Josh,
Your questions are not easily answered, because your underlying assumptions are false. It's just not as simple as "good" or "bad" ratings.
The Rockwell hardness test uses a pre-set force driving a small point into the steel. The depth of penetration is measured, and the Rockwell hardness is derived from that.
Edge holding and toughness can both be related to hardness, although that varies from steel to steel.
"What is a good hardness" is complicated by several factors:
- What are you using the knife for? If you're doing pure slicing, maybe you want the knife to be harder to emphasize edge holding, and don't care about toughness as much. For heavy chopping, you might want the knife softer so it'll be tough enough to stand up to the repeated impact.
- What kind of steel are you using? Given a certain use for a knife, the ideal hardness will vary from steel to steel. You cannot necessarily compare two different steels based on their rockwell hardness.
- What kind of heat treat was used? There can be different ways to get a steel to a particular hardness, and that can affect how the steel performs.
- Is the test even relevant for this type of steel? The Rockwell hardness test measures hardness by measureing penetration into the steel. It does not measure some important factors that will affect edge holding and toughness, such as amount and distribution of carbides. Talonite, which consists of lots of very hard carbides in a very soft matrix, comes out with a very low rockwell rating even though it holds an edge forever. To a lesser extent, the particle metallurgy steels also often come out with ratings a little low, but their edge holding can end up being incredible.
Given all those caveats, here is what most people use rockwell hardness for:
Given two knives of the same steel and roughly the same edge geometry and heat treat, the one with the higher rockwell hardness will probably hold an edge better but be more brittle.
Note that having "more edge holding and less toughness" (or vice versa) is neither a good thing nor a bad thing. It depends on the job. More edge holding and less toughness is appropriate for some jobs, inappropriate for others.
Anyway, hopefully I've disabused you of this notion that knowing the Rockwell hardness by itself can give you a wealth of information. It's one piece of information, but it can be extraordinarily misleading unless you also arm yourself with knowledge about the limitations of the test, and knowledge about other factors affecting hardness, edge holding, and toughness.
Joe
jat@cup.hp.com