Function of preoxidation
The preoxidation makes the surface of the workpiece obtain a layer of dense oxide film (3Fe + 2O2 → Fe3O4), which is then reduced by the carburizing atmosphere at the initial stage of carburizing, forming a clean primary iron (Fe3O4 + 2 [C] → 3Fe + 2co2), showing a high chemical activity and producing a large number of active sites that can adsorb the carburizing agent. At the same time, slight oxidation on the surface of the workpiece can properly improve the surface roughness of the workpiece, form micropores, artificially increase surface defects, also provide dislocation outcrops, steps and various surface defects, and form active centers with low “potential barrier”, which greatly increases the adsorption probability and adsorption capacity of the penetrant, thus accelerating the infiltration process of active carbon atoms and the formation of surface carbon concentration gradient It can effectively improve the carbon absorption activity of parts, adapt to the requirements of new carburizing process, and obtain qualified carburizing performance.
Purpose of pretreatment before carburizing
① The parts shall be preheated before carburizing, and the uncleaned grease can be burned at the same time, which is not easy to form carbon black;
② The reaction between oxygen and iron can produce Fe3O4 film, improve the surface activity of carburized parts, accelerate the speed of carbon atoms adsorbed on the surface of steel parts, and improve the carburizing speed and uniformity;
③ Fe3O4 is a dense oxide film, which can prevent further oxidation and decarbonization.
④ Stress relief and deformation reduction (the effect is not obvious when 500, 550-600 is better for stress relief, more than 500 is easy to be oxidized, resulting in oxidative decarburization. Preoxidation itself is not for stress relief. Indirectly, in order to remove stress, higher
⑤ Fe3O4 is a dense oxide film, which can prevent further oxidation and decarbonization.
⑥ Stress relief and deformation reduction (the effect is not obvious when 500, 550-600 is better for stress relief, more than 500 is easy to be oxidized, resulting in oxidative decarburization. Preoxidation itself is not for stress relief. Indirectly, in order to remove the stress, it is better to choose higher temperature and nitrogen as the protective gas.