01-02-2018, 06:36 PM
Mr. Lucius, tapered blades are often a problem. Many times people will use a coupon during heat treatment testing, with flat sides and polished. Going a step further is having it mounted and polished to allow the most accurate measure possible. Polishing and microhardness testing is going yet a step further.
I have never used hardness files. I have heard of blades skating a file, yet when checked under magnification, had a much softer structure than intended. Conversely, it's not unheard of for someone to fail to consider the decarburization that can happen on the surface of steel pieces, and try file testing only to come away with the impression their steel didn't harden. After filing more, they discover they just didn't go deep enough and are elated to find their steel did exactly what they wanted.
Assuming the steel in the shears was a fairly simple carbon steel, the hardness drop indicates temperatures in the range of 500 to 700 degrees F, depending upon steel, though to loose hardness in such a short time, it was probably higher. Stainless steels are generally more resistant to softening than that, and common kitchen knives are generally quite soft anyway, and so may not soften at all. There are a huge number of variables, from steel type, abrasive type, cooled, fast, slow, etc. Power sharpening without coolant doesn't always cause issues, and water or oil cooled sharpening can still cause burning. Sparks are no guarantee of overheating, as iron/steel powder (and many other metal powders) can auto-ignite during grinding, causing sparks. The only way to sort it out is to find a reliable way to evaluate the end result, which in our case on this forum is initial sharpness and it's longevity.
Again, I am surprised at how controversial this topic can become. I think it's perfectly reasonable for anyone who sharpens with powered equipment to consider overheating. It's possible to do without even knowing. Fortunately, using and/or testing the knife will reveal the problem. As long as there isn't gross overheating where color changes are evident, it can be remedied fairly easily.
All these issues are avoidable with enough experience, information, and a way to examine the results. Edge holding tests with the EOU sharpness devices are one way to examine the edge, both before and during use/testing.
I have never used hardness files. I have heard of blades skating a file, yet when checked under magnification, had a much softer structure than intended. Conversely, it's not unheard of for someone to fail to consider the decarburization that can happen on the surface of steel pieces, and try file testing only to come away with the impression their steel didn't harden. After filing more, they discover they just didn't go deep enough and are elated to find their steel did exactly what they wanted.
Assuming the steel in the shears was a fairly simple carbon steel, the hardness drop indicates temperatures in the range of 500 to 700 degrees F, depending upon steel, though to loose hardness in such a short time, it was probably higher. Stainless steels are generally more resistant to softening than that, and common kitchen knives are generally quite soft anyway, and so may not soften at all. There are a huge number of variables, from steel type, abrasive type, cooled, fast, slow, etc. Power sharpening without coolant doesn't always cause issues, and water or oil cooled sharpening can still cause burning. Sparks are no guarantee of overheating, as iron/steel powder (and many other metal powders) can auto-ignite during grinding, causing sparks. The only way to sort it out is to find a reliable way to evaluate the end result, which in our case on this forum is initial sharpness and it's longevity.
Again, I am surprised at how controversial this topic can become. I think it's perfectly reasonable for anyone who sharpens with powered equipment to consider overheating. It's possible to do without even knowing. Fortunately, using and/or testing the knife will reveal the problem. As long as there isn't gross overheating where color changes are evident, it can be remedied fairly easily.
All these issues are avoidable with enough experience, information, and a way to examine the results. Edge holding tests with the EOU sharpness devices are one way to examine the edge, both before and during use/testing.