Cast iron preparation has one job: keep the graphite where it is. Nodule count, nodularity and flake classification are all measurements of the graphite, and every one of them is destroyed by a preparation that pulls it out.

Why cast irons are difficult

The graphite is the measurement, and it is not attached to anything

Nodules and flakes sit in the matrix rather than being bonded to it. Any step that drags will lift them, and the hole left behind is counted as a smaller nodule or missed entirely. Nodularity falls, count falls, and the result is a report about the polishing.

Smeared ferrite closes the holes and hides the damage

The matrix is soft enough to flow over a pulled nodule. That makes an over-worked cast iron sample look acceptable while reading low on every graphite metric — the worst combination, because nothing on the image says it is wrong.

Etching is not what reveals the graphite

Graphite is read in the as-polished condition. If it is not there after polishing, no etchant will bring it back, which is why this material is the strongest argument on the site for following a published method exactly.

The published route

Akasel publishes this sequence for cast irons. 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, 9 µm	150 rpm	35 N	3:30 min
3 — diamond polishing	Aka-Silk	DiaUltra, 3 µm	150 rpm	30 N	2:30 min
4 — oxide polishing	Aka-Chemal	Colloidal Silica Suspension 50 nm Alkaline	150 rpm	15 N	1:00 min

Source: Akasel Aka-Brief #14, Cast Iron, 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.

Short times, and the shortest final step of any route here. One minute on the oxide step. Every additional second is a second in which the cloth can pull a nodule, and the incremental gain in surface quality after the 3 µm stage is small.

A silk cloth for the 3 µm stage, not a napped one. Nap grips the graphite. A low-nap woven cloth holds the abrasive without holding the nodule, which is the whole reason the method names it.

Check the as-polished sample before you etch. Count the graphite first. If nodularity looks low, the question is whether the casting is poor or the preparation was — and the only way to tell is to prepare a second sample to the figures below and compare.

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 #14. 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 #14, Cast Iron, 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 #14, Cast Iron, 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 cast iron for metallography