Pre-oxidation treatment temperature

Pre-oxidation treatment temperature

The protective effect of Al2O3 or Cr2O3 in the oxide film increases with the increase of temperature, the better the protective effect of the film. Generally, the content of the main oxide in the film should not be less than 90%.

The relationship between the heating temperature of the iron-chromium-aluminum electric heating alloy 0Cr25Al5 and the Al2O3 content in the film is shown in Figure 3-7.

The pre-oxidation temperature of the alloy should not be lower than 1150℃. As the aluminum content of the alloy increases, the pre-oxidation temperature should increase accordingly. On the contrary, the temperature should be appropriately lowered. To form a protective film mainly composed of Cr2O3 and NiO on the surface of nickel-chromium and nickel-chromium-iron electric heating alloys, the heating temperature should not be lower than 900°C.

The length of the holding time determines the thickness of the oxide film. The thickness of the protective oxide film is usually (1~2)×10-4mm. According to the oxidation rate of the alloy, it needs to be kept for 4~8h. Pre-oxidation treatment temperature and holding time of electric heating elements of various alloys

(1) It can slow down the permeation rate of oxidizing gas in oxidizing medium and reduce the rate of alloy oxidation.

(2) In non-oxidizing media (such as nitrogen, hydrogen, carburizing gas, sulfide, etc.), slow down the gas penetration rate and reduce the corrosion rate of the alloy.

(3) It can reduce the volatilization loss of electric heating alloy components under vacuum or low pressure conditions.

The protective oxide film is the only barrier to prevent carburization of electric heating elements. Once the oxide film is destroyed, the reducing gas will penetrate into the alloy through the damage and carburize. The affinity of chromium, iron and carbon in electric heating alloys is very strong, while aluminum, silicon, nickel, etc. are very weak. Therefore, when the carburizing gas contacts the alloy, the following carburizing reaction will occur.

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