Interesting stuff, Andy. I can add another - case hardening. That is, packing the iron in a container with carbon, heating and letting it diffuse into the surface for several hours. This forms a high-carbon steel as an outer skin that is very hard, and can be hardened further by quenching. This is still sometimes used, having the advantage that it can be done after the component is made from a softer steel. It's an old process, still used.
Also, induction hardening uses an induction coil to heat the outside surface of a component, usually medium carbon steel, which can then be quickly cooled or quenched to harden it.
Quite a few processes have been developed to make a hard outer layer or skin with a tough, ductile interior. It's pretty much an ideal tool material.
It's relatively recently that alloy steels (such as the fancy ones now used for planes) have been developed that are hard throughout but can be worked into shape without cracking. That is, for a factory with big expensive equipment, they can be relatively cheaply made as is is a one-step process and gives good marketing copy. As Custard and others have pointed out, it is not so obvious that they are useful in the workshop, since they are hard (literally) to sharpen. Hence also the multiplicity of threads on the Voldemart topic.
One could even argue that they are really aimed at well-off amateurs rather than pros. The professional might get the advantage of sharpening every hour or two rather than every 10-20 mins, but it's a harder (slower) job. The amateur will use the tool far less, and may only have to sharpen every month or two rather than every day, so will think that it's a miracle!
I'd personally go for a laminated blade rather than a super hard one, except on tools that get only occasional use such as the shooting plane. In this case the lower wear is useful as it has a heavy task, but is only used for a few strokes, once or twice per board. I've had one a few months and only needed to sharpen it once. (And yes, I know that it is not necessary, it's a treat).
Also, induction hardening uses an induction coil to heat the outside surface of a component, usually medium carbon steel, which can then be quickly cooled or quenched to harden it.
Quite a few processes have been developed to make a hard outer layer or skin with a tough, ductile interior. It's pretty much an ideal tool material.
It's relatively recently that alloy steels (such as the fancy ones now used for planes) have been developed that are hard throughout but can be worked into shape without cracking. That is, for a factory with big expensive equipment, they can be relatively cheaply made as is is a one-step process and gives good marketing copy. As Custard and others have pointed out, it is not so obvious that they are useful in the workshop, since they are hard (literally) to sharpen. Hence also the multiplicity of threads on the Voldemart topic.
One could even argue that they are really aimed at well-off amateurs rather than pros. The professional might get the advantage of sharpening every hour or two rather than every 10-20 mins, but it's a harder (slower) job. The amateur will use the tool far less, and may only have to sharpen every month or two rather than every day, so will think that it's a miracle!
I'd personally go for a laminated blade rather than a super hard one, except on tools that get only occasional use such as the shooting plane. In this case the lower wear is useful as it has a heavy task, but is only used for a few strokes, once or twice per board. I've had one a few months and only needed to sharpen it once. (And yes, I know that it is not necessary, it's a treat).
