Changes of Phosphating Film on Bearing Steel After Heating
Steel parts undergo chemical treatment in a phosphate solution containing zinc, manganese, calcium, iron and other elements, forming a water-insoluble crystalline phosphate conversion film on their surface. This chemical process is known as phosphating of steel. The phosphating film appears gray to grayish-black and has a porous structure. After being filled, oil-impregnated or painted, it exhibits good corrosion resistance in the atmosphere and is widely used in the spherical plain bearing industry. Adding a coating process such as spray painting or dry film spraying after phosphating can significantly improve the comprehensive performance of the film layer. In the coating process, heating at different temperatures is often required, and the properties of the phosphating film will change after heating.
The phosphating film is a chemical conversion film formed by chemical reactions between the metal surface and the solution. The spray painting or dry film spraying applied after phosphating forms a physical bond with the phosphating film, and no chemical reaction will occur between them even when heated. Therefore, the changes in the surface of the phosphating film after heating at different temperatures are consistent with those of the phosphating film after surface coating.
Conclusions
1.As the temperature gradually increases, the phosphating film changes from grayish-black to yellow gradually, the film layer gradually loses water, and the overall film weight shows little change.
2.With the gradual increase of temperature, the surface crystals of the phosphating film gradually crack. When the temperature reaches 250 ℃, the surface crystals of the phosphating film suffer severe cracking and even pulverization, leading to a gradual decrease in the corrosion resistance of the phosphating film.
After high-temperature manganese phosphating, the workpieces require heat treatment. The temperature should not exceed 150 ℃ and should be controlled below 120 ℃ as much as possible to maintain the integrity of the phosphating film crystals and its excellent corrosion resistance.
