Guides / Process

Stress Relief for Invar Machined Parts: When and How Shops Do It

September 20, 2026

Invar parts that skip stress relief can look perfect off the machine and then drift out of tolerance days or weeks later. The cause is internal stress left behind by roughing, and the fix is a heat treatment step most shops build directly into the machining sequence rather than treating as an afterthought.

Why Invar moves after machining

Roughing removes most of a part’s stock, and every cut leaves residual stress in the material near the surface. In a lot of alloys, that stress relaxes slowly enough that it never shows up as a real problem. Invar is different for two reasons: it work hardens readily under cutting forces, and the whole reason to specify it is that the part has to hold its shape and size to a degree other alloys cannot match. A part that moves 0.02 mm after machining might be a rounding error in a stainless bracket. In an Invar mirror mount, it can be the entire tolerance band.

The movement is not random. It follows the stress pattern left by roughing, which usually means the part relaxes toward its unstressed shape as internal stress releases, whether that happens naturally over weeks or is triggered on purpose in a furnace.

What a stress relief cycle actually does

A stress relief cycle heats the part to a temperature well below its transformation range, holds it there long enough for internal stress to relax, then cools it slowly and evenly. The exact temperature and hold time depend on the part geometry and the shop’s process, and the alloy spec or the drawing usually sets the target. The point of the cycle is not to change the material’s properties, it is to let the stress that roughing put in come out under controlled, even conditions instead of releasing unevenly over time on your bench or in the field.

Even heating and cooling matters as much as the target temperature. A part that heats unevenly, or that is loaded into the furnace resting on a point that constrains it, can come out with new stress instead of less.

Where it sits in the machining sequence

Most shops that cut Invar regularly follow the same basic sequence, explained in more depth in the Invar machining buyer’s guide:

1. Rough. Remove the bulk of the stock, leaving a consistent allowance on every face that matters.
2. Stress relieve. Heat the part, hold it, cool it slowly. This is where the bulk of the roughing stress comes out.
3. Finish. Light cuts with sharp tools bring the part to final size. Because the roughing stress is already relieved, the part that comes off the machine now is closer to the part you will still have in six months.
4. Stabilize and inspect for the most demanding optical or metrology parts, a further stabilization cycle follows, and the part is measured only after it has soaked to room temperature.

Doing the stress relief before finishing, not after, is the detail that matters most. If a shop finishes the part to final tolerance and then stress relieves it, the part will move during the cycle and there is no second finishing pass to correct for that movement.

What this costs and why it is worth asking for

A stress relief cycle adds a furnace run, handling in and out of the furnace, and a second machine setup for finishing. It is one of the larger line items on an Invar quote, covered in detail in why Invar 36 parts are costly. Shops usually batch parts into a single furnace load where they can, since a furnace run for ten parts does not cost much more than one for a single part.

Skipping stress relief on a part that does not need it is a legitimate way to save money, and a shop with Invar experience can tell you when that applies. The risk is skipping it on a part that does need it, because the failure mode is not visible at inspection. The part passes CMM the day it ships and moves out of tolerance in the customer’s hands.

What to specify on your drawing

If your part is thermally critical, say so explicitly rather than assuming the shop will infer it from the alloy callout. Useful things to include:

• Whether the drawing calls for stress relief only, or a full stabilization cycle
• Any features where post-machining movement would be unacceptable
• Whether inspection needs to happen after a soak at a controlled temperature

Say “shop to recommend” if you are not sure which cycle applies. A shop that has run stress relief on similar parts can usually match the cycle to the tolerance instead of you having to specify a furnace schedule you have no way to verify.

Send your drawing through our RFQ form and note the tolerance and any thermal requirements. We match you with shops that already run this sequence, so the quote you get includes the step your part actually needs.

Frequently asked questions

Keep reading

• Invar 36 Machining Parameters: Speeds, Feeds and Tool Wear
• Invar Drilling and Tapping: Hole Quality and Thread Practices