There are two possibilities for the formation of surface defects in
spiral steel pipes: one is that the plasticity of the material itself is not good during the deformation process, resulting in cracks and outward folding; Cracks and folds.
1. Thermal simulated tensile test results and analysis
To study the high-temperature plasticity of the material, a series of thermal simulation tensile tests were carried out. It can be found that 900-1 200 ° C is the high plasticity zone of 9Ni steel, and its tensile deformation can reach more than 90%. Comparing the deformation amount and deformation temperature of each stage of pipe rolling, it is not difficult to find that the two processes of piercing and cross-rolling are in the high plasticity zone, and the deformation amount is much smaller than the deformation capacity of the material. Although the temperature in the final stage of the sizing process is lower than 900 ° C, the previous analysis has shown that the defects on the surface of the pipe body are formed before the sizing. Therefore, it can be considered that the small outward folds and cracks in this rolling are not caused by the poor plasticity of the material itself.
2. High-temperature oxidation test results and analysis
The morphology of samples oxidized at 1 100 °C for different times was observed. It can be seen that although the surface of the oxidized sample is lubricated, fine-grain boundary oxidation appears between the oxide layer and the metal interface after 1 h. With the extension of oxidation time, the oxidation depth of the grain boundary is further deepened. At this moment, the grain boundary oxidation rate is greater than the internal driving rate of the oxide layer phase metal. When the grain boundary oxidation depth reaches a certain level, the thickness of the oxide layer further increases with the extension of oxidation time, but the grain boundary oxidation depth does not go any further. It can be seen that the speed of grain boundary oxidation and internal promotion of oxide layer phase metal has reached a balance at this moment.
