Heat treating machined parts changes the material's hardness, strength, or internal stress after or between machining operations, using processes such as through hardening, case hardening, nitriding, stress relieving, and precipitation hardening. The key decisions are which process and specification to call, and where heat treat falls in the machining sequence so the final dimensions still hold.
Heat treatment affects cost, lead time, and achievable tolerance. This page explains the common processes for machined parts, the typical sequence of machining and heat treat, and how to specify heat treatment on a drawing.
Common heat treat processes for machined parts
| Process | Typical materials | Purpose |
|---|---|---|
| Through hardening (quench and temper) | 4140, 4340, tool steels, 400-series stainless | Raise strength and hardness through the section |
| Carburizing and case hardening | Low-carbon alloy steels such as 8620 | Hard wear surface with a tough core |
| Nitriding | Alloy steels, some stainless | Hard surface with minimal distortion |
| Induction hardening | Medium-carbon steels | Local hardening of journals, gear teeth, or wear faces |
| Stress relieving | Steels, stainless, aluminum | Reduce residual stress to limit movement |
| Annealing | Steels, stainless, copper alloys | Soften material or restore ductility |
| Precipitation hardening | 17-4 PH, 15-5 PH, aluminum alloys | Age to a specified condition such as H900 or H1025 |
| Solution treatment | Austenitic stainless, nickel alloys | Dissolve precipitates and restore corrosion resistance |
Machining before or after heat treat
The sequence depends on the hardness required, the distortion expected, and the tolerance on the finished part.
Machine complete, then heat treat
Works when distortion from the process is small relative to the tolerance, as with nitriding, stress relieving, or aging 17-4 PH from condition A. Precipitation hardening grades shrink slightly and predictably during aging, which can be allowed for.
Rough machine, heat treat, then finish
The most common sequence for hardened steel parts with tight tolerances. The part is roughed with stock left on critical features, hardened, then finish turned, finish milled, or ground to size.
Machine from prehardened stock
Some alloys, such as 4140 prehardened to a moderate hardness, can be machined directly and avoid a heat treat step entirely, which saves lead time.
How to specify heat treatment on a drawing
- Call the process specification, such as an AMS or ASTM specification, where applicable.
- State the required hardness as a range and scale, for example 28 to 32 HRC.
- For case hardening, state effective case depth and surface hardness.
- For precipitation hardening stainless, state the condition, such as H1025.
- State whether hardness must be verified on the part or on a test coupon.
- State if dimensions apply after heat treat, and allow for grinding stock where needed.
- Require the heat treater's certificate with the shipment.
Heat treat and inspection
Hardness is usually verified by the heat treater with a Rockwell or Brinell test, either on the parts or on a coupon from the same load, and reported on the processor's certificate. Case depth for carburized or nitrided parts may be verified on a sectioned sample. After heat treat, the machine shop re-inspects dimensions, because distortion or growth can move features out of tolerance. For critical parts, state on the drawing where hardness may be tested, since a test indent on a finished functional surface may not be acceptable.
Heat treat, lead time, and cost
Heat treat adds transport to and from the processor, the processor's queue, and furnace time, often one to three weeks per operation. Most heat treaters also charge a minimum lot price, which affects small orders. If a design can use prehardened material or a precipitation hardening grade aged after machining, lead time may be shorter.
Heat treating at Wexmar
Wexmar does not heat treat in-house. Heat treatment is performed through qualified outside processors, and Wexmar manages the sequence: roughing, sending to heat treat, receiving with the processor's certificate, and finish machining or grinding. Final sizing of hardened diameters and flats can be done on the shop's centerless and surface grinders. Material certs and heat-lot traceability carry through the process. See our materials and capabilities pages.
Heat treat checklist
- Process and specification called on the drawing.
- Hardness range, scale, and test location stated.
- Machining sequence considered for distortion and final tolerance.
- Prehardened or PH material considered as an alternative.
- Processor certificate required with shipment.
Getting a quote
Include heat treat requirements on the drawing and Wexmar will quote the complete sequence, including outside processing. You can upload a drawing for an instant estimate or call (315) 400-2654.