- How deep should the case be?
- It scales with the load and the contact geometry: typically 0.5–1.0 mm for a small gear tooth, 1.0–2.0 mm for a heavily loaded one, up to 3–4 mm for a large bearing surface. Send the load case and we will confirm the depth against it rather than guessing.