Abstract: The causes of surface cracks in stainless steel seamless pipes made from 0Cr18Ni9 were analyzed and improvement measures were proposed. After analysis, it is believed that inclusions gotten into the round steel during smelting or pouring caused cracks in the perforated tube blank. The cracks further expanded in the cold rolling deformation process and eventually formed cracks that penetrated the wall thickness direction. It is recommended to check inclusions when accepting pipe blanks, strengthen the grinding of waste pipes, and add non-destructive testing procedures to control cracks in the production process of stainless steel seamless pipes. Surface cracks have a great impact on the quality of stainless steel seamless pipes in the manufacturing process and the safety of use after delivery, and are unacceptable defects. Therefore, it is necessary to analyze the macroscopic characteristics of surface cracks in stainless steel seamless pipes, find out the causes of crack formation, and make suggestions.
 
1. Macroscopic characteristics of surface cracks of steel pipes
Taking the stainless steel seamless pipe with a size of 10 mm×1 mm made of 0Cr18Ni9 as an example, the manufacturing processes are: round steel perforation → pickling → grinding → intermediate cold rolling → oil removal → natural gas furnace heat treatment (solid solution) → pickling → grinding → cold rolling of finished products → oil removal → bright heat treatment of finished products (solid solution) → straightening → pipe cutting to length and finished product inspection. The stainless steel seamless pipe leaked during the sealing test of the finished product. Longitudinal defects appeared on the outer surface by visual inspection, which were suspected to be cracks running through the wall thickness. After anatomical observation, there are indeed cracks at the corresponding positions, and penetration can be seen in the cross section, as shown in Figure 1.
 
2. Physical and chemical analysis
Samples were taken from the unqualified stainless steel seamless pipes of 10 mm×1mm that leaked during the sealing test, and physical and chemical analysis was conducted.
 
2.1 Chemical composition inspection
The chemical composition of the sample was analyzed in accordance with GB/T 11170-2016 "Determination of Multi-Element Content of Stainless Steel by Spark Discharge Atomic Emission Spectrometry". The results are shown in Table 1. After comparison, its composition meets the requirements for 0Cr18Ni9 in GB/T 14976-2002 "Stainless Steel Seamless Steel Pipes for Fluid Transport", and there are no obvious abnormalities in the main components of the matrix.
 

Figure 1 Macroscopic inner wall and cross-section of the cut stainless steel seamless pipe
 
Table 1 Chemical composition of stainless steel seamless pipes of 10 mm x 1mm  %

 
2.2 Analysis of mechanical properties at room temperatures
(1) Tensile properties
The room temperature tensile test was carried out in accordance with GB/T 228.1-2010 "Tensile Tests of Metal Part 1: Room Temperature Test Methods". The results are shown in Table 2. It was found that the strength and plasticity of the stainless steel seamless pipe with 10 mm × 1 mm were good and better than the standard requirements of GB/T14976.
 
Table 2 Room temperature mechanical properties of stainless steel seamless pipes with 10 mm x 1 mm

 
(2) Expansion and flattening tests
The expansion and flattening tests were carried out in accordance with GB/T 242-2007 " Expansion Test Methods of Metal Pipes" and GB/T 246-2017 " Flattening Test Methods of Metal Pipes". Select a conical top core with a top core angle of 60° and an outer diameter expansion rate of 10%. After the expansion, no cracks appear on the pipe wall; at the same time, no cracks appear on the inner and outer walls of the steel pipe after the flattening test.
 
2.3 Intergranular corrosion performance tests
Conduct copper-copper sulfate-16% sulfuric acid corrosion test according to GB/T 4334-2020 "Corrosion of Austenitic and Ferrite-austenitic (Duplex) Stainless Steel Intergranular Corrosion Test Methods for Metals and Alloys" method E. After the sample is bent at an angle of 180, there are no intergranular corrosion cracks on the surface.
 
2.4 Metallographic analysis
After wire cutting and inlaying, the defective parts were subjected to ultrasonic cleaning after grinding, polishing and electrolytic corrosion with 10% oxalic acid solution, and were observed under a metallographic microscope. The crack runs obliquely through the wall thickness at an angle of 45 with the pipe wall; there is a bifurcation in the middle of the crack, the crack width is wider on the inner side of the pipe wall than on the outer side, and there is obvious folding at the bottom of the crack on the inner side; carbon and fine grain have occurred near the middle of the crack, and the bifurcation is more obvious. Judging from the crack shape, the crack has undergone secondary expansion.
 
Take a longitudinal sample near the defect of the pipe body (position 1 in Figure 1); observe its metallographic structure, and measure it according to GB/T 10561-2005 "Standard Rating Microscopic Examination Method for Determination of Non-metallic Inclusion Content in Steel" method A for rating non-metallic inclusions. Except for Class B and Class D fine inclusions, which are Class 1.0 and Class 0.5, the rest of the structures are 0. It can be seen that the content of non-metallic inclusions in the matrix area is normal.
 
2.5 Scanning electron microscopy analysis
Scanning electron microscopy analysis found that there were many dotted and massive suspected inclusions in the defects. Select some points to perform energy spectrum analysis on the suspected inclusions. The defect morphology under the scanning electron microscope is shown in Figure 2, and the energy spectrum analysis results are shown in Table 3. Comparing the matrix composition, the above two points have abnormally high Si and Al contents. It can be judged that there are silicon oxide and aluminum oxide inclusions at these two points.


(a) Morphology 1 (b) Morphology 2
Figure 2 Defect morphology of stainless steel seamless pipes under scanning electron microscopes
 
Table 3 Energy spectrum analysis results of stainless steel seamless pipe defects %

 
3. Analysis of causes of cracks
Observing the crack morphology, the crack expanded diagonally at an angle of 45 with the inner wall. Since the pipe has good mechanical properties and ductility, it can be ruled out that it is caused by cold rolling. The width of the crack develops from thick to thin, and bifurcation appears in the middle, indicating that the through crack is not formed at once. In the cold rolling process, the steel pipe is subject to three-dimensional compressive stress, and the wall thickness direction is mainly affected by two-dimensional compressive stress and two-dimensional friction resistance. It deforms and extends under the action of stress, and the original crack expands under the action of stress. Since the bottom of the crack is irregular, it is easy to bifurcate, and the expanded crack is thinner than the original defect.
 
The structure near the crack was analyzed and it was found that there was a fine-grained layer at the bifurcation. The reason for this is that there are cracks in the pipe before cold rolling, and the rolling oil enters the cracks during cold rolling. The rolling oil in the crack gaps cannot be completely removed during degreasing, and the rolling oil and degreasing agent contain organic matter; the rolling oil remaining in the cracks will produce carburization after heat treatment. The higher the carbon content in the steel is, the higher the nucleation rate becomes, thus forming a fine-grained layer. This shows that the defect has already appeared in the previous pass and is not formed by the cold rolling of the finished product or the previous pass of the finished product. From the analysis of scanning electron microscope results, it can be judged that there are relatively concentrated aluminum oxide and silicon oxide inclusions in partial areas of the sample.
 
Such inclusions generally originate from smelting or casting, resulting in defects within the round steel. In the subsequent piercing and cold rolling deformation processes, they are affected by pressure processing, thereby forming cracks and penetrating the wall thickness, causing leakages during the sealing test. Therefore, the penetrating crack exists in the original tube blank itself, rather than expanding and cracking due to pressure during the sealing test.
 
4. Improvement measures
By formulating reasonable measures to control cracks in the production process of stainless steel seamless pipes, the quality of the products can be further improved.

(1) Raw material acceptance

Dedicated technical conditions for purchasing tube blanks are formulated for small-diameter stainless steel, and detailed quantitative provisions are made on inclusion content and outer surface quality. When re-inspecting raw materials, the focus is on inspecting the level of inclusions to prevent defects from occurring for finished products.

(2) Strengthening the repair and grinding of blank

Strengthen visual inspection; mark the defects found, and isolate the defective steel pipes; use endoscopes and other auxiliary visual inspections to locate defects on the inner wall of the steel pipes, and then use fixed-point grinding to polish the defects.

(3) Increasing non-destructive testing

Formulate reasonable acceptance criteria; conduct ultrasonic flaw detection on the finished steel pipes before cold rolling; mark and isolate the defective steel pipes for flaw detection, and repair and grind at fixed points to eliminate defects.

(4) Re-inspection
Use the same testing method for re-inspection of the steel pipes that have been ground at the above fixed points. If they are qualified, they will be put into production to prevent unqualified products from getting into the next process.
 
5. Conclusion
There was a leak in the 0Cr18Ni9 stainless steel seamless pipe with a specification of 10 mm × 1 mm during the sealing test of the finished product, and it was found that there were cracks on the outer surface which penetrated the wall thickness direction. Analysis shows that there are inclusions in the perforated tube blank itself, and cracks are formed during perforation, but the defects are not removed in the subsequent production process, causing the cracks to further expand in the cold rolling process and eventually penetrate the tube wall.