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If I wanted to for whatever reason, heat a blade up then let it air cool to room temperature, what temperature would be the max safe temperature so the blade's temper is not ruined? What if I wanted to, again for an irrelevant reason, cool a blade significantly (freezing or lower) and let it air warm up to room temperature, what is the min safe temperature?
The answer is it depends. What kind of steel is it? A carbon steel blade will lose hardness at much lower temperatures than stainless.
I'd not recommend more than 350 F for any reason for an unknown steel. If you only wanted to sterilize the blade use boiling water at 212 F. If you get the blade too hot, it will take on oxidation colors. Gold is still good, blue is bad, purple gray and the hardness is gone.
Minimum safe temperature would be absolute zero, if you could get it there. Most steels will tolerate liquid nitrogen without any problem. But, you will have to run a tempering cycle to convert the fresh martensite into tempered martensite. Or the blade will be even more brittle than it was before freezing.
+1 also be aware that temperature changes may cause the blade to warp by creating or releasing stress in the metal. More Toll and Die Making 101. You should get a Machinist Hand Book. That is the actual name. http://www.feedforward.com.au/machine_tools_sample.pdf for example.
To be on the safe side I try to not let the blade get too hot to touch. If buffing on a wheel or dremel tool I'll do 3 or 4 at a time and when one gets warm put it down and move to the next one and so on.
Please, please, please be extra careful when applying any sort of high heat to a straight razor.
Due to the extremely thin cross-section of the cutting edge, it is possible to build up way too much heat there before it can be 'wicked' away to the thicker portion of the razor. This cautionary note applies even more strongly to those straight razors with a very pronounced hollow grind (thinner).
The safe rule would be to NEVER apply heat to a straight razor using an open flame source. :nono: This list would include candles, bunson burners, propane torches and so forth.
Heat sources that might be considered safer (but never completely safe) would be ovens. A hot plate might be considered for use, but ONLY if one uses extra precautions to evenly spread the heat and at the same time hold the razor away from direct contatct with the heat source (via a screen platform or such).
In all cases, the application of heat should be SLOW and controlled. If you have no way to judge how much heat you are using then you are setting yourself up for a big disappointment as you might well draw the temper of your razor, thus rendering it useless.
When heating the steel of a straight razor blade, if you start to see a color change in the metal, you are already in dangerous territory as regards the strength of the steel. Heated steel exhibits color changes at particular temperatures. - - http://www.sizes.com/materls/colors_of_heated_metals.htm - - But, again, with respect to the very thin cross-section of a straight razor, you may have almost no time to react to this color change before it is too late. If the steel of your razor has started to turn yellow... on into blue... you have in all probability already affected the characteristics of the blade steel. Usually not for the better. :(
As a final note. Do not be tempted to think that placing a razor into boiling water is a good idea for any reason. I have heard a story (from a manufacturer) regarding a client who seemed to think that his brand new razor needed to be sterilized before use. Rather than simply wipe it down with some alcohol, he was silly enough to put said razor into a pan of boiling water on the stove... and succeeded in completely ruining not only the finish, but also the temper. The razor was a total loss. :cry:
The cautionary note is good Ignatz. I can see that just leaving a razor to lie on the bottom of a pan of hot water would expose it to the higher heat from the heat source at the bottom of the pan.
The 100 C/212 F temperature of a bath where the blade was suspended would not be enough by itself.
I, and some others, routinely heat treat steels using a molten salt bath. The quench is often as high as 150+C/300+F depending on the steel and the requirements of the tool. But the blades are suspended and removed from potential contact with the heat source. The legendary lead baths at Thiers and Dovo would be similar in principle.