Cutting and forming Invar 36 sheet without losing flatness

By Invar Machining editorial team· Published September 26, 2026 · How we write these guides

Invar 36 sheet is cut by fiber laser, abrasive waterjet, wire EDM or shearing. The right choice depends on how much heat and cold work the edge can carry. Laser is fastest and leaves a small heat affected zone. Waterjet and wire EDM leave almost none. Shearing is cheapest on thin strip but work hardens the edge. Whatever the method, a sheet part with a flatness or stability requirement still needs a stress relief, because the rolling stress in the sheet is released when it is cut.

What Invar 36 sheet and strip look like

Sheet and strip are cold rolled from hot rolled plate and supplied annealed or in a cold rolled temper. Typical thickness for sheet runs from about 0.3 to 6 mm, with strip below 1 mm sold in coils. Above 6 mm the product is plate and the economics change, as covered in Invar bar, plate and sheet stock. The chemistry is the same as bar, 36 % nickel with the balance iron, to ASTM F1684 or a UNS K93600 series designation, whatever trade name the supplier uses for Invar. The Invar specifications reference lists the equivalents.

Two properties of the sheet matter before any cutting starts. First, cold rolled sheet carries rolling stress, roughly balanced through the thickness, and any cut that breaks that balance lets the sheet move. Second, cold work raises the expansion of Invar 36. A cold rolled temper can sit above the annealed 1.2 to 1.6 ppm/°C, and the drawing should say which condition it wants. For a stability critical part, order annealed sheet and plan a stress relief after cutting and forming.

Laser cutting

A fiber laser cuts Invar 36 sheet much as it cuts stainless steel of the same thickness. Nitrogen assist gives a clean, oxide free edge. Typical practical thickness runs up to about 6 mm on a mid power machine, with the edge quality falling off above 4 mm.

The edge carries a heat affected zone. On thin sheet it is typically 0.05 to 0.2 mm wide, deeper on thick sheet and on slow cuts. Within that zone the material has been melted and resolidified at the surface, then heated through a range of temperatures below it. The recast layer is harder and can carry micro cracks, so an edge that will be bent should be deburred and, on thicker sheet, lightly dressed first. And the rapid heating and cooling leaves tensile stress at the edge, which matters when the cut edge is a datum or when a slot pattern is dense enough that the stresses add up across the part.

Laser suits most brackets, shims, shields and blanks that will be machined afterwards. Hole position across a 500 mm sheet is typically within ±0.05 to ±0.1 mm as cut, with the sheet warming during a long program.

Waterjet cutting

Abrasive waterjet cuts cold. There is no heat affected zone and no recast layer, which is why it is chosen for parts where the edge stress from laser would matter or where the sheet is thicker than a laser handles well. It cuts Invar 36 plate to 50 mm and beyond, slowly.

The limits are geometric. The jet lags and flares, so the kerf tapers, typically by 0.05 to 0.1 mm per 10 mm of thickness unless the head tilts to compensate. The kerf is about 0.8 to 1.2 mm wide. The edge has striations that get coarser toward the bottom of the cut. Garnet can embed in the edge, which is a concern on vacuum and cleanroom parts and should be called out for cleaning. Position accuracy is similar to laser, typically ±0.1 mm on a mid size table.

Waterjet is a good roughing method. Cut the blank 1 to 2 mm oversize, stress relieve, then mill or grind the edges and features that carry tolerance.

Wire EDM

Wire EDM cuts sheet and stacked sheet to the tightest tolerance of the four methods, typically ±0.005 to ±0.01 mm with skim passes. It leaves a recast layer 5 to 20 µm thick that skim passes reduce to a few microns. It is slow and priced per hour, so it is used for small precise parts, for stacks of thin shims cut in one go and for features such as narrow slots that a laser would overheat. The wire EDM for Invar guide covers the process in detail, including the residual stress the recast layer carries.

Shearing and punching

Shearing and punching are the fastest and cheapest methods on strip and thin sheet. They are also the ones that cold work the edge most. A sheared edge on Invar 36 shows roll over, a burnished band, a fracture band and a burr. The material within roughly one sheet thickness of the edge has been plastically deformed. Because Invar work hardens readily, described in the Invar work hardening guide, that band is harder and has a raised local expansion until it is stress relieved.

For tabs, spacers and parts with edges that carry no function, shearing is fine. For a part with an edge datum or a flatness call, shear oversize and machine the edge, or choose laser or waterjet.

The four methods at a glance

Method	Heat affected zone, typical	Edge stress	Thickness, typical	Position accuracy, typical
Fiber laser	0.05 to 0.2 mm	Tensile at edge	0.3 to 6 mm	±0.05 to ±0.1 mm
Abrasive waterjet	None	Very low	1 to 50 mm	±0.1 mm, taper with thickness
Wire EDM	Recast 5 to 20 µm	Low after skim passes	0.1 mm to stacks of 100 mm	±0.005 to ±0.01 mm
Shearing or punching	None	Cold work, about 1 t deep	0.1 to 3 mm	±0.1 to ±0.2 mm

Typical values for planning. The shop’s quote and its machine govern.

Forming and springback

Invar 36 forms much like a low carbon steel or an annealed austenitic stainless. Annealed sheet bends around an inside radius of about one sheet thickness without cracking, and tighter radii are possible on thin gauge. Springback is modest, on the order of 1 to 3 degrees on a 90 degree bend in annealed material, more in a cold rolled temper. Shops compensate by overbending, and a first article establishes the correction.

The point that is specific to Invar is not the springback. It is what forming does to expansion. A bend is cold work, concentrated in the outer and inner fibers of the radius. The expansion there rises above the annealed value, and the stress locked in the bend relaxes over time. A formed bracket that holds an optic will creep. So a formed Invar part with any stability requirement is stress relieved after forming, typically 315 to 345 °C for 1 to 2 h followed by a slow cool. The drawing governs the cycle. The stress relief sequence guide explains why the timing matters.

Flatness after cutting

Sheet arrives with a mill flatness that is rarely better than a few millimetres per metre. Cutting releases rolling stress and the blank moves again. A laser cut blank of 2 mm sheet, 300 mm across, may show 0.3 to 1 mm of bow as cut. Waterjet and EDM blanks move less, because they add no thermal stress, but they still release what the mill left.

Stage	Flatness, typical, 300 mm blank of 2 mm sheet
As received, cold rolled	1 to 3 mm
As laser cut	0.3 to 1 mm
As waterjet or EDM cut	0.2 to 0.6 mm
After stress relief between flat plates	0.1 to 0.3 mm
After stress relief and surface grinding	0.01 to 0.03 mm

Typical values. Actual results depend on the sheet lot and the part shape.

Flattening is done in the furnace. The parts are stacked between ground plates, weighted and stress relieved, which takes out both the cutting stress and most of the bow. Roller levelling adds cold work and is avoided on stability parts. If the drawing needs better than about 0.1 mm over 300 mm, plan on grinding after stress relief, as described in the Invar grinding guide, and leave stock for it.

When a sheet part still needs stress relief

The rule is short: if the part has a flatness call, a position tolerance under ±0.05 mm, a bend or any mention of thermal stability, stress relieve it after cutting and forming and before final machining or grinding. Skip it only for parts whose edges carry no function and whose tolerances are ±0.2 mm or looser.

For thin parts the furnace cycle is the same as for plate, and the cost is small per part because many blanks fit in one load. The real cost is a few days of lead time for the furnace and the cool down.

Worked example, hypothetical

Take a hypothetical thermal shield for an optical instrument: Invar 36, 150 × 100 × 2 mm, with a pattern of eight 3.2 mm holes at position ±0.05 mm, one 90 degree flange 15 mm high along the long edge and flatness of 0.1 mm on the main face. Quantity 40.

A sensible route is laser cutting from annealed 2 mm sheet with nitrogen assist, holes cut on the laser at 3.0 mm and left for a reamer, a first article to set the bend allowance, forming on a press brake with a 2 mm inside radius, then stress relief of all 40 parts stacked between plates at a typical 330 °C for 1.5 h with slow cool. After the furnace the holes are reamed to 3.2 mm on a drill fixture that locates from two of them, the flatness is checked on a surface plate and a sample is measured on a CMM at 20 °C.

The hole pattern ends up inside ±0.05 mm because the reaming happens after the part has finished moving. Had the holes been laser cut to size and the parts formed and shipped without stress relief, the pattern would have held as cut and then walked by a few hundredths as the bend relaxed.

When to machine from plate instead

Sheet routes stop making sense in three cases. When the part is thicker than about 6 mm, laser edge quality drops and waterjet blanks need so much finishing that a machined plate part costs the same. When the flatness requirement is under about 0.05 mm over the part, or a face carries a parallelism call, the part needs a machined and stress relieved face on both sides and it is easier to start from plate with stock on each face. And when the part has pockets, steps or bores with tight position, the sheet is only a blank and the real work is milling, so start from plate and follow the rough, stress relieve, finish sequence. The thin wall machining guide covers what happens when a plate part is milled down to sheet thickness.

What to put on the RFQ

Give the sheet thickness, the condition (annealed or cold rolled) and the spec, Invar 36 to ASTM F1684 or the UNS number. State which cutting method you will accept or write “shop to recommend”. Call out any edge that is a datum, any hole that carries position tolerance and the flatness on each face. Say whether the part is formed and whether it is stress relieved after forming. Add cleaning requirements if the part goes into a vacuum or cleanroom assembly, because garnet and laser oxide matter there. When you send the RFQ with a flat pattern or a 3D model, we match you with up to three shops that cut and form the alloy and can run the stress relief in the same sequence. See also the Invar 36 machining page.

Frequently asked questions

Reviewed against

• ASTM F1684, Standard Specification for Iron-Nickel and Iron-Nickel-Cobalt Alloys for Low Thermal Expansion Applications
• Wikipedia, Invar

Values in this guide are typical handbook figures for planning. The mill certificate for your heat lot and the shop's quote for your geometry govern.

Keep reading

• Choosing Invar Stock: Bar vs Plate vs Sheet · Materials
• Wire EDM and Invar: When to Use It Instead of Milling · Process
• Stress Relief for Invar Machined Parts: When and How Shops Do It · Process
• Thin-Wall Invar Parts: Machining Without Warping · Process

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