SS · stainless
Stainless Steel 420
Martensitic and hardenable to about 50 HRC, which is the point of it. Machines reasonably in the annealed state; after hardening it is a grinding job.
Quoting this material
- Typical lead time
- 10–15 working days
- Condition
- Annealed; harden and temper to 40–52 HRC
- Achievable finish
- Ra 1.6 µm annealed typical · Ra 0.2 µm (polished) 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.
Martensitic and hardenable to about 50 HRC, which is the point of it. Machines reasonably in the annealed state; after hardening it is a grinding job.
Properties as supplied
| Parameter | Value | Unit |
|---|---|---|
| Tensile strength (hardened) | 1,450 | MPa |
| Yield strength (hardened) | 1,250 | MPa |
| Elongation (hardened) | 8 | % |
| Hardness (max) | 50 | HRC |
| Thermal conductivity | 24.9 | W/m·K |
| Modulus of elasticity | 200 | 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; harden and temper to 40–52 HRC
Heat treatment
Harden by austenitising at roughly 980–1,050 °C, oil or air quench, then temper. Higher tempering gives toughness at the cost of hardness; a 50 HRC part is genuinely hard and a 40 HRC part is much tougher.
Achievable surface finish
Ra 1.6 µm annealed
Ra 0.2 µm (polished)
Polishes to a high gloss, which is why it is a cutlery and surgical-instrument grade; corrosion resistance improves as the surface gets smoother.
Tooling and cutting parameters
Cutting speed
- 120–200 m/min annealed (carbide) · 50–80 m/min hardened (CBN/ceramic)
Feed
- 0.10–0.25 mm/rev turning · 0.06–0.15 mm/tooth milling
Tooling
- Coated carbide annealed; CBN or ceramic for the hardened condition
Coolant
- Flood throughout, but never quench a hot part in coolant to speed the cycle
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
- Surgical and dental instruments
- Cutlery and blades
- Valve trim and seats
- Wear plates and guides
- Mould and die inserts
DFM notes for this grade
- Harden then finish: at 50 HRC the final dimension is a grinding or hard-turning operation, and the allowance for it has to be on the drawing
- Corrosion resistance is the weakest of the grades here - it is a hardenable steel that happens to be stainless, not a corrosion material
- Sharp internal corners in a 50 HRC part are a cracking risk in the quench; radius them in the pre-hardened geometry
Questions buyers ask
- What hardness should I specify for a wear part in 420?
- Enough to resist the wear mechanism and no more, because toughness falls as hardness rises. For a sliding guide, 45–48 HRC is often the balance. For an instrument blade, 50–52 HRC. A 52 HRC part is brittle and will chip at a sharp edge under impact.
- Is 420 stainless enough for a wet environment?
- It is the least corrosion-resistant stainless we machine regularly. In fresh water with regular cleaning it is acceptable; in chlorides, a 316L or 17-4 PH part will outlast it substantially, and martensitic grades pit badly once the passive layer breaks.
Grades we would quote instead
416
6/10The martensitic free-machining grade - the best-machining stainless on this list and the one that hardens after machining, unlike the austenitics.
17-4 PH
4/10In the annealed condition it machines like a tough 304; in H900 it is hard enough that finishing has to be done by grinding or hard turning.
D2
4/10Annealed it machines reasonably but it is abrasive on the tool because of the chromium carbides.