The iron oxide scale on the inner and outer surfaces of the hot-rolled tube stock or the in-process tube after the intermediate heat treatment should be removed before the cold working of the ordinary seamless tube. The existence of iron oxide scale, on the one hand, affects the quality of subsequent lubrication; on the other hand, because its hardness is higher than that of the base metal, it will press into the surface of the seamless steel pipe during cold working and cause defects. The result is that the friction force increases during processing and deformation, the tool wear increases, and the surface quality of the seamless steel pipe decreases, so that the cold working process cannot be carried out normally and effectively. Therefore, it is very important to remove the iron oxide scale on the surface of the pipe and the work-in-process. In the cold working production of seamless steel pipes, chemical methods are widely used to remove iron oxide scale, that is, pickling methods are generally used for carbon steel seamless pipes and low-alloy seamless steel pipes, and pickling or alkali are used for high-alloy seamless steel pipes. Acid compound washing method.
The iron oxide scale on the surface of ordinary carbon steel seamless steel pipe is mainly iron oxide (Fe2O3), ferric oxide (Fe3O4) and ferrous oxide (FeO). In the part of the iron oxide scale close to the air, the number of oxygen atoms is large, so Fe2O3 with more oxygen is formed (of which oxygen accounts for 30%), and Fe2O3 with moderate oxygen content is formed in the middle part (of which the oxygen content is 27.6%) , while in the part closest to the iron matrix, FeO with less oxygen content is generated (which accounts for 22.2% of oxygen content).
Generally, the iron oxide scale produced by ordinary seamless steel pipes at a temperature greater than 575 °C has a structure divided into three layers: the first layer is Fe2O3; the second layer is Fe3O4; the third layer is a mixture of Fe3O4 and FeO. The thickest layer of the third layer accounts for about 80% to 85% of the thickness of the iron oxide scale; the second layer accounts for about 4% to 18%; the thinnest layer of the first layer accounts for only 1% to 2%. However, the thickness of the iron oxide scale and the proportion of each layer are not fixed, but vary with the heating temperature, heating speed and oxidizing medium (steam, water, furnace gas).