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As I know tempering of hardened metal must be at temperature close to 150 celsius degrees.

So why exist so widespread idea that you must not use (good) knifes to cut hot products or to wash it in hot water?

(they telling that knife will lost it's cut properties... WHY?)

UPD: Absolutely sure that better do not clean good knifes in dishwashers - this knife will be dead from those moment in a ~ month period. Because of salt used in dishwashers. Salt will be cause of corrosion of sharpened part of knife. Corrosion of sharpened part = blunt knife.

As I understood, prohibition of usage knifes in temperatures higher than 45 degrees is because of the same reason:

  • O2 + salts

  • temperature is the cause faster corrosion because of salts

  • any water except distilled have salts

But time period for corrosion is longer than corrosion period of dishwashers wash corrosion.

BUT! I'm almost zero in chemistry and in phisics. That's why I'm asking here :)

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They are wrong. There are no solid-state microstructural transformation processes active at those temperatures. If the knives you have are stainless steel, then there are no corrosion processes active at those temperatures either which might dull the cutting edge of the knife.

If your knives are made from carbon steel (NOT stainless) then the knife will corrode at those temperatures only if it is in contact with acidic food like fresh tomatoes or tomato sauce. The knife surface will become discolored and brownish rust may form on it. You can stop this process by washing the knife (hot water and soap are OK!) and then wiping it lightly with cooking oil.

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  • $\begingroup$ But in the same time phisic properties of metal and phisic properties of sharpened metal can be different. As example: you will not see any problems with stainless steel pan if you wash it in dishwashers. But if you use dishwashers for any good enough knife - this knife will be dead from those moment in a ~ month period. Because of salt used in dishwashers. And because of sharpened metal have a little bit another phisic/chemical properties. That's why I'm not sure that you are right. $\endgroup$
    – Andrew_STOP_RU_WAR_IN_UA
    Commented Oct 5, 2020 at 12:19
  • $\begingroup$ as I understood, the idea of this process is because of combination of three things: salt(exist in any water except distilled), O2 and temperature(water is hot and phisical-chemical process of corrosion doing faster). So this can be the same reason that I explained above about dishwashers. But time period is much longer. And it can be filled much more on the sharpened part of knife. BUT! I'm almost zero in chemistry and in phisics. That's why I'm asking here :) $\endgroup$
    – Andrew_STOP_RU_WAR_IN_UA
    Commented Oct 5, 2020 at 12:44
  • $\begingroup$ I thought that atomic diffusion could occur at those temperatures. Is this wrong? Of course, it would be strange to have a knife where this process could be detrimental, but that is another thing. $\endgroup$
    – untreated_paramediensis_karnik
    Commented Oct 5, 2020 at 14:40
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    $\begingroup$ @thermomagneticcondensedboson, diffusion processes in metals do not become significant until you reach 1/3 to 1/2 the melt temperature in degrees absolute. $\endgroup$
    – niels nielsen
    Commented Oct 5, 2020 at 16:36
  • $\begingroup$ according to the following site: polymersolutions.com/blog/why-does-stainless-steel-rust corrosion from the hot water tank might cause even stainless steel to rust. This looks like have a sense. And especially this will be dangerous for sharpened metal - it is really can be the cause of making sharp knife to dull knife. Niel, can you give me your opinion about this, please? $\endgroup$
    – Andrew_STOP_RU_WAR_IN_UA
    Commented Oct 9, 2020 at 21:38

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