Steel Properties knifeopedia
The properties of a hardened steel relevant in kitchen knives are hardness, abrasion resistance, toughness and corrosion resistance. Unfortunately, hardness/abrasion resistance on one side and toughness/corrosion resistance on the other side are in reverse relationship to each other. By the nature of metallurgy, a harder steel becomes more brittle and more corrosion prone. And soft steel with less Carbon, but higher amounts of Chromium, can be truly stainless, but will not hold an edge well.
Hardness is the resistance of the steel to permanent distortion at microscopic level. It’s a desired property in a knife steel, as harder steel will hold an edge better than softer steel. During cutting, a previously sharp edge (pict. A) is worn by abrasion (contact with food and cutting board) and deformed (from pushing the edge on the board).
Abrasion will result in rounding of the cutting edge (pict. B), whereas deformation will cause the edge to roll (pict. C). Imagine a piece of paper vs. a metal sheet with equal size and thickness. If you push the paper vertically onto a table, it will roll, whereas the metal sheet won’t.
It’s logical that a harder material will be more abrasion resistant, as well as less likely to be deformed, so that the pointed state of an edge in pic A will be preserved and the knife remain sharp for a longer period of time.
Also, since the harder material is less likely to roll (pict. C), knives with harder material can be sharpened to a more acute angle. If the angle is more acute, the blade can be pushed through food more easily, which is what we feel as sharpness.
In addition, even if the edge is slightly worn, due to the acute angle, it will still be able to cut, which is why a narrow edge angle has a major impact on both sharpness and edge retention.
Unfortunately, harder materials tend to be more brittle and more prone to chipping or cracking.
A high abrasion or wear resistance is the key to edge retention or how long a knife will stay sharp, because much of the degradation of sharpness in kitchen use is due to abrasion, from friction between the blade and both food and cutting board. A higher abrasion resistance will result in less wear, and the sharpness of the knife will be kept for a longer period of time.
Whereas, as mentioned above, abrasion resistance is related to hardness and will increase with an increase of hardness, there is much more to it. More than sheer hardness, the material composition of a steel, the type of carbides formed, plus the size and volume of carbides have an impact on abrasion resistance.
Elements in steel like Vanadium or Tungsten form very hard and tough carbides and improve abrasion resistance (at the same given hardness) as compared to steel with lower amounts of those elements. Following the same logics, a higher volume of carbides also increases abrasion resistance. Because the harder the carbides are and the more carbides a steel contains, the longer it takes to abrade them.
Whereas the benefits of a knife with high abrasion resistance in terms of edge retention are clear, there is a downside to it. It’s more difficult to sharpen, which is again logical, as sharpening works through abrasion of material.
Toughness (also referred to as ductility) refers to a blade’s ability to withstand sudden impacts and forces that might otherwise produce cracks or chips. A high toughness is desirable, as it will reduce the sensibility of the knife in daily kitchen use, e.g. when cutting through harder objects. Toughness usually stands in a reverse relationship to hardness. One can improve toughness. e.g. by tempering at higher temperature, but will have to live with a lower hardness as a result.
Therefore it must be the objective of every knife maker to reach the maximum toughness at a given hardness or the maximum hardness at a pre-defined toughness. This can be impacted by the choice of material as well as heat treatment.
How tough a knife has to be largely depends on the intended use. A Yanagiba knife, made to cut Sushi, needs to be sharp, but not necessarily tough, whereas a cleaver, made to cut through smaller bones, needs to be very tough, but not necessarily so sharp.
Corrosion resistance is a blade’s ability to withstand contact with water and acids without a chemical reaction. Steel basically is iron with added carbon, thus highly corrosive. When cutting, the blade comes into contact with water and acids, causing a chemical reaction called corrosion. This will lead to stains on the blade or worse, pitting. Adding elements to a steel can reduce or increase the corrosiveness. As a rule of thumb, a higher carbon Content will increase the corrosiveness and a higher Chromium content will reduce it. In today’s knife steel, corrosion resistance is usually achieved by adding Chromium.
It’s commonly said that steel with more than 11% or 12% can be called stainless. This however only works as a rule of thumb. When Chromium is added to a steel, a chromium oxide passive layer is formed, which protects the steel from corrosion. For this to happen, a certain minimum amount of Chromium is needed. A further increase beyond this minimum level improves the strength of the passive film. Elements like Molybdenum or Nitrogen don’t create a passive film on their own, but adding them to the steel helps to strengthen the existing Chromium oxide passive layer.
But there is one more thing to it – the amount of Carbon in the steel. With increasing Carbon content, Chromium in knife steel is tied up in Chromium carbides formed during hardening, so that only a certain share of Chromium goes into solution and forms the oxide layer. E.g. steel with 12% Cr, but 1.5% C, in a hardened state doesn’t have enough Cr in solution to be called stainless. Subsequently, steel with a larger amount of Carbon requires more than 12% Chromium to have a reasonable corrosion resistance.
On the market, there are knives with varying degrees of corrosion resistance:
pure Carbon steel, with hardly any Chromium
“semi stainless” knives, with less than 11% Chromium in solution
truly stainless knives, with more than 11% Chromium in solution.
In daily kitchen use, the degree of corrosion resistance will show in how long it takes until stains or pitting can be seen on a knife, e.g. when leaving a knife uncleaned or in the wet kitchen sink for a period of time, or worst cleaning in a dishwasher. However, many knives considered as semi-stainless as per the above definition, will not stain in daily use unless put in a dishwasher.
So the level of corrosion resistance needed in a knife depends on the habits of using and cleaning. If a blade is always wiped clean immediately after even use, a user can be very happy with a carbon steel knife. But if knives are cleaned in a dishwasher, there is little choice to using a knife made of softer low carbon steel with high amounts of Chromium in it.