1. Test process and results
1.1 Appearance inspection: (a) 1# steel pipe sample is a non-annealed rough pipe, (b) 2# steel pipe sample is an annealed rough pipe, (c) 3# steel pipe sample is a non-cracked cold-rolled steel pipe, and (d) 4# steel pipe sample is a cracked cold-rolled pipe. No external folding defects were observed on the steel pipe surface in the macroscopic morphology. No quality problems were found on the rough pipe after pipe penetration and the finished steel pipe after cold rolling. Therefore, it can be preliminarily inferred that the raw materials of this batch of steel pipes do not have surface quality problems that affect customer use. Observing the macroscopic crack morphology of the steel pipe, it was found that the cracks were extended from the initial cracks on the inner wall of the steel pipe to the surface after expansion. Therefore, it can be preliminarily concluded that the crack source is on the inner wall of the steel pipe, and there may be some defects on the inner wall of the steel pipe that cause stress cracking.
1.2 Chemical composition inspection: Spectral component analysis was performed on 4 sections. The analysis results show that the carbon content of the 3 sections is on the upper limit, but all meet the standard range. According to the test results, it can be preliminarily judged that these three samples may be from the same batch of steel. In order to analyze whether it is caused by the segregation of raw material components, 4 points are randomly selected on each steel pipe section for spectral analysis, as shown in Table 3. From the spectral analysis data in Table 3, it can be seen that there is no obvious chemical composition segregation quality problem in the 4 steel pipe sections submitted for inspection.
1.3 Non-metallic inclusion inspection: Non-metallic inclusion inspection was performed on 4 steel pipes according to GB/T10561-2005. From the analysis of the non-metallic inclusion test results, it can be seen that the non-metallic inclusion indicators of the 4 steel pipes in this batch are good and meet the corresponding standard requirements. It can be preliminarily concluded that the quality problems of this batch of steel pipes are not caused by non-metallic inclusions.
1.4 Banded structure inspection: The banded structure and Rockwell hardness of the 4 steel pipes were tested. From the test data in Table 5 and the metallographic structure analysis in Figure 3, it can be seen that the banded structure of the rough pipe after pipe penetration is uniform and there is no obvious segregation structure; and the banded structure after annealing is level 2.5 (due to annealing, the banded structure increases, which is caused by annealing), and the banded structure after cold finishing is level 2, both of which are within the allowable range of the standard. Therefore, it can be inferred that the quality problem of this batch of steel pipes is not caused by the segregation of the banded structure of steel.
1.5 Metallographic structure inspection: In the process of testing the banded structure of 4 steel pipes, it was found that the 1# rough pipe had a serious Widmanstatten structure. According to GB/T13299-1991 and GB/T6394-2017, the grain size and Widmanstatten structure were rated. Note: Due to the serious deformation of 3# and 4# cold-finished steel pipes, it is impossible to perform the Widmanstatten structure and grain size rating. Comprehensive analysis shows that the blank tube has a serious Widmanstatten structure before annealing (1# blank tube), while the Widmanstatten structure disappears after annealing (2# blank tube), and the grain size is smaller. This change indicates that the heating temperature of the steel tube billet is too high when the steel tube is pierced, which causes the blank tube to have a 4-level Widmanstatten structure during cooling. However, after high-temperature annealing of this blank tube, the grains of the steel are refined from 6.5 to 8.5 due to recrystallization, and the Widmanstatten structure is eliminated. Comparing the pearlite content in the metallographic structure of 1# steel tube and 2# steel tube, it can be found that the pearlite content of 2# steel tube after annealing is significantly increased, and the hardness also increases from 7HRC before annealing to 10HRC. It can be inferred that the cooling rate of a 2# steel tube is faster after heating, and the precipitation of ferrite is suppressed. The annealing heat treatment process temperature is 780℃, but according to the metallographic structure morphology analysis, the actual heating temperature may be far higher than the 780℃ specified in the process. The heat treatment process is suspected to be normalizing heat treatment rather than annealing heat treatment.
1.6 Inspection of the inner wall structure of the steel pipe: Since the cracks start from the cracks on the inner wall of the steel pipe, it is judged that the crack source should exist on the inner wall of the steel pipe. In order to verify whether there are some defects in the inner wall of the steel pipe that cause the cracks of the steel pipe, the inner wall of the 4 steel pipes was analyzed for metallographic structure. The inner wall of the 1# rough tube is smooth, and no cracks or decarburization are found; no cracks are found in the 2# rough tube after heat treatment, but obvious decarburization is found compared with the 1# sample, which indicates that the heat treatment heating temperature of the steel pipe should be above 800℃, proving that the actual heating process of the customer does not conform to the specified annealing process; a large number of cracks are found on the inner walls of the 3# and 4# cold-rolled steel pipes, with a maximum depth of about 100μm, and obvious deformation streamlines can be observed at the cracks, indicating that the steel pipes are subjected to large deformation at the cracks during the cold rolling process. In order to show the morphology of the cracks more clearly, the cracks of the cold-rolled steel pipes are observed at a higher magnification. The cracks are cracked along the ferrite grain boundaries at the decarburization of the inner wall. Due to the low fracture strength of ferrite, the cross-sectional shrinkage rate of the cold-rolled steel pipe during the cold-rolling process is about 53.1%, and the deformation is large. The inner wall is subjected to large stress during the deformation process and cracks are formed.
1.7 Metallographic inspection of cracks on the inner wall of the steel pipe: The metallographic structure of non-metallic inclusions and tissues was observed at the cracks on the inner wall of the steel pipe. It was found that there were no obvious non-metallic inclusions around the cracks on the inner wall of the steel pipe, so it can be judged that the cracks were not caused by non-metallic inclusions of the steel grade. The inner wall of the steel pipe is the source of the cracks. In addition to the main cracks, there are some small cracks on the inner wall. The main crack extension area presents a spider web shape, suspected to be intergranular cracking. After the crack extension area is corroded with a nitric acid alcohol solution, its microstructure morphology. From the analysis of the metallographic structure of the crack extension area, the crack extension area is indeed intergranular cracking, and no decarburization or other abnormal tissues are found in the metallographic structure around the crack. Therefore, it can be inferred that the main crack at the root of the crack is a combination of multiple intergranular cracks.
2. Result analysis
According to the verification of the production process, a series of test results such as the chemical composition, non-metallic inclusions, and metallographic structure of the steel can be concluded:
(1) This batch of steel pipes does not have serious chemical composition segregation defects that affect the quality;
(2) Through the observation and detection of crack formation, the cracks of the cold-rolled steel pipes extend from the inner wall to the outer wall, and the crack source should be on the inner wall of the steel pipe;
(3) Through the inspection of the inner wall of the steel pipe, it was found that there were a large number of fine cracks along the grain on the inner wall of the cold-rolled steel pipe, which was the cause of the cracking of the steel pipe;
(4) Through the hardness test of the steel pipe, it was found that the hardness of the rough pipe after annealing was higher than that before annealing;
(5) Through the metallographic structure test, it was found that the rough pipe had obvious decarburization after heat treatment, while there was basically no decarburization before heat treatment;
(6) The 1# rough pipe had a serious Widmanstatten structure, which proved that there was overheating at high temperatures during the production of steel pipes.
In summary, the chemical composition, hardness, metallographic structure, inner wall decarburization, inner wall cracks, and other aspects of the steel were analyzed and tested, and it was concluded that the main cause of the steel pipe cracks was the presence of fine cracks on the inner wall of the rough tube.
In addition, according to the comparison of the steel tube structure, it can be determined that the heating temperature used for this batch of steel pipes after being threaded into rough tubes was much higher than the austenitizing temperature of 20# steel, which was inconsistent with the 780℃ annealing process specified in the heat treatment heating process. The speed of the rough tube cooling was too fast, resulting in increased hardness incomplete release of stress, and obvious decarburization on the inner wall of the steel pipe, which reduced the fracture strength of the inner wall. During cold finishing rolling, large deformation produced a large amount of plastic deformation, severe lattice distortion, and a sharp increase in dislocations inside the grains. After a large number of roughness and retained slip bands were formed, the strength of the grains themselves decreased, and cracks were initiated along the grain boundaries at the stress concentration point on the inner wall. The cracks cracked under the action of external stress [6][[7].
3. Conclusion
(1) The quality problem of steel pipe cracking is not caused by the quality problem of raw materials.
(2) The fundamental reason for the cracking of cold-rolled steel pipe expansion is the cracks on the inner wall of the cold-rolled steel pipe.
(3) The generation of inner wall cracks is related to the heating temperature being higher than the austenitizing temperature and the fast cooling speed. (The temperature control of the heating furnace should be strengthened in the future)
Post time: Feb-05-2025