CNC Machining Heat Treatment Service: How to Choose
Heat treatment sets hardness, toughness and dimensional stability after cutting, so it decides whether a part survives service or fails early. This guide is for engineers and buyers comparing suppliers. Read it to judge process fit, distortion risk, quote scope and inspection evidence.
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Key takeaways
Matching heat treatment to the part
Use this when the drawing names a hardness but not a process.
| Goal | Typical process | Watch out for |
|---|---|---|
| Through hardness, toughness | Quench and temper | Distortion on thin sections; temper after, never before |
| Hard skin, tough core | Gas nitriding | Long cycle; mask threads and bores you must re-cut |
| Relieve residual stress | Stress relief anneal | Do it before final finishing, or the part moves later |
| Maximum surface wear resistance | Carburizing and hardening | Case depth scatter; specify a range, not a single value |
| High strength-to-weight ratio | Solution treat and age | Aluminum alloys only; furnace steps are time sensitive |
| Softening for later forming | Full anneal | Slow cooling; grain growth if held too long |
What a cnc machining heat treatment service actually controls
A cnc machining heat treatment service sits between cutting and final inspection. The furnace cycle changes the microstructure of the metal, and that change is what the drawing is really asking for when it lists 40 HRC or a 0.5 mm case. You cannot get the same result by choosing harder stock.
Three properties matter in most jobs. Hardness resists wear and indentation. Toughness resists crack growth under shock or fatigue. Dimensional stability keeps the part at size months after it leaves the machine. A single cycle usually trades one against the others, which is why the specification has to say what the part must survive.
For aluminum, aging after solution treatment raises strength without a quench that distorts geometry. For steel, quench and temper gives the widest spread of hardness and toughness. For parts that need a hard surface and a ductile core, nitriding or carburizing adds a case while the core stays soft.
The boundary matters too. Very thin walls, long unsupported shafts and parts with sharp internal corners are poor candidates for a quench. In those cases, talk about a case-hardening route or a redesign before the drawing is fixed.
Judging distortion risk before you commit
Distortion is the most common reason a heat-treated lot gets scrapped, and it is rarely a surprise. Section thickness drives it. A part with a 3 mm wall next to a 25 mm boss will cool at different rates and bend toward the heavy side.
Ask the supplier how they plan to handle it. Good answers mention stock allowance, support fixtures, controlled cooling and a post-treatment grind or hard mill. Weak answers are a hardness number with no plan for geometry.
Long parts are the hardest case. A 400 mm shaft quenched vertically can bow several tenths of a millimeter. Straightening after temper helps, but it leaves residual stress. If the drawing calls for ±0.005 mm over that length, plan a finish cut after treatment and budget the extra pass.
Thin plates behave differently. They warp at the edges and twist across the face. Stress relief before final machining removes most of the movement, and clamping the plate flat during the cycle limits the rest.
Sequencing machining and heat treatment in one shop
When cutting and treatment happen under one roof, the sequence can be tuned instead of guessed. We rough machine with 0.2 to 0.5 mm of stock on critical faces, send the part through the furnace, then finish to the final tolerance. That order keeps the hardness and the size.
Split shops make this harder. The machinist finishes to print, the part travels to a treater, and it comes back oversize or out of flat. Someone then decides whether to re-cut a hardened surface, which is slow and expensive.
An integrated flow also shortens the loop when something goes wrong. If hardness comes back at 38 HRC instead of 42 HRC, the part can be re-treated or re-cut before it reaches final inspection, not after it has been packed.
For production quantities, we hold a first-article through the full sequence and measure hardness, case depth and critical dimensions. That record becomes the baseline for the rest of the run. Around 15 years of this work has shown that the sequence, not the furnace brand, decides most outcomes.
How to read a heat treatment quote
A quote that says "heat treat included" tells you almost nothing. Ask for the process, the target hardness range, the case depth range if there is one, and the inspection method. Those four lines separate a real quote from a placeholder.
Price drivers are cycle time, fixturing and testing. Nitriding runs for many hours and often needs masking, so it costs more than a simple stress relief. Hardness testing on every lot adds a small amount but removes a large argument later.
Lead time should be stated per step. A typical flow is quotation and DFM feedback within 12 hours, production starting within 24 hours, and parts shipping in 3 to 5 days for standard jobs. Treatment outside that window should be listed as a separate line.
Watch for quotes that exclude straightening or rework. Those are the two items that appear on the invoice after a bad lot. If the supplier will not name them up front, treat the number as a starting point, not a final price.
Evidence, certification and material fit
For aerospace, automotive and medical work, the paperwork carries as much weight as the part. Ask for batch traceability, furnace cycle records and a hardness report tied to the lot number. Certifications to look for include ISO 9001:2015, IATF 16949:2016, ISO 13485:2016 and ISO 27001:2022.
Material fit decides whether a process is even available. Common stainless grades such as 303, 304, 316L, 17-4PH and 440C respond differently to the same cycle. Tool steel and 4140 harden well; 1018 needs a case to reach high surface hardness. Aluminum 6061-T6 and 7075 age harden, while most plastics and magnesium alloys need a different route entirely.
Titanium such as TC4 (Ti-6Al-4V) and Inconel are usually stress relieved or solution treated and aged. Both are sensitive to oxygen at temperature, so the furnace atmosphere is part of the specification, not a detail.
If the part carries a hardness callout with no process note, send the drawing and the application. We can suggest a range that is achievable on the geometry, rather than a number that looks good on paper and cracks in the quench.
Step by step: from drawing to treated part
Follow this order to keep hardness and tolerance in the same part.
- 1Read the callout against the functionFind out what must resist wear, fatigue or impact. Convert that into a hardness range in HRC or HV and a case depth in mm if needed.
- 2Choose the process and flag the risksQuench and temper, nitriding, carburizing or aging. Note thin sections, sharp corners and long unsupported spans before quoting.
- 3Set the stock allowanceLeave 0.2 to 0.5 mm on faces that will be finished after treatment. On long shafts, allow more and plan a straightening step.
- 4Rough machine and stress relieveCut to near-net shape, then relieve residual stress if the part is thin or has tight flatness. Skip this and the quench will find the stress.
- 5Run the furnace cycleRecord temperature, time at temperature and cooling method. Control atmosphere for titanium and for any part that must avoid decarburization.
- 6Verify before finishingCheck hardness and case depth on a sample or the part itself. If the numbers miss the range, re-treat now, not after grinding.
- 7Finish machine or grindCut hardened surfaces to the final tolerance, targeting ±0.005 mm where the drawing requires it, then inspect 100% before shipment.
Questions buyers ask
Can a part be heat treated after it is finished to size?
Usually no. Quenching and tempering move the part, and hardened surfaces are slow to re-cut. Finish to size only after treatment, or accept a looser tolerance on the treated surface.
If the geometry will not allow a post-treatment cut, design for a process with low distortion, such as nitriding, and set the tolerance to match what the furnace can hold.
What hardness range should I put on the drawing?
Give a range, not a single value. A 40 to 45 HRC band is manageable; a single 42 HRC target invites arguments over a two-point miss.
The range should come from the application. Wear surfaces need the upper end, and parts under shock load often do better at the lower end with more toughness.
Does heat treatment change the surface finish?
Yes. Scale, discoloration and light pitting appear on untreated surfaces. Finishes applied before treatment may need to be repeated.
If the drawing calls for Ra 0.2 to 0.8 μm, plan the final polish after treatment. As-machined surfaces at Ra 1.6 to 3.2 μm are less affected but still need a check.
How do I know the case depth is right?
Ask for a case depth measurement on a test piece or a sectioned sample, reported in mm with the method named. Hardness alone does not prove case depth.
For nitrided parts, a common target is 0.2 to 0.5 mm. For carburized parts, it depends on load and contact stress, so the number should come from the design side.
What is the smallest quantity you will treat?
There is no minimum order quantity. One prototype and a 10,000-part run both go through the same documented sequence.
Small lots still get a first-article check. The furnace cost per part is higher, but the inspection and traceability do not change.
Can you treat parts made from titanium or Inconel?
Yes, with the right atmosphere and cycle. TC4 (Ti-6Al-4V) and Inconel are typically stress relieved or solution treated and aged.
These alloys pick up oxygen at temperature, so the furnace atmosphere and hold time are specified and recorded. Expect a longer cycle than carbon steel.
Send the drawing, get a treatment plan
We review the hardness callout, the geometry and the material, then quote the machining and the heat treatment together with the inspection steps named.
12-hour quote and DFM feedback100% inspection before shipmentNo minimum order quantity
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