Two possibilities for 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 outward folds; the other is that the surface oxidation of the material causes surface defects, and the surface defects are amplified during the deformation process. Cracks and folds.

1. Thermal simulation tensile test results and analysis:
To study the high-temperature plasticity of the material, a series of thermal simulated tensile tests were conducted.
It can be found that 900-1 200℃ 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 pipe rolling, it is not difficult to find that both the piercing and cross-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°C, the previous analysis has shown that defects on the surface of the pipe body are formed before sizing. Therefore, it can be considered that the small external folds and cracks that appeared 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 the samples oxidized at 1100°C at different times.
It can be seen that although the surface of the oxidized sample is lubricated, subtle grain boundary oxidation appears between the oxide layer and the metal interface after 1 hour. As the oxidation time extends, the oxidation depth of the grain boundaries further deepens. At this time, the grain boundary oxidation speed is greater than the internal pushing speed of the oxide layer phase metal. When the grain boundary oxidation depth reaches a certain level, as the oxidation time extends, the oxide layer thickness further increases, but the grain boundary oxidation depth does not go further. It can be seen that the speed of grain boundary oxidation and internal push of the oxide layer phase metal has reached a balance at this time.