Work hardening: the failure mode that ruins superalloy parts

Ask a shop why it will not quote your Inconel bracket and the honest answer is usually work hardening. Not the material’s hardness — its habit of becoming harder exactly where and when you do not want it to.

What is happening

Austenitic stainless steels, nickel superalloys and cobalt alloys all have a face-centred cubic structure that deforms by slip. Push a cutting edge into that structure and the material immediately ahead of and below the edge plastically deforms before it shears. That deformation multiplies dislocations, and the dislocations make the metal harder.

The numbers are not subtle. Inconel 718 can go from roughly 36 HRC to over 50 HRC in the layer a single pass leaves behind. 316 stainless can nearly double its surface hardness. Hastelloy C-276 hardens fast enough that a tool which pauses for a second is cutting a different material when it resumes.

That hardened skin is typically 0.05 to 0.3 mm deep. Thin enough to ignore in a drawing note, thick enough to eat the next cutter.

The three ways it shows up

Notch wear. A groove worn into the insert exactly at the depth-of-cut line — where the edge runs along the hardened skin the previous pass created. It is the signature failure on nickel alloys, and it is why varying the depth of cut between passes extends tool life.

Sudden edge failure. Not gradual flank wear but a chipped or collapsed edge, often minutes into a cut that started fine. The edge has been rubbing in work-hardened material, heating, and losing its coating.

Surfaces that will not finish. A finishing pass that skims a hardened layer tears rather than cuts. You get a rough, inconsistent surface that no amount of slowing down improves, because slowing down is the cause.

The rules that beat it

Keep the feed up. This is the one people get wrong. When a cut sounds bad the instinct is to reduce feed, but below roughly 0.05 mm per tooth the edge stops cutting and starts burnishing — which is exactly the deformation that creates the hardened layer. Feed into the material, not against it.

Never dwell. A tool that stops moving while still engaged is hardening the material it sits in. Program the retract. Peck drills. Avoid spring passes.

Cut under the previous layer. Each pass must take a depth of cut greater than the hardened depth of the last one, so the edge works in fresh material rather than skimming the skin. Depths under about 0.25 mm on a finishing pass in nickel alloy are often the problem, not the solution.

Vary the depth of cut between roughing passes so notch wear does not concentrate at one line on the insert.

Keep the edge sharp. A worn edge has an effectively larger radius, which deforms more material before it shears. On these alloys a tool change at a scheduled interval is cheaper than the scrapped part a worn edge produces.

Use pressure, not just volume. High-pressure through-tool coolant breaks the chip and pulls heat out of the interface. Flood coolant washing over the outside of the cut does very little. High-pressure coolant covers why.

What this means for your drawing

Work hardening is the reason a few drawing decisions cost far more on a nickel alloy than they would on steel.

Deep narrow slots and pockets force light radial engagement, which pushes the process toward exactly the rubbing conditions that harden the surface. If a slot can be wider or shallower without losing function, it should be.

Very fine surface finishes require light finishing passes, which are the passes most prone to burnishing. Specify Ra where it matters and open it elsewhere — see surface finish on exotic alloys.

Thin walls deflect away from the tool and spring back behind it, which means the edge rubs even when the programmed depth looks correct. Thin-wall titanium parts has the detail.

Reaming and tapping in work-hardening alloys are far less forgiving than in steel. A hole that hardened during drilling will destroy a tap.

Further reading on the mechanism: work hardening.

If your part has been scrapped for surface defects or short tool life, send the drawing — the cause is usually visible in the geometry before anyone touches a machine.