Where does heat treatment fit in a CNC machining sequence?

The short answer

The default route: rough, treat, finish

Heat treatment is a stage inside the machining sequence, not a service bolted onto the end of it. Roughing removes most of the material while it is cheapest to cut and leaves stock on the faces that have to end up accurate; the treatment then hardens the part, or relieves the stress that roughing put into it, and moves the geometry a little; finish machining or grinding then brings the part to its final size, so the movement is removed rather than inherited. A 4140 plate finish-milled at 30 HRC and then quenched to 45 HRC will typically move 0.1 to 0.3 mm across a 300 mm length, which is far outside any fine tolerance. Where the part is thin-walled, long, wide or asymmetric, an intermediate stress relief between roughing and semi-finishing is often added, because it stabilises the part before the final cuts are taken. The whole plan, including allowance, fixturing and inspection, should be agreed before the first cut, not after the first out-of-tolerance report.

Six treatments and where each one sits

Annealing and normalising are done on the stock, before any cutting, because they soften the material and make it machinable; if the mill already supplies the grade in an annealed, normalised or pre-hardened condition that matches the drawing, the step can be skipped and the lead time with it. Stress relief goes after roughing and before finishing, holds the part at roughly 550 to 650 degrees Celsius to let the internal stresses redistribute, and is the cheapest insurance there is against a part that creeps out of flatness after delivery. Through hardening, quench and temper, is placed after roughing and before finishing so the quench distortion can be machined away; carburising follows the same logic and ends with a grinding or hard-turning operation to clean up the case. Nitriding and induction hardening are shallow and move a part much less, but they still belong before the final finishing pass, because the case itself cannot be cut back afterwards without losing the property you paid for. For aluminium, solution treatment with artificial aging is the equivalent step, and 2xxx, 6xxx and 7xxx grades all respond to it.

How much metal moves, and how much stock to leave

Distortion cannot be eliminated, because hardening involves a phase change and a release of residual stress, so the practical aim is to budget for it. On a small, well-supported part the movement after treatment is commonly held within about 0.05 mm. On a long or asymmetric part it is much larger, which is why a calculated allowance of roughly 0.2 to 0.5 mm is left on the critical surfaces before hardening and removed afterwards by grinding or hard turning. The allowance has to be large enough to clean up the post-treatment shape and small enough not to waste cutting time, and it is worth telling the heat treater how the part will be supported and quenched, because the racking position and the quench direction move the result as much as the furnace does. If a part repeatedly fails on flatness, the fix is usually a stress relief cycle and a support change rather than a tighter tolerance.

Hardness decides whether you can still cut it

The hardness a treatment produces sets what happens next. Below about 35 HRC, carbide runs at normal parameters and the process is straightforward. Between 35 and 45 HRC, cutting speeds come down and the depth of cut per pass gets smaller, so the cycle lengthens and the tooling bill rises. Above roughly 50 HRC the part is in hard milling or grinding territory, which needs a very stiff setup to avoid chatter and is often better handled by grinding for a simple flat or cylindrical face. Surface finish follows the same ladder: Ra 1.6 to 3.2 micrometres is a normal as-machined steel surface, 0.8 to 1.6 micrometres comes from a controlled finish-milling pass, and 0.2 to 0.8 micrometres generally means a ground surface. If a drawing asks for a hardness figure that pushes the part past the point where the critical features can still be machined, the treatment and the tolerance have to be reconciled before the order is placed.

What to ask for, and what to check on arrival

Give the heat treater the alloy grade and starting condition, the target hardness range rather than a single figure, the case depth and treated area where those apply, and the heat-treatment standard your own contract invokes; those four lines decide whether the result is repeatable between batches. On arrival, check the property and the geometry separately, because a part that hits the hardness figure can still be outside tolerance and a part that measures perfectly can have missed its target hardness. Hardness is normally verified by Rockwell, Brinell or Vickers, with a microhardness traverse to confirm case depth on carburised or nitrided parts; geometry needs the same instruments used for the final datum, and a CMM where position or form is critical. Hardness reports and treatment records should be part of the inspection pack, and the measurement method should be agreed before the job starts, because a CMM result and a micrometer result can differ by more than a fine tolerance.

See CNC machining services for parts routed through roughing, treatment and finishing in one sequence, surface finishing for the nitriding, plating and coating options that follow machining, and die casting for cast housings and dies whose tooling steel is heat treated on the same logic.