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Analysis of the causes of cracking of 20# seamless steel pipe

Fracture cracking occurred in 20# seamless steel pipe when cold forming was performed according to the process requirements. The chemical composition of the material at the fracture, force degree energy and microstructure analysis. The results show that the cold bending fracture is due to the low final rolling temperature. For the cold bending system fracture cracking situation to put forward some preventive transformation measures.

20220531080609 94744 - Analysis of the causes of cracking of 20# seamless steel pipe

The company produced a batch of Φ76×5, 20# steel, GB/T8163 seamless pipes and supplied them to the user. When the user was cold bending and forming according to the process requirements, three of the pipes fractured and cracked when they were bent to about 150° angle, and the fractures are shown in Figure 1 and Figure 2 below. The user immediately sent the fractured pipe section back to the company. In order to find out the cause, the section of the pipe was tested and analyzed.
20220531094518 73998 - Analysis of the causes of cracking of 20# seamless steel pipe
Figure.1 fracture
20220531094536 75721 - Analysis of the causes of cracking of 20# seamless steel pipe
Figure.2 fracture

1. Physical and chemical inspection

(1) Observation of the fracture, in the area of bending under tensile stress, steel pipe wall thickness thinning, indicating the presence of plastic deformation, but the radial line is very short, the shear zone is long, indicating insufficient plastic deformation.
(2) In this section of the steel pipe to take the chemical composition analysis specimens, the analysis results are shown in Table 1.

Table.1 Chemical composition of 20# seamless pipe (%)
Project C Si Mn P S Cr Ni Cu

Measured value
0.21 0.24 0.47 0.020 0.016 0.02 0.15 0.13

Standard value
0.17~0.23 0.17~0.37 0.35~0.65 ≤0.035 ≤0.035 ≤0.25 ≤0.30 ≤0.25

 Chemical composition in line with GB/T8163 on 20 # steel chemical composition requirements.
(3) Tensile specimens are taken near the fracture position for tensile testing, and the results of the mechanical properties are shown in Table 2.

Table.2 Mechanical properties testing results of 20# seamless steel pipe
Sample batch No Yield strength
(MPa)
Tensile strength
(MPa)
Elongation
(%)
Yield ratio

1
435 545 15 0.80

2
440 550 14 0.80

3
440 545 15 0.81

Standard value
≥295 410~550 ≥20

As can be seen from Table 2, the yield strength of the three batches of material specimens are high, the tensile strength is close to the upper limit of the standard value, the elongation is much lower than the standard value, and the flexural strength ratio is relatively high.

(4) The longitudinal metallographic specimens were taken near the fracture position for microstructure observation, and the organization is shown in Figure 3 and Figure 4.

20220531094946 25983 - Analysis of the causes of cracking of 20# seamless steel pipe

Figure.3 100×

20220531095008 33517 - Analysis of the causes of cracking of 20# seamless steel pipe

Figure.4 400×

Grain size:6 – 7, organization:F+P, no obvious Wei’s organization. The distribution of ferrite and pearlite is not uniform, the diameter difference of pearlite cluster is large, the shape is very irregular, and there are sharp edges in the transition part around the cluster boundary, and some ferrite is distributed along the longitudinal direction in long strips of needles.
(5) The remaining pipe section near the fracture position is about 3∞mm long for 910℃, 25min heat preservation and normalization treatment.
Take a tensile specimen test, the results in Table 3.
Table.3 20# seamless steel pipe mechanical properties test results

Sample batch No Yield strength
(MPa)
Tensile strength
(MPa)
Elongation
(%)
Yield ratio

1
335 480 28 0.70

2
330 475 27 0.70

3
340 475 28 0.71

Standard value
≥295 410~550 ≥20

As seen in Table 3, after re-normalization, the yield strength and tensile strength of the specimen are restored to normal levels, elongation of about 28% to meet the standard A ≥ 20% of the provisions of the flexural strength ratio is also relatively low.

(6) The longitudinal metallographic specimens were taken on the normalized pipe section and microstructure was observed as shown in Figure 5 and Figure 6 below.

20220531095247 70517 - Analysis of the causes of cracking of 20# seamless steel pipe

Figure.10

20220531095307 58793 - Analysis of the causes of cracking of 20# seamless steel pipe

Figure.6 400×

Grain size: 8.5~9.5, organization: F+P. The distribution is equiaxed and uniform, the diameter of pearlite group is not much different, the shape is regular, the periphery is straight, no sharp edges, the size of ferrite particles is also more uniform, there is no obvious longitudinal distribution along the longitudinal characteristics of needle-like.

2. Analysis and discussion

From the physical and chemical inspection results, it can be seen that the 20# seamless pipe is in line with the technical requirements only from the chemical composition and microstructure, but from the mechanical property testing and microstructure characteristics comparison, it can be seen that the 20# seamless pipe cold bending fracture is related to the heat treatment process.
Check the original production records, this batch of steel pipe is hot-rolled out of the finished pipe, which is in line with the above Figure 3, 4 shows some of the characteristics of the metallographic organization (grain size coarser than normal normalized organization, pearlite group exists sharp sharp angle, ferrite along the rolling direction is elongated, etc.). Check the original records of inspection, this batch of steel pipe tensile properties of a one-time test pass, no re-inspection records, the original records of its tensile properties: Rel: 395/415MPa, Rm: 525/515MPa, A: 24%/27% 0 combined with the user in the use of only three steel pipe bending fracture phenomenon, proving that the batch of tubes in production is part of the abnormal state.
From the organizational characteristics shown in Figure 3, 4 and the above original records can be seen, the pipe in the hot rolling process, the final rolling temperature is low, that is, in the two-phase area below Ar3, due to the two-phase area, austenitization is not complete, resulting in the final final rolling deformation caused by metal crystallographic defects such as dislocations, lattice distortion and other residual stresses, and the post-process can not be eliminated through the reply, recrystallization. In other words: the strengthening and hardening of the metal material caused by the processing process cannot be eliminated, and the final
The tube ends up in a high residual stress state due to the lack of proper normalization.
However, when the tube is subjected to bending tensile stresses in the same direction in the high residual stress state, the residual stresses are superimposed on the normal tensile stresses, resulting in some localized stresses that are so great that micro-cracks occur in the tube under minor external forces, causing the tube to fracture early.

3. Conclusion and suggestion

The above analysis shows that the cold bending fracture of the 20# seamless pipe is due to the high residual stress generated by the low final rolling temperature during the hot rolling process, and the superposition of the tensile stress generated during the cold bending process into the shape, resulting in the fracture of the pipe. To avoid similar quality problems occur:

  • (1) In the steel rolling process, strict control of the final rolling temperature to avoid rolling in the two-phase area below Ar3;
  • (2) Fruit due to improper control, has appeared similar phenomenon, by the above normalization treatment of tensile properties and metallurgical organization can be seen, for low carbon steel: will be rolled out of the finished tube for a 910 ℃ normalization treatment, can make the flexural strength ratio reduced, elongation significantly increased, so as to ensure that the product in the plastic deformation process to improve the ability to resist thinning, cold into an enhanced ability to meet the performance requirements of the standard. The above analysis report is for the thermal deformation process.

The above analysis report is aimed at analyzing the causes of cracking of hot-tied seamless steel tubes, and we can also use the same reasoning to analyze the causes of precision rolled tubes for seamless steel tubes.

20# precision seamless pipe cracking is what causes

Steel pipe cracking phenomenon, the most important reason is that its internal stress, its later use or processing process at some point triggered this stress if the base material of the steel pipe is not a problem recommended heat treatment such as annealing precision steel pipe for oxygen-free annealing treatment to eliminate the purpose of its internal stress.
Another reason is that the material of the steel pipe is problematic may be poor quality steel whose internal non-metallic inclusions are too much resulting in material instability, very easy to crack.

20# seamless steel pipe cracking cause analysis

Fine cracks inside the steel pipe

There is also a possibility that the material is also not a problem, the production process of precision steel pipe is problematic the most important reason may be that in order to so-called reduce the cost of steel pipe rolling using a strengthening process, two or even three processes are combined into one rolling completed resulting in the steel pipe internal fine cracking such steel pipe is very easy to crack.

The production of steel pipe is a rigorous process, can not be sloppy and careless, so take it seriously, not for a little bit of small benefits, self-deception. In the end, the bad luck is still their own.

Source: China 20# Seamless Steel Pipe Manufacturer – Yaang Pipe Industry Co., Limited (www.epowermetals.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.)

If you want to have more information about the article or you want to share your opinion with us, contact us at [email protected]

References:

  • 1. Zhu Fuxian, Zhao Qian, etc. Pretreatment process test for eliminating angular cracks in cold forming of 20 steel seamless pipes [J]. Steel Pipe, 2007, 36(1):21-25
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