Case Hardened Knife
At Life's Forge, we make (along with other pieces) custom hand-forged blades. Some clients want historically accurate reproductions. In historical times, a few blades were case hardened, but most were made of higher carbon steel, pattern welded, or not hardened at all. It's important to know the difference, because case hardened blades must be sharpened in a particular way to hold their edge.
Case hardening is an ancient 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 hardenable steel. It was (and is) sometimes 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 highest carbon fraction, cast iron, could be melted and poured with very high heat, but would crumble or break if the smith attempted to work it with a hammer. The lowest 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 the surface of his wrought iron into steel, giving a piece with the toughness of the wrought iron core and the hardness of the outer steel layer.
The pattern-welded blade sandwiches hard and soft layers; sharpening exposes the hard layers at the edge.
The case-hardened blade has thin skin of hard steel and a core of soft wrought iron or mild steel.
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.
There is a problem with a case hardened knife that isn't found in pattern-welded or homogenous steel knives. The hardenable 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.
With modern steelmaking technology (from the second half of the 1800s), steel has become both much cheaper and much better understood. Modern steel is of much more reliable quality than the older products. Now we use mild steel in place of wrought iron. It has some differences, but neither one will harden enough to form a good edge. Case hardening works just as well on mild steel as it does on wrought iron.
When sharpened from both sides, the hard steel outer layer is worn away and the soft core is exposed at the edge.
When sharpened from one side only, the hard steel outer layer remains at the edge on the unsharpened side.
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 or with a pattern-welded blade.