The higher the melting point of the metal material, the higher its usable temperature limit. If the thermodynamic temperature is used to express the melting point, 60% of the metal melting point Tm is defined as the theoretically usable temperature upper limit Tc, that is, Tc=0.6Tm. This is because as the temperature increases, the mechanical properties of metal materials decrease significantly, and the tendency of oxidation and corrosion increases accordingly. Therefore, general metal materials can only work for a long time at 500~600℃, and can work at a high temperature of >700℃. The working metal is generally called heat-resistant alloy, and "heat-resistant" means that it can maintain sufficient strength and good oxidation resistance at high temperatures.