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 during the deformation process, resulting in the formation of cracks and external folds; the other is that the surface of the material is oxidized to cause surface defects, and the surface defects are enlarged 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 simulation tensile tests were carried out.

It can be found that 900-1 200 ℃ is the high plastic region of 9Ni steel, and its tensile deformation can reach more than 90%. Comparing the deformation amount and deformation temperature of each stage of rolling, it is not difficult to find that the two processes of piercing and skew rolling are in the high plastic region, 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 indicated that the defects on the surface of the pipe body are formed before the 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

The morphology of the samples oxidized at 1 100 ℃ for the 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 1h. With the extension of the oxidation time, the grain boundary oxidation depth is further deepened. At this moment, the oxidation speed of the grain boundary is higher than the pushing speed in the oxide phase metal. When the grain boundary oxidation depth reaches a certain level, with the extension of oxidation time, 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 grain boundary oxidation and the internal promotion of the oxide phase metal has reached a balance at this moment.