Heat Treatment for Machined Parts: Rough, Harden, Then Finish

A 4140 shaft comes back from the heat treater at 30 HRC, and the bearing journal that was turned to within 0.01 mm is now 0.06 mm out of round. The shop blames the treater, the treater blames the steel, and the schedule loses a week.

In most of these cases the sequence was simply wrong. Heat treatment is not a step you bolt onto the end of a process plan; it is a step that has to sit in a specific place, with a specific amount of material left on the part for it.

This article covers where that place is for the common alloys, how much movement to plan for, and the hardness level at which cutting stops being practical.

The sequence decision in one sentence

Rough machine, stress relieve if the part is thin or heavily pocketed, heat treat to final hardness, then finish machine only the surfaces that must be accurate.

Everything else is detail. Any thermal cycle that changes microstructure also changes volume, and on an asymmetric part that produces distortion. Cut to final size beforehand and you inherit it in your tolerance.

Rough, relieve, finish

A workable process plan for a hardened steel part looks like this:

1. Cut the blank with 1.5 to 3 mm of stock per surface on large parts, 0.5 to 1.0 mm on small ones.
2. Rough machine, leaving the stock allowance plus 0.2 to 0.4 mm of grind stock on critical fits.
3. Stress relieve if the part has removed more than about 40 percent of its section, or has thin walls under 3 mm.
4. Heat treat to the specified hardness.
5. Semi-finish, then finish. Grind or hard mill the fits that must hold 0.01 mm.

Stress relief for 4140 is typically 2 hours at 650 °C followed by slow furnace cooling. It costs little and relaxes the residual stress from roughing. On a thin, heavily pocketed part it is often the difference between a straight part and a banana.

How much movement to expect

Nitriding is the exception. It runs at 500 to 570 °C and introduces nitrogen by diffusion rather than a phase change, so distortion is almost nil, but the surface grows 5 to 15 µm as the lattice expands. That is enough to close a 10 µm bore tolerance and to require masking on threads.

Hardness decides the process

Carbide end mills cut comfortably up to about 45 HRC. Between 45 and 55 HRC, cutting is still possible with high-alumina or AlTiN-coated micrograin carbide, reduced chip loads and a very rigid setup, but tool life falls and the process becomes fragile.

Above 55 HRC, plan on grinding, wire EDM or die-sinking EDM. Trying to mill a 60 HRC carburized surface is not a shortcut; it is a way to burn through a dozen tools and scrap the part.

This is why the hardness call matters as much as the alloy. A part at 28 to 32 HRC can be finished by hard milling in the same setup family as the roughing. A part at 60 HRC needs a grinding house and typically an extra week.

For fits, hardened surfaces below 45 HRC can be milled to ±0.02 mm; the same surface at 58 HRC is normally ground to ±0.005 mm with a better finish.

Three alloys that behave differently

4140 is the standard pre-hardened mold and machine part steel. Supplied pre-hardened at 28 to 32 HRC, it machines well with carbide and needs no further treatment for many applications. If you need it harder, specify the tempering temperature rather than just "harden": 400 °C temper gives roughly 45 HRC, 540 °C gives roughly 35 HRC, 600 °C gives roughly 30 HRC.

17-4 PH is supplied in Condition A at about 30 HRC and ages to final properties in a single low-temperature step. H900 (1 hour at 480 °C) reaches 40 to 44 HRC; H1150 (4 hours at 620 °C) reaches 30 to 34 HRC with much better toughness. The aging shrink is predictable, 0.0004 to 0.0006 mm per mm, so program the allowance rather than discovering it.

Aluminum does not need heat treatment after machining unless it has been welded or formed. 6061-T6 welded in the heat-affected zone drops toward annealed strength, roughly 125 MPa against 310 MPa, and only a full solution treat and age cycle restores it. 7075-T6 plate carries residual stress from quenching, so heavy asymmetric pocketing will bow it; specify 7075-T7351 or 7050-T7451 when flatness matters.

Case hardening and nitriding specifics

Carburizing diffuses carbon into a low-carbon steel to a depth of 0.5 to 1.5 mm, then quenches to 58 to 62 HRC at the surface with a tough core. Distortion is significant and not fully predictable, so leave 0.8 to 1.5 mm of stock plus grind allowance, and never carburize a part with finished threads.

Nitriding produces a very hard, wear-resistant case without a phase change, which is why it is used on gear teeth, guide rails and extrusion screws. Decide before machining which surfaces must be protected: threads, splines and fits are usually masked with copper plating. The white compound layer is brittle, and on fatigue-loaded parts it may need a light grind.

What to put on the drawing

State the process, the hardness and where it applies, plus the sequence. A note such as "4140, harden and temper to 30 to 34 HRC, stress relieve after rough machining, grind bearing journals after heat treatment" gives the shop enough to plan properly.

Vague notes like "heat treat" push the decision onto the shop, which will choose whatever is cheapest.

Say whether dimensions apply after heat treatment. A ±0.01 mm bore and a 58 HRC case with no operation sequence is a conflict waiting to happen.

When we quote parts like this at JSHS, we sequence the operations and price heat treatment as part of the routing rather than a line item added later, and the quote notes where the tolerance is held. If you already have a heat treat specification, send it with the model and we will flag any conflict with the geometry.

Frequently asked questions

Should I machine before or after heat treatment?

Rough machine before, finish after. Final size before hardening means inheriting distortion you cannot remove. Leave 0.5 to 1.5 mm depending on section, then finish the critical surfaces once hard.

How much distortion does 4140 have after hardening?

Typically 0.1 to 0.3 mm on a 200 mm part, more if the section is uneven or the part is long and slender. Stress relief after roughing reduces it, and leaving adequate stock makes it recoverable.

Can hardened steel be machined?

Yes up to about 55 HRC with carbide, and up to 65 HRC with ceramic or CBN tooling at low speeds. At 58 to 62 HRC, grinding or EDM is usually cheaper and more accurate than milling.

Does nitriding change part dimensions?

The surface grows by roughly 5 to 15 µm because nitrogen expands the lattice. That is small but not zero, so bores with tight fits and threads should be masked, and final machining should be completed before nitriding.