The issue of heat during sharpening is a surprisingly controversial one. The issue is not whether the body of the blade gets too hot, but whether the edge gets too hot. Most people who power sharpen hold or regularly touch the blade bare handed, and know enough to look for color creeping up the blade. However, if the color formed on just the edge, in a strip only 1/32" wide, would we see it if we didn't think to look for it?
I have 4 knives that led me to stop using my power sharpening equipment. One is in 1055 steel, another is something like 1095, one was A2, and another is S7. All 4 were sharpened on my HF 1x30 belt sander with 220 to 320 grit belts, and power stropped with a leather belt using compound on the same sander. All 4 had edges that would dent during use afterward. Hand sharpening stopped the issues. Additionally, the 1095ish blade was sharpened on a water cooled wheel followed by the Sharpmaker. Then it was sharpened on the HF and power stropped. It was used on the same work after both, and was damaged significantly by edge denting following the HF sharpening. Changing speeds or adding coolant would probably solve the problems, but that required a step up in complexity. Going back to stones was much less complicated. All 4 of these blades had denting issues. If you suspect your current sharpening procedure is causing this sort of damage, I would recommend the bamboo skewer test. Chop a bamboo skewer into pieces. If the edge dents, you've got an issue. If not, then any softening that did occur is secondary other factors and may not be noticed in use.
Something else to think about. The colors seen during grinding/sharpening are oxides that form on the steel surface. Anything that affects how these form affects the color seen when sharpening. For a 1095 blade, a blue color indicates a fairly narrow temperature range. What is the temperature range for turning a stainless steel blue? I don't know, but I can only imagine it is considerably higher. Here is another question that Jan referred to above. If the blade at a point 1/2" away from the edge gets warm to touch (100 to 120 F), how hot did the very edge get, as that is the point of entry of all the heat available to raise the temperature of the whole blade.
One final thought on the grinding heat topic. A fair amount of research has been done into how much heat is generated while grinding. The most reliable study I've read shows a temperature of somewhere near 1000 F generated during grinding. Some other first hand conversations indicate the temperature can (not is or will be, but can) be much higher. I know GE regularly does tests to see if grinding their parts has resulted in excess grain growth in nickel alloys meant for high temperature applications.
Is overheating possible? Yes.
Is it always detectable before use? No.
Do we know how much softening has occurred? Typically no.
Would I knowingly pay someone to sharpen one of my blades used for fine cutting with high speed and/or uncooled power equipment? No.
Note this question had several qualifiers; knowingly, fine cutting, high speed, etc. If you want to use an angle grinder to sharpen a blade used to cut shingles, I don't see a problem other than needing to buy a new knife really soon.
In the end, is a sharp blade that's a little softer better than a dull blade? Depends on the user, but I'd say yes.
I have 4 knives that led me to stop using my power sharpening equipment. One is in 1055 steel, another is something like 1095, one was A2, and another is S7. All 4 were sharpened on my HF 1x30 belt sander with 220 to 320 grit belts, and power stropped with a leather belt using compound on the same sander. All 4 had edges that would dent during use afterward. Hand sharpening stopped the issues. Additionally, the 1095ish blade was sharpened on a water cooled wheel followed by the Sharpmaker. Then it was sharpened on the HF and power stropped. It was used on the same work after both, and was damaged significantly by edge denting following the HF sharpening. Changing speeds or adding coolant would probably solve the problems, but that required a step up in complexity. Going back to stones was much less complicated. All 4 of these blades had denting issues. If you suspect your current sharpening procedure is causing this sort of damage, I would recommend the bamboo skewer test. Chop a bamboo skewer into pieces. If the edge dents, you've got an issue. If not, then any softening that did occur is secondary other factors and may not be noticed in use.
Something else to think about. The colors seen during grinding/sharpening are oxides that form on the steel surface. Anything that affects how these form affects the color seen when sharpening. For a 1095 blade, a blue color indicates a fairly narrow temperature range. What is the temperature range for turning a stainless steel blue? I don't know, but I can only imagine it is considerably higher. Here is another question that Jan referred to above. If the blade at a point 1/2" away from the edge gets warm to touch (100 to 120 F), how hot did the very edge get, as that is the point of entry of all the heat available to raise the temperature of the whole blade.
One final thought on the grinding heat topic. A fair amount of research has been done into how much heat is generated while grinding. The most reliable study I've read shows a temperature of somewhere near 1000 F generated during grinding. Some other first hand conversations indicate the temperature can (not is or will be, but can) be much higher. I know GE regularly does tests to see if grinding their parts has resulted in excess grain growth in nickel alloys meant for high temperature applications.
Is overheating possible? Yes.
Is it always detectable before use? No.
Do we know how much softening has occurred? Typically no.
Would I knowingly pay someone to sharpen one of my blades used for fine cutting with high speed and/or uncooled power equipment? No.
Note this question had several qualifiers; knowingly, fine cutting, high speed, etc. If you want to use an angle grinder to sharpen a blade used to cut shingles, I don't see a problem other than needing to buy a new knife really soon.
In the end, is a sharp blade that's a little softer better than a dull blade? Depends on the user, but I'd say yes.
