Adam Smith Knives

Observations on the knife performance system (with thanks to the work of Dr. Larrin Thomas)

Most of this comes from my reading of Knife Engineering by Dr. Larrin Thomas. Knifemakers should read that book cover to cover, more than once. Knife users may be able to take some benefit from this summary of what I feel are critical points.

Dr. Thomas has not endorsed this summary, any errors in my reading his book or other writings, are obviously my own. I am grateful for his contribution to the knife community's metallurgical resources. It is an improvement by orders of magnitude to what was available when I started learning in this field, in the late 70's.

1. Geometry is the most important factor in edge holding. Finer edges hold much longer than coarser edges. It is a much bigger effect than steel grade or hardness. (Though hardness is important in allowing for a fine edge, see below).

2. There are three major ways for a knife to lose its edge:

chipping or other fracture mechanisms,
edge rolling (or other deformation), and
direct edge wear (like a shoe heel rubbing on pavement).

There is a fourth edge loss factor, comparatively minor in most circumstances – edge loss from corrosion.

Edge rolling, chipping and corrosion can all be thought of as premature failure. We'd like to design our knife system (steel and heat treat choice, edge angle, and sharpening regimen) so that our knife loses its edge by wear rather than chipping or rolling.

3. There are competing properties required from steel to address the different edge loss mechanisms. To minimize chipping/fracture we want the steel to be as tough as possible (toughness is the opposite of brittleness, impact strength, not the same thing as tensile strength). To minimize rolling we want it to be as strong as possible (which normally correlates with hardness). To minimize wearing we want it to be as hard as possible, and with significant percentage of hard carbides. Carbides are compounds of carbon and metal, and are formed in steel if enough carbon is present and conditions are right. The carbides we are concerned with are harder than the base hardness of the matrix in which they are formed. The mental image of concrete: microscopic stones in cement matrix, is a useful one.

Once again, these are competing properties. Increasing hardness normally reduces toughness (within a given steel type). High carbide percentage has some effect of reducing toughness (but this is a larger effect for large and/or weak carbides than small strong ones). Vanadium carbide is much harder than chromium carbide which is much harder than ferric (iron) carbide. Some stain resistant steel grades are formulated so that all of the chrome is in solution (dissolved) in the iron, and no significant amount of chromium carbide is formed, the steels with this design are among the toughest available to the knifemaker but show much lower wear resistance than steels with lots of carbide. Particle metallurgy techniques allow steels to be formulated with significant amounts of vanadium or chromium and other carbide forming elements, while keeping the grain size including carbide size small. As a result toughness is not harmed as much in PM steels as would be the case for conventional melt processes with the same composition.

4. Given the above we are looking to select the steel (and heat treatment) that is best suited to hold the finest possible edge geometry, relative to a particular use. The use dictates the required amount of toughness and strength. We want to go as hard as we can with as much hard carbide as we can, without reducing the toughness to the point that we have edge chipping (or outright breakage). Stain resistance may also play a role in steel selection. Once the knife is made, we can only address chipping or rolling by increasing the edge angle, which will have the effect of reducing edge retention.

5. Wear resistance, as such, matters most in cutting materials that are somewhat abrasive (e.g. cardboard, mud soaked moose hide, carpet, electrical insulation, roofing material). Pure wear resistance matters less if non-abrasive materials are being cut (like most kitchen use, barring the use of abrading cutting boards). My theory, not yet fully proven, is that kitchen knives mostly want to be hard enough for fine edges, and once that is accomplished with adequate toughness, high carbide steels contribute less to overall edge stability than for "outdoor" and "hard use" applications.

6. Selection and execution of the heat treatment is critical to get the design properties from a given steel. The details of this are beyond the scope of this summary, but it is important to know that a given steel grade can go from an optimal to a terrible performance profile depending on heat treat selection and execution. In particular, over hardening (too hot or too long) will damage a blades performance.

6. Forging blade blanks can contribute, in a minor way, to toughness and alignment of grain direction with the edge. Improper forging (too hot or cold, improper or inadequate normalization or annealing) can greatly harm blades. Heat treating in a forge will rarely, if ever, give an optimal heat treatment for a given steel analysis. Lower alloy steels are more forgiving of imprecise (forge) heat treatment than higher alloy steels. It is absolutely the case that laminating different metal types (normally by forge welding) is the most effective way of protecting a blade from outright breakage while providing a hard steel edge. If you want to be able to bend a blade 90 degrees, lamination is the best way to get there. My own feeling is that outright breakage is not a significant factor for knives, barring abuse or using the wrong knife for a high strength task (batoning a fine cutting knife, prying a fine point).