There are two possibilities for the formation of surface defects of spiral steel pipes: one is that the material itself is not plastic in the process of deformation, resulting in cracks and outward folding; the other is that the surface of the material is oxidized and causes surface defects, which are enlarged on the deformation process. Cracks and folds.

1. Thermal simulation tensile test results and analysis

To study the high-temperature plasticity of materials, a series of thermal simulation tensile tests were carried out.

It can be found that 900-1 200℃ is the high plastic zone of 9Ni steel, and its tensile deformation can reach more than 90%. Comparing the deformation and deformation temperature of each stage of the rolling tube, it is not difficult to find that both the piercing and cross-rolling processes are in the high plastic zone, and the deformation 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 indicated that the defects on the surface of the tube constitute before the sizing. Therefore, it can be considered that the small outward folds and cracks at this rolling are not caused by the poor plasticity of the material itself.

2. High-temperature oxidation tests results and analysis

Observe the morphology of the oxidized samples of 1 100°C for different times.

It can be seen that although the surface of the oxidized sample is lubricated, there is a slight grain boundary oxidation between the oxide layer and the metal interface after 1 hour. As the oxidation time extends, the depth of grain boundary oxidation further deepens. At this moment, the grain boundary oxidation speed is greater than the pushing speed in the oxide phase metal. When the grain boundary oxidation depth reaches a certain level, as the oxidation time extends, the thickness of the oxide layer further increases, but the grain boundary oxidation depth does not go further. It can be seen that the speed of the grain boundary oxidation and the internal pushing of the oxide phase metal has reached equilibrium at this moment.