D2, SKD11 and CPM CruWear: the semi-stainless middle
Between “stainless” and “carbon” steel there is a band that neither label fits. D2 is its best-known representative. It has about 12% chromium, so formally it passes the definition of stainless, and it rusts.
Why 12% chromium does not save it
The definition of stainless steel is surprisingly loose: different sources put the chromium threshold at 10.5, 11 or 12%. But those definitions were written for low-carbon steels, and in tool steels what decides the matter is not how much chromium there is but what state it is in.
What protects against rust is not chromium itself but the oxide film it forms on the surface. To form that film, the chromium has to be in solution. D2 has a lot of carbon, and a large share of the chromium ends up bound in chromium carbides. Bound chromium is already busy and gives no film. Hence the paradox: a steel with 12% chromium rusts, while CPM 3V with 7.5% chromium turns out to be better in corrosion resistance — because there almost all the chromium is in solution.
Among rusting steels, though, D2 is known precisely for good corrosion resistance. Not stainless, but not a carbon steel that goes dark from onion in ten minutes either. The semi-stainless middle is not a marketing term but an accurate description.
And it is not “carbon steel”
In enthusiast talk, any non-stainless steel is called carbon steel. Strictly speaking, carbon steel is a narrow category: alloyed only with carbon, manganese and silicon. That is 1084, 1095, W1, White #1. Steels with small additions of alloying elements are alloy steels: 52100, 5160. Steels with substantial additions are tool steels: A2, D2, CPM 10V, Vanadis 8. The borders are arbitrary in places, but the scale of the difference is real.
How real is visible in the pair 1095 and 10V. Both non-stainless, both “carbon steel” in everyday speech. 1095 needs a water quench or a fast oil quench, gets its wear resistance from cementite, of which there is little and which is relatively soft, and is worked by a smith without fuss. 10V hardens in air, contains a lot of hard vanadium carbide, and machines as badly as any stainless. All they have in common is the absence of enough chromium. That is exactly why the “stainless versus carbon” argument is meaningless: the category is too broad to mean anything.
SKD11: the same thing in Japanese
SKD11 is a Hitachi Japanese tool steel identical to AISI D2. The same properties are also sold under the names DC11 and SLD. If you see one of these markings on a Japanese kitchen knife, it is D2.
The hardness achieved on it is serious: 60–64 HRC. Some makers harden to 64, many stop at 62. SKD11 can be taken very sharp, and that is its main lure. It is also the main trap: the steel is prone to chipping, and most often the cause is not the steel but the edge angle. Tool and high-speed steels should not be sharpened at too narrow an angle at all; for SKD11 the sensible range is 22–24 degrees. A person who thins the edge further “so it cuts better” gets chips and blames the metal.
There is a factory answer to the same problem: SLD Magic, a reworked version of ordinary SLD, which is chip-prone. It hardens to 60–62 HRC and has a good reputation, but it is rare in kitchen knives — poor availability and a high price.
Powder does not fix everything
It is reasonable to assume that a powder version of D2 solves the chipping question. It solves it partly. Carbide size in a powder steel is very small to begin with, but the carbides then grow — a natural process that goes faster the higher the temperature. And the steel passes through high temperatures during hot pressing of the powder, during forging and during rolling.
This is where the difference between carbide types shows up. Chromium carbides are less stable than vanadium carbides and coarsen faster. So CPM-D2 has larger carbides than Vanadis 8, even though the total carbide content is the same. Larger carbide means lower toughness. Powder metallurgy improved D2 but did not change the chemistry: a steel whose wear resistance rests on chromium carbides is doomed to a worse balance than a steel built on vanadium carbides.
CPM CruWear: the same band, done right
CPM CruWear lives in the same band between stainless and carbon steel — 7.5% chromium, against the 4–5.5% of typical rusting steels. It does have chromium carbides, but few enough not to spoil the toughness.
CruWear’s closest relative is CPM 3V; the steels are nearly the same and differ mainly in carbon content. Because of that CruWear comes out somewhat harder, and 3V tougher and more corrosion-resistant. Both belong to the narrow group of steels with a very attractive combination of toughness and edge retention, together with Vanadis 4 Extra and CPM-M4.
This is essentially where knife metallurgy has arrived: the best figures come from a powder rusting steel alloyed mainly with vanadium. Rusting — because then no chromium has to be spent, and the whole carbide budget goes to vanadium.
Who needs it
Semi-stainless steels demand discipline, but not much: wipe the blade dry after work, do not leave it wet in the sink. That is less than a true carbon steel demands and more than zero. If you are not up for it, the question is closed — look for stainless.
The second demand concerns sharpening. A narrow angle does not work on D2 and SKD11, and that limit has to be accepted rather than worked around.
Finally, it is worth knowing what you pay for CruWear and steels like it: the same thing you pay for any high-wear-resistance steel — time on the stone. That is exactly why professional kitchens routinely get by with cheap carbon steel that is trued on a steel in ten seconds. Powder tool steels are an option for someone who sharpens rarely and is in no hurry.