Powder metallurgy: where the super stainless steels came from

Powder metallurgy did not appear for the sake of knives and was not solving a knife problem. But it is what shifted the limit that knife steel had been hitting for decades.

The limit

It all comes down to carbides. They give wear resistance and they give brittleness: carbides are hard and brittle, a crack starts in them easily, and the more of them there are, the more readily the edge chips and the tip breaks. More carbide means it cuts longer and breaks more easily.

There is only one way out of the dilemma, and it lies not in quantity but in quality. Carbide hardness usually does not affect toughness. So if you take harder carbides — vanadium ones, for instance, among the hardest there are — and keep them small, you get more wear resistance at the same amount of carbide. Or, looked at from the other side, more wear resistance at the same level of toughness.

“Keep them small” — this is where ordinary metallurgy ends.

How it is done

In the powder process molten steel is atomised through a nozzle by jets of nitrogen gas and solidifies as very fine particles. The carbides in them are very small to begin with — simply because they have no time to grow. The powder is then consolidated into a solid ingot by hot isostatic pressing, HIP.

Then comes the unpleasant part. Carbides grow on their own through a natural process called Ostwald ripening. The higher the temperature, the faster it goes. And high temperature is unavoidable in production: during HIP, during forging and during rolling the steel spends time where carbides slowly but surely coarsen.

They grow at different rates. Chromium carbides are less stable than vanadium carbides and therefore coarsen faster. That is why CPM-D2, where the carbide is chromium, has coarser carbides than Vanadis 8, where it is vanadium — at the same carbide volume.

What came of it

The best combination of toughness and wear resistance comes from powder steels alloyed mainly for vanadium carbide: CPM-1V, CPM-3V, Vanadis 4 Extra, CPM-4V, CPM-10V, K390, Vanadis 8. These are non-stainless steels, and the difference from the stainless ones is not cosmetic: in powder non-stainless steels the toughness at a given level of edge retention is close to double.

The practical meaning of double toughness is direct. The edge chips less often; you can raise the hardness without losing strength; you can grind the edge thinner and get a better cut.

Powder stainless steels do not get this. They always contain a substantial amount of chromium carbide — at least 9–10%, with total carbide of at least 15%. Above fifteen per cent carbide, high toughness is no longer available.

Niobium

The logical next step is a carbide even more stable than the vanadium one. There is such a carbide: niobium. It coarsens more slowly during HIP and hot rolling, and in the finished steel niobium carbides come out even finer than vanadium ones. A modified version of CPM-3V alloyed mainly with niobium rather than vanadium was in fact made: carbide size went down, toughness improved. It never went into production.

Physics gets in the way. Niobium is a very strong carbide former; it starts forming carbides at high temperatures, while the steel is still liquid. The more niobium, the higher that temperature. At some point the carbides form before the melt can be atomised into powder — and there they grow fast, because it is liquid and hot. They can also clog the nozzle through which the molten steel passes into the nitrogen jets. The practical ceiling is around 3% niobium.

A workaround was invented: first atomise the steel with no carbon added at all, so that FeNb forms instead of niobium carbide, then mix the powder with graphite before pressing it into an ingot. Bohler’s patent on such high-niobium powder steels was filed back in 2009. In one of the compositions chromium was left at only 12%, and corrosion resistance came out better than that of M390 — probably in part thanks to the raised molybdenum. There are still no products on the market.

Nitrogen instead of carbon

The second direction is replacing carbon with nitrogen. A nitrogen atom behaves in the crystal lattice much like a carbon atom and raises hardness the same way, but does far less damage to corrosion resistance. On top of that, chromium and vanadium nitrides coarsen more slowly than their carbide counterparts. When a particle contains both carbon and nitrogen it is called a carbonitride.

Nitrogen has been used in small amounts for a long time: S30V, released in 2001, has about 0.2%. Cronidur 30, also known as LC200N, contains 0.4% and was developed in the late 1980s not for knives at all, but for bearings.

Vanax went furthest. Molten steel dissolves nitrogen poorly, so it is atomised with a relatively low nitrogen content and the powder itself is then nitrided — before HIP. That is how it reaches 1.55% nitrogen. The comparison is telling: Vanax and Elmax are very similar in composition, except that Elmax has 1.7% carbon and about 0.1% nitrogen, while Vanax has 1.55% nitrogen and 0.35% carbon.

No magic happened. Vanax still has plenty of carbonitrides — about 10% chromium and 4% vanadium, that is, the same total as the carbides in other stainless steels — and its toughness is nothing outstanding. What it does have is exceptional corrosion resistance together with excellent edge retention, and Vanax sharpens fairly easily.

Both tricks also work together. S45VN contains 0.5% niobium and about 0.17% nitrogen, and has only slightly more chromium carbide than S30V or S35VN, even though its chromium is 16% against fourteen.

The super stainless steels proper

A separate branch is the steels whose resistance to staining and corrosion is far higher than that of ordinary stainless steels. They are austenitic and non-magnetic, and were made for environments where ordinary stainless gives up: salt water, high humidity, rainforest, swamp.

The compositions there are of a different order: from 26% to 42% chromium, from 10% to 22% nickel and from 1.5% to 10% titanium, tantalum, vanadium, niobium, aluminium, silicon, copper or molybdenum — or combinations of these.

• H1, made by Myodo Metals, Japan. Spyderco puts it in knives for salt water and diving; Benchmade used it too, later switching to X15TN.
• X15TN, a French steel patented by Aubert & Duval, originally developed for the medical industry and jet engine ball bearings. According to the company’s data sheet it meets the EN 1.4123 standard (designation X40CrMoNV16-2) and UNS42025. A martensitic stainless steel with a high nitrogen content, remelted for an optimal structure.
• Vanax, made by Uddeholm — the third-generation powder steel in which carbon is largely replaced by nitrogen.
• LC200N, also known as Z-FiNit, Cronidur 30 and N360, made by Zapp Precision Metals — a high-nitrogen tool steel giving superb corrosion resistance together with high toughness even at hardness up to 60 HRC.

None of this has much to do with the kitchen. A salt-water knife needs corrosion resistance more than edge retention; at home that trade is not required.

The kitchen part

In the kitchen powder steels show up in a different guise — as grades with very high carbon and very high hardness. SG-2, also known as R2, from Takefu gives 62–63 HRC. The Japanese SRS-15, with very high carbon plus tungsten and vanadium, hardens to 64–65 HRC. Cowry-X from Daido — 3% carbon and 20% chromium, 64–65 HRC. ZDP-189 from Hitachi — the same carbon and chromium, but with molybdenum, tungsten and vanadium, and some makers take it to 65 and even 67 HRC.

The price is not only money. Such steels are very hard to sharpen compared with others, and at extreme hardness they are relatively prone to chipping. Powder metallurgy shifted the boundary of the possible, but it did not abolish the wear resistance versus toughness dichotomy — it only moved it.