Observing the crack morphology, the cracks extend obliquely at an angle of 45° to the inner wall. Since the pipe itself has good mechanical properties and high ductility, it can be ruled out that it is caused by cold rolling. The crack width develops from coarse to fine, and a bifurcation appears in the middle, indicating that the through crack is not formed in one go. During the cold rolling process, the steel pipe is subjected to three-way compressive stress, and the wall thickness direction is mainly deformed and extended under the action of two-way compressive stress and two-way friction resistance. The original crack expands under the action of stress. Due to the irregular bottom of the crack, it is easy to bifurcate, and the expanded crack is finer than the original defect.
Analyzing the structure near the crack, it is found that there is a fine grain layer at the bifurcation. The reason for this phenomenon is that there are crack defects in the pipe before cold rolling, and the rolling oil enters the crack during cold rolling. The rolling oil in the crack gap cannot be completely removed during degreasing. The rolling oil and degreasing agent are rich in organic matter. The rolling oil remaining in the crack produces carburization after heat treatment. The higher the carbon content in the steel, the higher the nucleation rate, thus generating a fine grain layer. This shows that the defect has already appeared in the previous pass, rather than in the finished product or the last pass of the finished product.
From the analysis of the scanning electron microscope results, it can be judged that there are relatively concentrated aluminum oxide and silicon oxide inclusions in the local area of the sample. Such inclusions generally come from “smelting or casting, resulting in defects inside the round steel. In the subsequent perforation and cold rolling deformation process, they are affected by pressure processing, thus forming cracks and penetrating the wall thickness, causing leakage during the air tightness test. Therefore, the through crack exists in the original tube itself, rather than expanding and cracking under the influence of pressure during the air tightness test.
Post time: Oct-09-2024