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What a can of worms you opened! Rc is Rockwell hardness rating. I'm not sure what the numbers are calibrated to, but most blade steels will run from upper 50's (57 or so) to low 60's (60 or 61). Different steels can be brought to different hardnesses, and these hardnesses serve a different purpose. Too hard and the blade is brittle and will not bend, but break. Too soft and the edge dulls quickly and there isn't enough springiness to the blade. Every steel has a good balance. As far as which steel is best, you can't answer that. Different steels excel in different ways. It also depends on the thickness of the blade, hardness, type of grind, angle of the edge, width of the blade, etc. There are way too many factors, and no one steel is going to do every cutting task "the best" because there is more to it than just steel.
A good source for steel types (and also sharpening) is the FAQ section of this site. Joe Talmadge listed almost any steel used in knives today. Another source for Rockwell rating is Steve Bottorff site at http://www.ameritech.net/users/knives/index.htm
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.
Josh, the best way to learn about steels and which ones are the best for you and the way you use a knife is right here on this forum. Read the FAQ's and keep an eye on all the various threads and posts.
In the meantime, you won't go wrong by purchasing any of the quality knives you see mentioned in these posts. The steels that are used are of a much higher quality than most of the 'generic' knives on the market. Pick a knife you like from one of the top manufacturers: Spyderco, Benchmade, Cold Steel etc. You can rest assurred that you will get a steel that is much better than average.
"Walk softly and carry a big folder... and a small folder... and a SAK... and a multi-tool..."