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-grained layer. This shows that the defect has already appeared in the previous pass, rather than 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. This type of inclusion generally comes from “smelting or casting, resulting in defects inside the round steel. During the subsequent perforation and cold rolling deformation process, it is 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.
Improvement measures for surface cracks of small-diameter stainless steel seamless pipes
By formulating reasonable measures to control cracks in the production process of stainless steel seamless pipes, the quality of the product can be further improved.
1. Raw material acceptance. Formulate special tube procurement technical conditions for small-diameter stainless steel, and make detailed quantitative regulations on inclusion content and external surface quality. , when re-inspecting the raw materials, focus on inspecting the level of inclusions to prevent defects during the processing of the finished product.
2. Strengthen the grinding of rough pipes. Strengthen visual inspection, mark the defects found, and isolate the defective steel pipes; use endoscopes and other auxiliary visual inspections to locate the defects on the inner wall of the steel pipe, and then adopt fixed-point grinding to grind the defects.
3. Increase non-destructive testing. Formulate reasonable acceptance criteria, conduct ultrasonic flaw detection on the steel pipes before cold rolling of the finished products, mark and isolate the defective steel pipes for flaw detection, and eliminate the defects by fixed-point grinding.
4. Use the same inspection method to re-inspect the steel pipes after the above fixed-point grinding. If qualified, put them into production and use them to prevent unqualified products from flowing into the next process.
Post time: Oct-09-2024