A general note on rockwell tested on blades.
A blade may be tested at,say Rc62, but that is the hardness where the testing stylus can be applied. The actual edge is often much lower, and there is no simple way to get an accurate Rc of the edge. I know that we all try to be as careful as possible when grinding a hardened blade, but the physics involved in removing the hardened steel from the edge make huge stresses and can severely alter the temper, hardness, and grain structure at the edge.
In the case of localized heating ( you may not see it, but it is there), the edge is heated to high temperature by the friction of the belt grit running over the edge.This happens in a microscopic area at the edge, but the damage can carry back several millimeters on a fine edge. In rough hogging (on unhardened steel) you often see the edge turn bright red. That is often between 1200F and 1400F. Even with care in grinding the edge may heat to near 1000F on a blade with no sign that anything is happening. The heat is rapidly absorbed into the thicker blade body which gets hot...... you may think that it only got to a couple hundred degrees, but the edge is the source of much of the heat that warmed the blade up. You can grind bare handed and the blade will only get good and warm, but the edge can still momentarily attain high temps.
All this can result in a Rc62 blade with a Rc52 edge.
In the case of an embrittled edge, the same forces may create an overly work hardened and/or temper embrittled edge. This edge may seem wickedly sharp, but not last worth a hoot, due to micro-chipping. In this case a Rc62 blade may have a softer or harder, but brittle edge.
Magnified examination of the edge after sharpening, and again after testing will tell a lot about what is actually going on. Use a good 10-20 power loupe to check the edge out. Use strong light and examine the edge carefully.CAUTION: you are putting a very sharp object close to your face. Pay attention and be careful! If the edge looks good after sharpening, but looks all serrated ( microscopically) after testing, then it is either too acute, or too brittle. If it looks rolled ( microscopical) or rounded, it is too soft ( or too obtuse). The brass rod test is a non-magnified test for these conditions. Even after a brass rod test, magnified examination of the edge can reveal a lot.
I mentioned grain structure earlier. Grain growth occurs when grains grow into each other, absorbing the boundaries and becoming larger. At high heat this can happen easily. At temperatures below the Mf, it is static, and growth does not occur.However, any addition of energy to the grain boundaries can create grain growth. Heating to 1600F is one way to rapidly add energy.Even heating to above the Ms can allow slow grain growth . Adding kinetic energy in the form of friction between grains or friction between the metal surface and another surface can also cause grain growth. This creates heat energy,too, but is much more localized ( at the grain boundary) than heating the whole object. If you have ever wanted to cut a coat hanger and didn't have a pair of diagonal cutters handy - what did you do? You bent it back and forth until it suddenly broke apart in your hands. What you did was create larger and larger grains at the site of the bend, until the grains were so large that they had insufficient structural integrity to stay together. Imagine concrete made with pebble size rocks compared to concrete made with basketball sized rocks. Both are the same hardness, but the smaller grained one is many times stronger, as there are millions of times the number of grain boundaries sharing the physical load.
OK, enough physics....it will make JT's head hurt.
Stacy
