There are two possibilities for the formation of surface defects in spiral steel pipes: one is that the material itself lacks plasticity during deformation, leading to cracks and outward folds; the other is that surface oxidation causes surface defects, which are amplified into cracks and outward folds during deformation.

1. Results and Analysis of Hot Simulation Tensile Tests on Spiral Steel Pipes.
To study the high-temperature plasticity of the material, a series of hot simulation tensile tests was conducted. It was found that 900-1200℃ is the high plasticity zone for 9Ni steel, where the tensile deformation can reach over 90%. Comparing the deformation amount and temperature at each stage of pipe rolling, it is easy to see that both piercing and skew rolling processes occur in the high plasticity zone, and the deformation amount is much smaller than the material’s deformation capacity. Although the temperature in the final stage of the sizing process is below 900℃, the previous analysis has indicated that the surface defects of the spiral steel pipe form before sizing. Therefore, it can be concluded that the small outward folds and cracks appearing in this rolling process are not caused by the poor plasticity of the material itself.

2. High-Temperature Oxidation Test Results and Analysis of Spiral Steel Pipes.
The morphology of samples oxidized at 1-100℃ for different times was observed. It can be seen that although the surface of the oxidized sample is smooth, fine-grain boundary oxidation appears between the oxide layer and the metal interface after 1 hour. With the extension of oxidation time, the grain boundary oxidation depth further increases. At this point, the grain boundary oxidation rate is greater than the internal driving rate of the oxide layer phase within the metal. When the grain boundary oxidation depth reaches a certain level, with the extension of oxidation time, the oxide layer thickness further increases, but the grain boundary oxidation depth no longer increases. It can be seen that at this point, the rates of grain boundary oxidation and internal driving of the oxide layer phase within the metal reach equilibrium.