Machining hardened steel and specialty alloys

The sequence decision

Every hardened part has at least two valid process routes, and they have completely different cost curves.

Machine soft, then harden. Annealed tool steel runs at 80–150 m/min with ordinary coated carbide. Cheap tooling, fast cycles, complex geometry is straightforward. The cost is that heat treatment moves the part — and the risk is that quench cracking or distortion beyond allowance destroys it after most of the machining money has already been spent.

Grade	Typical dimensional change through hardening
A2	±0.05 % — low, which is its selling point
D2	−0.06 % to +0.10 %, varies with section
O1	+0.10 % to +0.20 %, and distortion-prone
H13	±0.05 % to 0.10 %
M2	+0.05 % to +0.15 %

On a 300 mm die plate, 0.1 % is 0.3 mm. That has to be left as grind stock on every critical feature, with a finishing operation afterwards.

Buy hard, machine hard. No distortion after the fact, one operation, dimensions are what you measured. Slower cycles, CBN tooling, higher hourly rate — and a hard ceiling around 65 HRC where turning and milling give way to grinding and EDM.

Hybrid, which is what most real parts do: rough soft, harden, finish hard or grind. An extra setup, and it removes most of the risk from both routes.

Hard turning against grinding

For rotational features above 45 HRC, CBN has taken a great deal of work away from grinders — but not all of it, and the threshold is worth knowing.

	Hard turning	Grinding
Practical hardness	45–65 HRC	to 70 HRC and beyond
Achievable roundness	2–5 µm	under 1 µm
Typical finish	Ra 0.2–0.8 µm	Ra 0.05–0.4 µm
Material removal rate	3–5× grinding	baseline
Setups on a multi-feature part	often one	often several

The single-setup argument is usually the real reason to hard turn, and it is not about cycle time. A hardened shaft with two journals and a bore, ground conventionally, needs a cylindrical grinder, an internal grinder and careful re-datuming between them — and each re-chuck introduces concentricity error. Hard turned, every feature comes off one chucking and the runout between them comes free.

Where grinding still wins: tolerance below about IT5, finish under Ra 0.2 µm, hardness above 65 HRC, or geometry a lathe cannot reach.

Condition codes are the specification

Precipitation-hardening stainless is the family buyers most often under-specify. A drawing saying “17-4 PH” and nothing else omits the decision that controls strength, toughness, corrosion behaviour and machining cost.

Condition	Hardness	Yield	Character
A (solution annealed)	~38 HRC	~760 MPa	Supply condition, not a service condition
H900	40–47 HRC	~1170 MPa	Strongest, most notch-sensitive
H1025	35–40 HRC	~1000 MPa	Balanced, common choice
H1150	28–33 HRC	~795 MPa	Toughest, best stress-corrosion resistance

H900 gets specified by default because it is the strongest, and it is also the most susceptible to stress corrosion cracking in chlorides. For anything seeing seawater, sour gas or cyclic loading with stress raisers, H1025 or H1150 is usually correct even though the datasheet number looks worse.

The specialty alloys

Some materials in this bracket are hard to machine for reasons that have nothing to do with hardness.

Invar 36 and Kovar are nickel-iron alloys bought for dimensional stability and glass-sealing respectively. They are soft, gummy, and the real work is the stress-relief sequence — buying Invar and then machining it carelessly wastes the reason you bought it.

Molybdenum and tungsten have a ductile-to-brittle transition at or above room temperature, so the edge propagates a crack rather than shearing a chip. Chips come off as powder, edges break out at cut exits, and interrupted cuts are genuinely dangerous.

Tantalum and niobium are the opposite — extraordinarily ductile, smearing rather than cutting, welding to the tool.

For all of them, material cost usually dominates machining cost, which inverts the normal optimisation: it is worth spending cycle time to reduce scrap risk.

What to write on the drawing

Material: AISI A2 tool steel, supplied annealed. Harden and temper to 58–60 HRC after rough machining. Finish grind features marked G after heat treatment. Hardness verified on the finished part, three locations.

Grade, supply condition, target hardness, sequence, acceptance test. “D2, hardened” leaves all five open, and every open question widens the quote.

Also state whether white layer — the thin re-hardened, brittle surface a worn CBN tool can leave — is acceptable or must be checked. That turns a vague quality expectation into a measurable one.

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