Stainless steel: what chromium actually does

It is convenient to talk about stainless steel as if it were one substance with one property. In fact it is a collective name, and the property is conditional.

Technically, there is no such thing as stainless steel. All steels rust if they are not looked after properly; some resist corrosion better than others. The English term stainless — steel that does not stain — is more accurate than the Russian one, and “rustless” is best understood as “stain-resistant.”

Where the line runs

The definition of stainless steel is surprisingly vague. The minimum chromium content is given as 10.5, 11 or 12 per cent, depending on what you read. Usually steels with no more than 1.2% carbon and no less than 11% chromium are counted as stainless; knives at 11% chromium and above do have good rust resistance.

The trouble is that these definitions were written for low-carbon stainless steel. In the world of tool and knife steels, the influence of the other elements is too large for chromium alone to be the measure.

Chromium in solution and chromium in carbide

Chromium works like this: it forms an oxide layer on the surface of the steel that prevents rust. But only chromium dissolved in the steel can do that. Chromium bound to carbon in a carbide forms no surface oxide — it is already busy.

Hence the most common example: D2, with roughly 12% chromium, is not stainless. It has so much carbon that a lot of chromium carbide forms, and none of the chromium is left for the surface.

It works the other way round too. S110V with 15.25% chromium is no worse in corrosion resistance than M390 with 20% — because what matters is the whole composition, not one number. Most knife steels hold 10–13.5% chromium in solution; M390, for all its twenty per cent, has only about thirteen in solution, the rest bound up in carbides.

CPM-3V shows the same logic from the other side: it has only 7.5% chromium, formally a rusting steel, but almost all of that chromium is in solution, and in corrosion resistance it beats D2 — which, incidentally, is known precisely for good corrosion resistance. Typical rusting steels contain 4–5.5% chromium, so 7.5% is already borderline territory.

What stainlessness costs

The requirement to add a lot of chromium is a design constraint like any other. Accept one requirement and you sacrifice properties somewhere else.

What gets sacrificed is the carbides. With a lot of chromium it becomes harder and harder to make the carbides in the steel be vanadium carbides. Chromium carbides form instead, and they are worse at everything a knife cares about.

• They are softer than vanadium carbide — so wear resistance and edge retention are lower.
• In powder steels they are larger — so toughness is lower and the edge chips more readily.
• Since they are softer, you need more of them for the same edge retention — and toughness drops again.

Plus simple arithmetic: the more chromium there is, the less vanadium carbide forms for the same amount of vanadium, and the more chromium carbide.

How this is fought

The first way is to lower the chromium and balance the composition so that all the remaining chromium goes into solution. That was the reasoning in 1995 behind S90V with 14% chromium instead of the earlier steels at 16–20%. The same base 14% later underpinned S30V, S35VN and S125V.

The second is molybdenum. It raises corrosion resistance at a given chromium content, which is why S30V, S35VN, S125V, S110V and S45VN contain 2% molybdenum or more. S35VN and S90V are both at 14% chromium, but S35VN has noticeably better corrosion resistance — thanks to the molybdenum.

The third way does without powder metallurgy altogether: keep the total amount of carbide very low, so that the chromium carbides stay small and the toughness stays high. That is how AEB-L, 12C27 and 14C28N are built. The metallurgists carefully balanced carbon and chromium to get both high hardness and corrosion resistance without breeding chromium carbide. The microstructure comes out very fine, the toughness is excellent, and the wear resistance and edge retention are lower than in powder stainless steels.

A curious detail: edge retention on AEB-L and 14C28N is still better than on the low-alloy 52100 and 1095, because chromium carbides are harder than iron carbide. In that particular pairing, the stainless steel has a better balance of properties than the “carbon” one. The argument about which is better in general turns out to be pointless again: the question is always which specific grades are being compared and on which property.

A little vanadium or niobium can be added to such low-carbide stainless steels to get higher wear resistance for a small loss of toughness. The nearest available example is Niolox, but its carbide size is relatively large and the toughness did not come out as hoped; the largest carbides there are, again, the chromium ones. Niobium, meanwhile, is a stronger carbide former than vanadium and can form its own carbides even in the presence of large amounts of chromium, so the direction works.

Why any of this matters to a buyer

Stainless steel is preferred in knives for a prosaic reason: it needs less maintenance to stay free of rust. And it is not only cosmetic — an edge loses sharpness from corrosion, not only from cutting. Large companies making production knives fit stainless partly because that is what the average customer expects.

A carbon-steel kitchen knife and the maintenance that comes with it do not suit everyone, so choosing stainless is not a bad choice. There are stainless steels today that are harder and more wear-resistant than some carbon steels. The chance of rust appearing never goes away entirely.

One last thing: the presence of chromium does not by itself make a knife cut. The 420 series has less than 0.3–0.5% carbon and is simply too soft for the kitchen, though it resists rust beautifully. X50CrMoV15 with its 0.5% carbon is very rust-resistant and nothing more. Chromium is responsible for stains. Everything else is responsible for the cut.