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Heat Treatment for Machined Parts: Sequencing, Grades and Distortion Control

Heat treated quenched steel shafts and hardness testing after CNC machining

A machined part can be dimensionally perfect on delivery and still fail in service because nobody planned the heat treatment — or because it was applied at the wrong stage. Heat treatment decides hardness, strength, dimensional stability and machinability, and its position in the process chain (before or after machining) changes both the price and the final geometry. This guide covers the treatments that matter for machined steel and aluminium parts and when to specify each.

The Four Families of Heat Treatment

Treatment	What it does	Typical machined use
Annealing / normalising	Softens steel, relieves stress, evens grain	Pre-machining: makes heavy stock removal stable and predictable
Quench & temper	Hardens through the section, then toughens	Through-hard parts: shafts, gears, tooling up to ~55 HRC
Case hardening (carburising, nitriding)	Hard shell, tough core	Wear surfaces: pins, bushings, cam followers, crank parts
Stress relieving	Removes machining residual stress	After roughing on distortion-prone or precision parts

Before or After Machining? The Sequencing Question

This is the decision that most often goes wrong on drawings. Three working patterns:

Rough → heat treat → finish. The standard route for through-hardened parts: rough machine in the soft state, heat treat with grinding stock left on critical faces, finish machine or grind to size. Distortion is absorbed by the finishing allowance.
Machine fully, then treat. Works for nitriding and for stress relief — both run at temperatures low enough (roughly 500–580 °C for nitriding) to leave geometry intact. Nitrided parts often need no post-machining at all, which is why it is the favourite for finished-dimension wear parts.
Machine fully, then through-harden. Only viable on simple, symmetric, low-precision parts — quenching distorts and grows parts by 0.05–0.3%. If your drawing has ±0.02 mm features and calls for 50 HRC, expect the quote to include finishing after treatment.

Case Hardening vs Through Hardening

Through hardening gives uniform hardness but sacrifices core toughness — a fully hardened shaft snaps instead of bending. Case hardening puts 0.3–1.5 mm of hard case on a soft, tough core: carburising (then quench) reaches 58–62 HRC on the skin for gears and cam surfaces, while nitriding achieves 65–72 HRC-equivalent with zero quench distortion, at the price of shallower case and slower cycle. Choose by duty: impact loading wants case + tough core; static wear without shock can go through-hard.

Aluminium: A Different Story

Aluminium doesn't quench-harden like steel — its strength comes from precipitation. T6 temper (solution treat + artificial age) is the standard high-strength condition for 6061 and 7075, as covered in our 6061 vs 7075 guide. Two machining-specific notes: stress-relieved tempers (T651, T7351) exist precisely because mill-stock warps when you machine one side away — for parts with heavy asymmetric material removal on aluminium, buying T651 plate or doing a post-roughing stress relief is cheap insurance. And never plan on "heat treating after machining" to fix a soft stock mistake: solution treatment temperatures near 530 °C will trash machined tolerances.

Distortion: The Real Cost Driver

Heat treatment cost itself is modest; the cost lives in distortion management. What we do about it:

Design symmetrically. Balanced sections distort least; a deep pocket milled into one face of a plate is a built-in banana.
Leave grind stock only where needed — 0.3–0.5 mm on faces that finish at tight tolerance, nothing elsewhere.
Specify stress relief after roughing on long, thin or asymmetric parts (steel around 550–650 °C, slow cool).
Quench method matters: oil, polymer or vacuum-gas quenching distort progressively less; on thin precision parts, vacuum hardening plus finishing is the sane route.

The geometric consequences belong on the drawing: if faces must stay flat and true after treatment, reference the tolerance to the post-treatment state and say so — the practice recommended in our GD&T guide.

What to Put on the Drawing

Process + grade, not vibes: "AISI 4140, quench & temper to 28–32 HRC" beats "hardened".
Case depth where case hardening applies (e.g. "carburise 0.8 mm effective case, 58–62 HRC").
Sequence: which faces may carry grind stock, which must be finished post-treatment.
Hardness as a range (e.g. 28–32 HRC, not "approximately 30").
Whether hardness testing is destructive or on-part (HRC spot checks vs a test coupon).

How Renjiu Handles Heat-Treated Work

We route through-hardened parts through certified local heat-treat partners with material certs carried through to shipping, and re-clamp for finish machining and grinding afterwards — the full flow is described in our QC process. Common treated-material work includes 4140/42CrMo4 shafts and pins, 4340 high-strength fittings, P20 and H13 tool components, and stress-relieved aluminium housings for automotive and robotics customers. Surface hardening pairs naturally with our surface treatment services for corrosion protection on the treated surfaces.

Frequently Asked Questions

Can you harden 6061 aluminium?

Not by quenching in the steel sense — it reaches T6 by solution treating and ageing, done on raw stock. Machined 6061 parts can be aged further only marginally; strength is bought at the mill, not after machining.

Which treatment has the least distortion?

Nitriding (no quench) for steel, and ageing (T6/T651) for aluminium. Vacuum quenching sits between them; water quenching is the worst offender.

Does heat treatment affect the surface finish?

Quenched parts can discolour and slightly roughen; nitriding leaves a clean, matte surface. Any finish-critical face on a through-hardened part should be planned for post-treatment finishing — see our Ra guide for spec levels.

Can heat treatment be combined with corrosion protection?

Yes — stainless through-hardening is limited, so for hard + corrosive duty we typically case-harden a coated carbon steel or nitride a martensitic stainless. Tell us both requirements early and we design the route.