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Rockwell Hardness Explained
The process of Rockwell testing was patented back in the mid 20's by a couple of guys named Hugh and Stanley Rockwell to determine quality control on the bearing races that they manufactured. They needed to know (like me) if they were producing a good and consistent product. Hugh and Stan developed it for themselves but found a commercial use for it and later patented the machine.
Every so often I take my woodcarving knives and hunting knives to a local manufacturing plant and have the Rockwell hardness tested. I do this because I want to know if my hardening methods are still effective and consistent. It really is a neat process and I will attempt to explain in simple language.
How is it done?
Well, it really is pretty simple. I give the testing person a flat heat treated (hardened and tempered) sample. It must be flat or the test is inaccurate. He then puts it on the bed of a machine that looks a lot like a desktop drill press. He sets it up by pushing some buttons to program the test. The machine starts by pushing down a small cylinder with a point that looks like a pencil. It first places minor pressure into what would be the blade edge and then takes a second pressure test in the same exact spot with a more major load. 150kgf, (150 kilograms of force) or if you're like me and haven't learned the metric system, approximately 331lbs of force on a tip of a diamond point. That's like having your brother-in-law standing on your knife blade. Yikes! This leaves a nice little dent in the blade. The depth of this dent (Shallow = hard, Deep = Soft) gives us a measurement of hardness. The difference between the two resistance numbers (major load and minor load) is the Rockwell hardness. This company does it on two places on the same blade to give me an average for that blade.
Most of the time, I bring 4 blades that I have pulled randomly from my inventory, have them each tested and take the average. If my blades fall too far (soft) or way too high (Hard), I have to take a look at what may have changed in my heat treating process. It could be that my quench oil is too old and isn't hardening anymore or my heat treating oven isn't holding temperature or... well, let's just say that it begins a whole new investigation.
Great, you've got a Rockwell hardness number now what does this mean? Well, I shoot for a hardness between 60-64 because I like the characteristics of a blade that hard. It sharpens fairly easy and stays sharp a little longer and has some toughness. I don't want to constantly sharpen while carving. Most knives fall into a range of 50RC up to 65RC. Now potentially steel can be hardened to 70 but it really is not useful for wood carving, because a fine edge would chip out on a nice piece of winter-cut basswood. Some carving knives hold a Rockwell number of 55 and they sharpen easily, but you sharpen often.
Rockwell testing isn't limited to steel you can test plastics, ceramics, brass, copper, sheet metal etc. The letter behind "Rockwell" but before the number determines what kind of point or "indenter" was used. Different indenters for different materials. So for example in my case, Rockwell 62C or 62RC. When testing my knives a diamond cone is used. If a different indenter was used, the letter C is different.
Well there you go. At your next Carving Club meeting, you can astound your friends with you knowledge of Rockwell hardness testing.