Stainless steels are, simply put, carbon steels with enough added alloys to make them corrosion resistant. However, there’s more to it than that because there’s an added benefit in that Stainless Steels will, generally, produce the better knife when compared to Carbon Steels.
Stainless knife steel, although not truly stainless, will not generally rust or corrode because it contains a high percentage (over 10.5%) of Chromium. The spin-off of this property of stainless steel is that it must also contain carbon (usually 0.6% – 1.1%) otherwise it cannot be hardened. Some of this carbon combines with some of the chromium to make chromium carbides, very hard microscopic particles embedded in the steel. Chrome carbides are much harder than the iron carbides found in carbon steels, so stainless steel knives will stay sharper far longer than carbon steel knives because of this “edge holding ability”.
However, heat treating stainless steel is a whole different ball game when compared to heat treating carbon steel, and the pitfalls are many and deep. It is very easy for the un-initiated to get a good hardness, but only mediocre toughness level, without realising they’ve done anything wrong. I believe that this may be why some knife makers still believe that carbon steels are better than stainless.
Stainless steels for knife making in general use in South Africa are limited. The best two, in my opinion, are Sandvik 14C28N (this is replacing 12C27 but is almost the same) and Bohler N690. The former makes a good knife and can be mirror finished. The latter does not take such a good polish and is better suited for satin finish knives. N690 is more highly alloyed though and, if heat treated correctly, gives a knife with superb edge holding abilities. D2 tool steel also makes a good knife and although it contains almost 12% chrome it is very high in carbon (±2%) and is less stain resistant than the above steels.
Powder metallurgy stainless steels (eg Bohler M390) make the “ultimate” knife blades in respect of performance. They are the most highly alloyed materials available (the powder metallurgy production route allows for far higher alloy content levels than would be possible by conventional steel manufacture) but they do not sacrifice toughness as would be the case with conventional steel-making techniques. PM steels are, however, very expensive.