Austenitic stainless work-hardens under the abrasive that is meant to be cutting it, and duplex adds a second phase that polishes at a different rate from the first. The published route answers both with a short sequence that ends on an alkaline oxide step rather than on a fine diamond.

Why stainless and duplex steels are difficult

It work-hardens while you prepare it

Austenitic stainless has no phase change to absorb deformation, so energy the abrasive puts in goes into the lattice. The surface gets harder as the step proceeds, which means a grit that started by cutting ends by rubbing. That is why the plane step runs at a high load and a high speed and is over quickly: the useful part of it happens early.

Duplex polishes at two rates at once

Ferrite and austenite have different hardnesses, so any step long enough to level one will start to relieve the other. Relief at the phase boundary is the characteristic duplex preparation failure and it is not recoverable by polishing longer — longer is what causes it.

The damage hides under a surface that looks finished

A smeared stainless surface reflects well and reads as clean in bright field. It shows up when you etch, by which point the sample is spent. This is the argument for the chemical-mechanical final step: it removes the deformed layer instead of flattening it.

The published route

Akasel publishes this sequence for stainless and duplex steels. 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	Aka-Piatto 220	Water	300 rpm	30 N	Until plane
2 — fine grinding	Aka-Allegran 3	DiaUltra, 6 µm	150 rpm	35 N	2:30 min
3 — diamond polishing	Aka-Daran	DiaUltra, 3 µm	150 rpm	30 N	2:00 min
4 — oxide polishing	Aka-Chemal	Fumed Silica Suspension, 0.2 µm	150 rpm	15 N	2:00 min

Source: Akasel Aka-Brief #7, Stainless and Duplex Steels, 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.

Aka-Piatto 220+ — the method specifies the + disc, which carries twice the diamond concentration; we list the standard disc.
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.

Four steps, and only one of them is aggressive. Plane grinding does the removal; everything after it is about undoing what the previous step did. Compressing the grinding rather than stepping through five grits reduces the number of times the sample is loaded, and on a metal that records every loading that matters more than a gradual ladder.

The force falls and the speed halves after step 1. 300 rpm while material is coming off, 150 rpm for everything else, and the load steps down to 15 N by the oxide step. Late force is what produces the deformed layer the last step then has to remove, so the sequence spends its aggression early, on material later steps will take away anyway.

The last step is chemistry, not abrasion. Two minutes on an alkaline silica suspension is not long mechanically and is not meant to be. The maker’s own technique note matters here: wet the cloth before the holder touches it, and flush with water for the last ten seconds so neither sample nor cloth dries with silica on it. Dried silica is the most common way this step goes wrong.

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 #7. 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:

Aka-Piatto 220 — the method specifies the + disc, which carries twice the diamond concentration; we list the standard disc
Aka-Allegran 3
DiaUltra
Aka-Daran
Aka-Chemal
Fumed Silica Suspension

Where these figures come from

The preparation route is Akasel’s, published in Akasel Aka-Brief #7, Stainless and Duplex Steels, 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 #7, Stainless and Duplex Steels, 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 stainless and duplex steels for metallography