Nickel-base superalloys are tough rather than hard: they resist cutting, work-harden while resisting it, and hold the deformed layer that results. The published route runs longer at every stage than the equivalent steel route and finishes on an oxide step.

Why nickel superalloys are difficult

Tough is not the same as hard, and it is harder to prepare

A superalloy at 350 HV cuts far less readily than a steel at the same hardness. The abrasive deforms the surface instead of removing it, and the deformed layer is then what the next step has to deal with.

It work-hardens under the abrasive

Like austenitic stainless and for related reasons. The surface is harder at the end of a step than at the start, so a step run too long stops removing material and starts burnishing.

Gamma-prime and carbides sit in a softer matrix

The strengthening phases are harder than what surrounds them. Over-polishing relieves them, and a relief measurement reported as a phase measurement is a real and common error on these alloys.

The published route

Akasel publishes this sequence for nickel superalloys. The figures are theirs. Times are quoted for a 300 mm preparation system and forces for a single 40 mm diameter sample.

Step	Surface	Abrasive or suspension	Speed	Force	Time
1 — plane grinding	Rhaco Grit	Water	300 rpm	25 N	Until plane
2 — fine grinding	Aka-Largan 9	DiaUltra, 9 µm	150 rpm	35 N	3:00 min
3 — diamond polishing	Aka-Silk	DiaUltra, 3 µm	150 rpm	30 N	3:00 min
4 — oxide polishing	Aka-Chemal	Fumed Silica Suspension, 0.2 µm	150 rpm	15 N	2:00 min

Source: Akasel Aka-Brief #20, Superalloys, 2022 edition, page 1. Every figure in this table is printed on that page; none of them is ours.

Where the method and this catalogue differ

We say what the method specifies and what we list, and we do not put the nearest thing we sell in its place.

Water — no consumable — the disc runs under water.

Reading the sequence

The shape of the route says more than any single number in it. This part is our reading, not a manufacturer statement.

Longer times than the steel routes, at comparable loads. Three minutes at each of the two diamond stages against two to three for stainless. The extra time is removal that a tougher matrix does not give up quickly, not additional refinement.

Plane grinding is on paper at 25 N. Higher than the aluminium and copper routes, lower than the fixed-disc steel routes. Paper presents fresh grit as it wears, which is what a work-hardening alloy needs.

It ends on alkaline fumed silica. Same argument as stainless: the deformed layer on a work-hardening alloy is removed chemically far more reliably than it is abraded away. The technique note applies here too — wet the cloth first, flush for the last ten seconds.

The same sample, after each step

Akasel photographs the same specimen after every stage of this route and publishes the images on page 2 of Aka-Brief #20. We have not reproduced them here. The titanium guide shows what that sequence looks like for a material we do hold the images for.

Scaling it to your machine

Smaller platen, longer steps. The maker’s guidance is to add roughly a third to the times on a 250 mm system and to double them on a 200 mm one. The abrasive does the same work over less swept area.
Bigger samples, more force; smaller samples, less. The quoted loads are per 40 mm mount. Scale with mounted area rather than guessing, and if your machine cannot deliver the figure, extend the step instead of exceeding it.
Head speed is fixed. 150 rpm on the sample holder throughout, constant across every published method in this series.

What the route needs

6 consumables, and the sequence does not work without them:

Where these figures come from

The preparation route is Akasel’s, published in Akasel Aka-Brief #20, Superalloys, 2022 edition, page 1. The reading of it — why the sequence is shaped this way, what goes wrong, what to watch — is ours, and it is an interpretation rather than a manufacturer statement. We have not run this method in our own laboratory and we do not claim to have. Where the method calls for something we do not list, the guide says so above rather than substituting the nearest thing we sell.

Sources

Akasel Aka-Brief #20, Superalloys, 2022 edition, page 1 — every figure in the step table.
Akasel product pages and data sheets, cited row by row on each linked product page.
The reading of the method, and everything under “Why” and “Reading the sequence”: P2 Industrial.

Preparing nickel superalloys for metallography