Sharpening a Case-Hardened Knife

Case hardening is an old process for giving a piece of pure iron or low-carbon steel a harder surface layer. It's used for gun parts like springs and sears, for gears and crankshafts, and can be used for nearly any piece that needs an outer skin of hard steel. It's used for knives, when the maker doesn't want to, or can't, use the more expensive and harder to produce high carbon or tool steels.

In the Iron Age, smiths discovered that their bloomeries and blast furnaces could produce different grades of iron. The higher carbon fraction, cast iron, could be melted and poured, but would break if the smith attempted to work it with a hammer. The low carbon fraction, wrought iron, could be worked with a hammer, but was too soft to keep a good edge. Steel, with a carbon content in between the two, was more difficult to make, but could be quenched and tempered to make it harder than cast or wrought iron. The harder steel was made, however, the more brittle it became. When case hardening was discovered, the smith had a process that could make his wrought iron into steel, at least on the surface layer, giving a piece with the toughness of the wrought iron core and the hardness of the outer steel layer.

When used to make a knife, this is a nearly ideal blend of qualities. The knife is very hard on the surface, making it easy to bring to and keep a sharp edge. The tough inner core makes it much less likely to break. Pattern welded blades (the better 'folded' Japanese swords, some Viking swords, some Malay kris blades, modern 'Damascus' blades) have a similar effect, in which soft and hard layers are alternated; the blade has hard layers to hold the edge and soft layers for toughness.

diagram, case hardened knife layers diagram, sharpening both sides of case hardened knife diagram, single side sharpening of case hardened knife
Before sharpening, a case-hardened blade has a skin of hard steel and a core of soft iron. When sharpened from both sides, the hard outer layer is worn away and the soft core is exposed at the edge. When sharpened from one side only, the hard outer layer remains at the edge on the unsharpened side.

There is a problem with a case-hardened knife, though. The hardened edge-holding layer on the outside of the knife is very thin, typically a few thousandsths of an inch or less than a tenth of a millimeter, and sharpening the knife wears away a portion of this hard outer layer. After a little sharpening, the soft inner core of the knife is exposed. If the knife were sharpened in the usual way, with equal amounts of sharpening on both sides of the knife, the edge would be composed entirely of the soft core metal, and would no longer hold an edge well. Pattern welded blades do not usually have this problem.

The solution is to sharpen a case-hardened knife on one side only. If only one side of the edge is worn away, then the other side's hard layer extends all the way to the edge. This edge will still be made of the hard outer layer, and will continue to hold an edge as well as when it was first made.

The single-side sharpening will result in an asymmetric blade, which will tend to drift to one side while cutting. It's often better than trying to cut with an edge that dulls quickly, which is what happens when the case-hardened blade is sharpened on both sides. The drift can be useful; when cutting thin slices, chefs can use this drift to counter pressure from the mass of the piece. If the drift is bothersome, replace the blade with one made of better steel throughout.