Two possibilities for the formation of surface defects of spiral steel pipes

There are two possibilities for the formation of surface defects of spiral steel pipes: one is that the material itself is not plastic enough during the deformation process, resulting in cracks and external folds; the other is that the surface oxidation of the material causes surface defects, and the surface defects are magnified into cracks and external folds during the deformation process.

1. Results and analysis of thermal simulation tensile tests

To study the high-temperature plasticity of the material, a series of thermal simulation tensile tests were carried out. It can be found that 900-1200℃ is the high plasticity zone of 9Ni steel, and its tensile deformation can reach more than 90%. Comparing the deformation amount and deformation temperature at each stage of the rolling tube, it is not difficult to find that both the perforation and oblique rolling processes 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℃, the previous analysis has shown that the defects on the surface of the tube body are formed before sizing. Therefore, it can be considered that the small external 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

Observe the morphology of samples oxidized at 1100℃ at different times. It can be seen that although the surface of the oxidized sample is smooth, slight grain boundary oxidation appears between the oxide layer and the metal interface after 1 hour. As the oxidation time increases, the grain boundary oxidation depth further deepens. At this time, the grain boundary oxidation rate is greater than the driving rate of the oxide layer phase metal. When the grain boundary oxidation depth reaches a certain level, as the oxidation time increases, the oxide layer thickness increases further, but the grain boundary oxidation depth does not increase further. It can be seen that at this time, the speed of grain boundary oxidation and the driving rate of the oxide layer phase metal has reached a balance.