In my post on the components of a great knife, I talked about how steel was just one of the important decisions in making a knife. There are certain things to consider when trying to decide on what steel to use.
When choosing a steel type, there are 4 (sometimes 5) main properties to look at for knife making. They are strength, toughness, wear resistance, and edge holding, and, occasionally, stain resistance.
Strength: Sometimes also described as hardness (a different can of worms, see below), strength is the ability for the steel to take a load without permanently deforming. When cutting, stresses are applied to the blade edge, and strength is what keeps the blade from bending, thus completing the cut. Strength is certainly the property of knife steels that gets the most attention, and although important, also needs to be balanced by toughness.
(A note on hardness: Hardness is a tricky word, as it is determined as a single number, the Rockwell hardness, which measures the strength of the steel matrix, but does not take into account carbides. Carbides can make a less strong steel functionally “harder” due to increased wear resistance.)
Toughness: This is the ability for steel to take an impact without damage. And by damage I mean, chipping, cracking, or breaking. Usually the damage is, instead, in the form of deformation (such as bending or rolling an edge). Toughness is (generally) inversely proportional to strength in steel. A blade that is too strong will chip out or break, where a blade that is too tough will regularly just lose its edge. Toughness is important in heavy jobs such as chopping, and vital any time the blade hits a hard impurities in a material being cut.
For the most part, knife makers must decide on a good balance between strength and toughness. As in any part of blade design, this balance is completely dictated by the end use of the knife. A sushi knife, with it’s exacting nature, and particular set of constraints, would be best made much stronger than a meat cleaver, which would need to have high toughness. A chefs knife would aim for a nice middle ground.
Wear resistance: a steels ability to withstand abrasion. For the most part, wear resistance is determined by the amount, type, and distribution of carbides within the steel. Again, wear resistance is often confused with strength, lumped together as hardness. It is possible to have extremely hard carbides sitting in a weak steel matrix, in which case the carbides will become a point of failure, chipping out of the weak steel at a microscopic level.
Some of the most common carbide forming elements in knife making steel are chromium, molybdenum, tungsten, and vanadium. Each has different characteristics, both pros and cons, which I’ll talk about in another post. Carbides themselves are a double edged sword. On the one hand, they do provide increased wear resistance and give certain blades a sort of microscopic saw action, on the other hand, carbides consume free carbon in the steel matrix, and, depending on the amount present, can considerably lower the “active” carbon content in a steel, reducing hardenability. Some sorts of carbides form within the grain boundaries of the steel, increasing brittleness. Carbides like chromium carbide (Cr3C2) are considerably larger than individual atoms in the steel matrix, eliminating the ability to ever get a smooth razor edge.
Edge holding: like it sounds, is the ability of a blade to hold an edge. Commonly used interchangeably, wear resistance and edge holding are the same. Edge holding can be effected by wear resistance, but they are not the same.
Edge holding is a job-specific property. Edge holding is a function of wear resistance, strength, and toughness, as well as edge geometry. Each job requires different properties for edge holding. In a meat cleaver, toughness is extremely important, because of all the hard bits and pieces – micro-chipping is the main reason for edge degradation. For wood chisels or whittling knives, strength is most important as the primary reason for edge degradation is edge rolling and impaction. Wear resistance is important for edge holding when cutting very abrasive materials, like sheets of sandpaper. For commercial prep kitchens, corrosion resistance would be important, as all the acidic foods would quickly degrade the blade.
Stain resistance (or rust resistance / corrosion resistance): stain resistance is the ability to withstand rust (oxidation). This can be helpful in corrosive environments, such as salt water (dive knives) or high moisture environments (swamps). Also, when the main function of the knife is regularly cutting acidic things (e.g.,various foods). Micro-oxidation can damage the blade and lead to edge loss in a very short amount of time. In “stainless” steels (stainless is actually a lie, these all will rust given the right circumstances and time – stain resistant is a much more realistic term) stain resistance affected most by free chromium (not chromium carbides). Some common stain resistant steels have upwards of 13% Cr content, (11% being the common baseline for a steel to be considered “stainless”).
Overall, it’s all about knowing your end use.
