The Rockwell Hardness Rating & Knife Steel
The Rockwell hardness testing is done by a gauge that forces a diamond-tipped probe, often called a cone, into the metal being tested. The depth of penetration for a given amount of force is measured and converted to a scale of relative hardness. This scale is called the Rockwell C scale. The hardness rating is usually abbreviated Rc.
There are other hardness tests for other types of materials. Because some high-tech knife blades are made out of ceramic, you will hear the mohs scale used. That relates to the hardness of minerals and is not appropriate for steel. Since a ceramic knife blade is a stone, mohs is used for that application alone. The rest of the time, the Rc scale applies.
In the 1960s, a trend was started among custom knifemakers. They began using the Rc rating of their blades as a sales gimmick. That started a race for ever-higher Rc ratings that is still going on today. You don't necessarily want the highest Rc rating you can find. When the rating is in the middle 60s, the steel becomes so hard that it can shatter like glass!
It's too simple to say that the higher Rc rating, the longer the knife will hold its edge. The type of steel has a lot to do with edge-holding ability. Each steel alloy has a hardness range that is optimum for it when used in a knife blade.
You'll notice that the hardness of stainless steel is usually lower than the hardness of carbon steel. That illustrates what was said about the various types of alloys having optimum hardness ranges.
With larger manufacturers, like Boker, Case, Ka-Bar, etc., you can be sure that they are using the optimum hardness range for the steel in their blades. These big companies have often had government contracts, where their blades were tested far more severely than any commercial blade ever would be. And, the government gets real serious when it comes to the metallurgical composition of their products!
The time to worry about Rc is when you buy a knife from an individual maker that brags that he makes all his blades from rasps and hardens them over the kitchen stove! You may get a fine knife from a maker like this - but just as easily you could get something that is worse than useless.
The thing to remember is that Rockwell hardness is one component of knifemaking, just as steel alloy is another. Your best bet is to stay with a qualified knifemaker whose products you can trust.
I've discussed Rockwell hardness as though it magically happens, but, of course, it doesn't. There is a process of heating the steel, quenching it and then tempering it that turns out a product within a certain hardness range. On Tuesday, I'll write about that process and what it means to you.
Copyright 2005-2006 by Cheaper Than Dirt. Reprinted with permission
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