Analysis of reaming cracking defects of 20# steel finish rolled seamless steel pipe

In this paper, through chemical composition analysis, hardness testing, metallographic inspection and other methods, the 20# steel finishing (cold-rolled) seamless steel pipe flare cracking defects produced by our company was inspected to analyze its causes, and the causes and mechanisms of cracking were studied. The results show that the main reason for the cracking of the steel pipe is the presence of small cracks in the inner wall of the hairpin. The generation of fine cracks due to the heating temperature is much higher than the austenitizing process 780 ℃, the rapid cooling rate of the capillary tube, resulting in the stress is not fully released, increasing the propensity to cracking. During cold finishing rolling, a large amount of plastic deformation is produced, and the lattice distortion is serious, resulting in cracks along the grain boundaries at the stress concentration in the inner wall, and cracks are cracked under the action of external stress.

Our company uses 20# steel hot-rolled billet steel sawing down → heating → threading → sizing → annealing → soaping → burr → cold finishing → flaw detection → sawing section → end reaming process to produce 20# steel pipe, after finishing sawing section reaming, the steel pipe cracking, and in the subsequent inspection found that the steel pipe surface hardness is not uniform (different branches 8 – 17HRC), for this phenomenon, the cause of 20# steel pipe reaming cracking of the reasons for the test analysis [1-4].

1. Sample program

In order to check the cause of cracking, in the tube, annealing, cold rolling 3 processes were sampled, sampling number 1#, 2#, 3#, 4#. Sample sampling information is shown in Table 1, the steel pipe samples were tested and analyzed for chemical composition, banding, inclusions, organization, etc.
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2. Test process and results

2.1 Appearance inspection

Specimens are shown in Figure 1, where (a) 1# steel pipe specimens, non-annealed state of the gross tube, (b) 2# steel pipe specimens, annealed state of the gross tube, (c) 3# steel pipe specimens, no cracking cold finishing tube, (d) 4# steel pipe specimens, cracked cold finishing tube. The macroscopic morphology of the steel pipe is not observed on the surface of the steel pipe with outward folding type defects, after the piercing of the gross tube and cold finishing rolled on the finished tube by flaw detection, no quality problems exist, so it can be initially inferred that the raw material of the batch of steel pipe does not have surface quality problems that affect the use of customers.
Observing the macroscopic crack morphology of the steel pipe, it was found that the crack was extended to the surface by the initial crack in the inner wall of the steel pipe, see Figure 2. Therefore, it can be initially concluded that the source of the crack is in the inner wall of the steel pipe, and there may be some defects in the inner wall of the steel pipe leading to stress cracking [5].
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(a) 1# annealed before the gross tube
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(b) 2# after annealing hair tube
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(c) 3# cold finishing tube
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(d) 4# cracked cold-rolled tube

Figure.1 macroscopic shape of steel tube specimen

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Figure.2 4# cold finishing tube cracked macroscopic shape

2.2 Chemical composition test

The spectral composition analysis of the four material sections sampled, the analysis results are shown in Table 2. From the analysis results in Table 2, it can be seen that the carbon content of the 3 material sections is on the upper limit, but they are in line with the standard range, and according to the test results can be initially judged that these three samples may be the same furnace batch of steel.
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In order to analyze whether it is due to the impact of raw material composition bias, in each steel pipe material section randomly take four points for spectral analysis, as shown in Table 3. From the spectral analysis data in Table 3, it can be seen that the four steel pipe sections sent for inspection do not have obvious problems with the quality of chemical composition segregation.
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2.3 Non-metallic inclusions test

4# steel pipe according to GB/T10561-2005 non-metallic inclusions test, test data are shown in Table 4. from the analysis of non-metallic inclusions in Table 4 no test results can be seen, the batch of 4 steel pipe non-metallic inclusions index is good to meet the corresponding standard requirements, can be initially concluded that the quality of the batch of steel pipe is not caused by non-metallic inclusions.
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2.4 Strip tissue inspection

4# steel pipe strip organization and Rockwell hardness testing, test data are shown in Table 5 and Figure 3. from Table 5 of the test data and Figure 3 of the metallurgical organization analysis can be seen, after wearing the gross tube strip organization uniform, no obvious bias organization; and after annealing strip organization for 2.5 (due to annealing, strip organization is elevated, is caused by annealing), after cold-rolling strip organization for 2, are allowed in the standard Within the range, it can be inferred that the quality of the batch of steel pipe is not caused by the steel ribbon organization deviation.
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Note: If the steel hardness is lower than 20HRC, Rockwell hardness test is not allowed, the data in the table is for reference only

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(a) 1# before annealing hair tube
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(b) 2# after annealing hair pipe
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(c) 3# cold-rolled tubes
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(d) 4# cracked cold-rolled tube
Figure.3 Steel tube strip organization

2.5 Metallographic organization test

In the process of testing 4 steel pipe strip organization found 1# gross tube there is a serious Weiss organization (see Figure 4), according to GB/T13299-1991 and GB/T6394-2017 for grain size and Weiss organization rating (see Table 6).
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Note: 3#, 4# cold-finished rolled pipes can not be rated for Weiss organization and grain size due to the existence of severe deformation.
From the comprehensive analysis in Figure 4, it can be seen that the gross tube before annealing (1# gross tube) there is a serious Weiss organization, and after annealing (2# gross tube) Weiss organization disappeared, and the grain size grain is finer. This change shows that the steel pipe perforation billet heating temperature is high, and prompted by the presence of 4 Weiss organization in the cooling of the hair tube, but this hair tube after high-temperature annealing due to recrystallization of steel grain refinement, from 6.5 refinement to 8.5, and eliminate the Weiss organization.
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Figure.4 1# steel pipe Weiss organization
Comparison of 1# steel pipe and 2# steel pipe metallographic organization in the pearlite content can be found, after annealing treatment of 2# steel pipe pearlite content increased significantly, the hardness also increased from 7 HRC before annealing to 10 HRC, it can be inferred that 2# steel pipe in the heating and cooling rate is faster, ferrite precipitation has been inhibited.
Annealing heat treatment process temperature of 780 ℃, but the actual heating temperature may be judged according to the metallurgical tissue morphology analysis far beyond the 780 ℃ process, the heat treatment process is suspected of normalizing heat treatment rather than annealing heat treatment.

2.6 Tube wall organization inspection

As the crack is cracked from the inner wall of the steel pipe, so the source of the crack should be judged to exist in the inner wall of the steel pipe. In order to verify whether there are some defects in the inner wall of the steel pipe caused by cracking, the inner wall of the four steel pipes were analyzed for metallurgical organization, see Figure 5.
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(a) 1# annealed before the gross tube
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(b) 2# after annealing hair tube
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(c) 3# cold finishing tube
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(d) 4# cracked cold-rolled tube
Figure.5 tube inner wall metallographic organization
As shown in Figure 5, the inner wall of 1# gross tube is smooth, no cracks and decarburization are found; 2# gross tube after heat treatment is not found to be cracked, but compared with 1# specimen there is an obvious decarburization, 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 and the specified annealing process does not match; 3# and 4# cold finishing tube are found to have a large number of cracks on the inner wall, the maximum depth of about 100μm, and the cracks can be observed at the obvious deformation streamline, indicating that the steel pipe in the cold finishing process at the crack by a large deformation.
In order to more clearly show the morphology of the crack, the cold finishing tube cracks are observed at a higher magnification, see Figure 6. It is clear from the figure that the crack is along the inner wall decarburization at the ferrite grain boundary cracking, due to the low fracture strength of ferrite, and the cold finishing rolling process cold
Due to the low fracture strength of ferrite, and the cold finishing rolling process, the section shrinkage of the tube is about 53.1%, the deformation variable is large, and the inner wall is subjected to large stresses during the deformation process and cracks are formed.
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(a) 3# finishing tube inner wall cracks enlarge
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(b) 4# finishing tube wall crack enlargement
Figure.6 The internal crack morphology of the refined rolled tube after enlargement

2.7 Tube wall cracks at the metallographic examination

The crack in the inner wall of the steel pipe at the non-metallic inclusions and the organization of the metallographic observation, see Figure 7, Figure 8. observation found that the crack in the inner wall of the steel pipe around no obvious non-metallic inclusions exist, you can judge the crack is not caused by non-metallic inclusions in the steel.
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Figure.7 crack shape at the inner wall of the steel pipe
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Figure.8 Crack expansion area inside the steel pipe
As can be seen from the figure, the inner wall of the pipe is the source of cracks, in addition to the main crack, there are some small cracks in the inner wall. The main crack extension area shows a spider web pattern, suspected of cracking along the crystal, the crack extension area using nitric acid alcohol solution after corrosion, its microstructure morphology is shown in Figure 9.
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(a) Magnification 200 times
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(b) Magnification 500 times
Figure.9 Crack extension area tissue morphology
From the analysis of the metallographic organization of the crack extension area, the crack extension area is indeed cracked along the crystal, and no decarburization or other abnormal organization is found in the metallographic organization around the crack, so it can be inferred that the main crack at the root of the crack is a combination of multiple cracks along the crystal.

3. Analysis of the results

According to the production process verification, the chemical composition of steel, non-metallic inclusions, metallographic organization and a series of test results can be derived from.

  • (1) The batch of steel does not have a serious chemical composition deviation defects that affect the quality.
  • (2) Through the observation of crack formation and testing verification, cold finishing tube cracking from the inner wall to the outer wall expansion cracking, crack source should be in the inner wall of the tube.
  • (3) Through the inspection of the inner wall of the steel pipe, it is found that there is a large number of small cracks along the crystal cracking of the inner wall of the cold finishing tube, which is the cause of the cracking of the steel pipe.
  • (4) Through the steel tube hardness test, found that the hardness of the hairy tube after annealing is higher than the hardness before annealing.
  • (5) Through the metallographic organization test, found that the heat treatment of the capillary tube produced a significant decarburization, while the heat treatment before the basic no decarburization can not be seen; (6) 1# of the capillary tube exists after annealing.
  • (6) 1# gross tube there is a serious Weiss organization, proving that there is a high temperature superheating situation when producing the tube.

In summary, from the chemical composition of steel, hardness, metallurgical organization morphology, inner wall decarburization, inner wall cracks and other aspects of inspection and analysis: the main cause of steel cracking is the presence of small cracks in the inner wall of the capillary tube.
Another comparison of steel pipe organization can be determined after the batch of steel pipe into the burr tube, the heating temperature used is much higher than the 20# steel austenitizing temperature, and heat treatment heating process specified in the 780 ℃ annealing process does not match the speed of the burr tube cooling and too fast, resulting in increased hardness, stress is not fully released, and the formation of significant decarburization in the inner wall of the steel pipe, reducing the fracture strength of the inner wall; cold finishing rolled large deformation produced A large number of plastic deformation, serious lattice distortion, a sharp increase in internal dislocations in the grain, roughness and residual slip zone after the formation of a large number of grains itself, the strength of the grain decreases, cracks in the inner wall stress concentration along the grain boundaries sprouted, cracks in the action of external stress to produce cracking [6] [[7].

4. Conclusion

  • (1) The quality problem of cracking steel pipe is not caused by the quality of raw materials.
  • (2) The root cause of cold finishing tube reaming cracking is due to cracks in the inner wall of the cold finishing tube.
  • (3) The inner wall cracking and heating temperature is higher than the austenitization temperature, the cooling rate is fast. (The temperature control of the heating furnace should be strengthened in the future to control)

Source: Network Arrangement – China Seamless Steel Pipes Manufacturer – Yaang Pipe Industry Co., Limited (www.pipelinedubai.com)

(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)

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  • [5] Cao Jingjing, Zhao Wenwu, Li Yang, et al. Failure analysis of 37Mn5 hot-rolled steel pipe with cracked inner wall[J]. Metal Heat Treatment, 2016, 41(1):228-232.
  • [6] Yu S J, Chen M, Yuan P B. Heat treatment of 30CrMnMo steel pipe. Analysis of the causes of heat treatment cracking in 30CrMnMo steel pipes[J]. Steel Pipe, 2017, 46(3):40-44.
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