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The quenching crack of seamless steel pipe causes and preventive measures

According to the characteristics of manufacturing processes of seamless steel pipes, combined with macroscopic morphology and microstructure analyses, it is believed that the main causes of quenching cracks in seamless steel pipes are surface defects, stress cracks and surface recarburization. The formation mechanism of different quenching cracks is analyzed, and corresponding preventive measures are proposed. Improving the surface quality of the pipe, optimizing the quenching process parameters and improving the uniformity of the billet’s composition can effectively reduce the occurrence of quenching cracks of seamless steel pipes through the analysis.

The main processes of seamless steel pipes are: refining → continuous casting of solid steel bars→ heating of solid steel bars→ piercing → hot rolling → reducing diameters → heat treatment. In the heat treatment process, quenching cracks sometimes appear. A quenching crack is a kind of crack which is perpendicular to the surface of the steel pipe. The crack tip is sharper and of less decarburization, which is caused by structural stress generated by rapid cooling and thermal stress exceeding the strength of the pipe. According to the different morphology of quenching cracks, the causes are also various. The causes of several typical quenching cracks are analyzed and corresponding improvement measures are proposed in this article to improve the quality of seamless steel pipes.

1. Types and causes of quenching cracks

1.1 Quenching cracks induced by surface defects

When a 26Cr-Mo4S alloy structural pipe with a size of 114.30mm x 8.56mm is rolled, small quenching cracks often appear on the inner wall, as shown in Figures 1 and 2. The polished micro-topography photo shows that there are many surface defects such as pits and peeling skin with a depth of less than 0.2 mm on the inner wall of the pipe. These defects produce stress concentration under the quenching stress and become the cause of quenching cracks.
Quenching cracks caused by surface defects mostly appear in pipes with small diameters and thin walls. The rolling elongation of steel pipes with small diameters is great, and the original defects such as pits and scratches are prone to appear on the inner and outer surfaces of the pipe; at the same time, the stress concentration effect generated by the surface tensile stress is more significant in the process of quenching and cooling processes of steel pipes with thin walls, so quenching cracks caused by surface defects easily happen. Besides, when rolling pipes in steel 20# with a size of 88.90mm x 6.45mm, we found that quenching cracks caused by surface defects were on the inner and outer surfaces.

1.2 Stress cracking type of quenching cracks

Stress cracking is a common type of quenching crack. It is a kind of crack caused by the surface’s tensile stress exceeding the material strength in the quenching and cooling process.
figures 1 2 - The quenching crack of seamless steel pipe causes and preventive measures
Figure.1 Quenching cracks induced by pits on the inner wall of 26crmo4s alloy structural pipe and  Figure.2 Quenching crack induced by outer wall defect of 26crmo4s alloy structural pipe
The morphology and surrounding structure of quenching cracks on surfaces of casing in steel N80Q with a size of 177.80mm ×11.51mm are shown in Figures 3 and 4. It can be seen from Figures 3 and 4 that pipes with stress cracking have smooth and flat surfaces, without original defects, and the microstructure is uniform and small. Cracks are caused by excessive tensile stress on surfaces. Stress cracking type of quenching cracking is completely perpendicular to the surface of the pipe body and extends along the direction of wall thickness, which also shows that this type of crack is entirely caused by excessive tensile stress on surfaces.

figures 3 4 - The quenching crack of seamless steel pipe causes and preventive measures

Figure.3 Quenching crack morphology of N80Q steel grade (26crmo4) casing outer wall and Figure.4 Microstructure around quenching crack on outer wall of N80Q steel (26crmo4) casing

In addition, when casing in P110 steel grade 29MnCr6 with a size of 139.70mm×10.54mm is rolled, stress cracking type of quenching cracking also appears.
Excessive stress on the surface of the pipe body is mostly caused by uneven cooling of the inner and outer walls of the steel pipe. Therefore, through experiments, the effect of different cooling methods on tensile stress on surfaces of seamless steel pipes in the quenching process is analyzed. 
The quenching cooling method of a heat treatment production line is external water spraying plus internal spraying water quenching, and the internal spraying is delayed by 3 seconds compared with the external spraying. The delayed quenching process with internal spraying will cause tensile stress in the circumferential direction of the pipe body; the water flow will be too great, and the cooling rate will be faster, resulting in generating great residual stress. The quenching test was carried out on the rolled Cr-Mo alloy steel pipe with a size of 127.00mm × 9.19mm. The sample was heated to 890℃ and kept for 30 minutes before leaving the furnace. The No. 1 sample was internally sprayed and then cooled for 10 seconds, and then the whole pipe was water hammered. The No. 2 sample was directly subjected to the whole water quenching and cooling. The results of residual stress measurement (Figures 5 and 6) show that the residual stress of the pipe body of the No. 1 sample is -105.8MPa (compressive stress), and that of the pipe body of the No. 2 sample is 219.8MPa (tensile stress). It can be seen that improper cooling methods will lead to great residual tensile stress on the surface of the pipe body and increase the risk of stress cracking type of quenching cracking.

figures 5 6 - The quenching crack of seamless steel pipe causes and preventive measures

Figure 5 No.1 sample (compressive stress)          Figure 6 No.2 sample  (tensile stress)

1.3 Surface carburized quenching cracks

When the medium carbon Cr-Mo microalloy steel with a C content of about 0.30% is used to produce seamless steel pipes, quenching cracks often appear on the surface of the pipe body, as shown in Figure 7. The results of microscopic analysis showed that there was recarburization in the structure around the quenching crack (Figure 8), and the depth of the recarburization layer was 0.5 to 2.0mm. The reason for the formation of the quenching crack is that there is recarburization on the partial outer surface of pipes, which leads to excessive stress in the recarburization part in the quenching process, thereby forming a quenching crack.

figures 7 8 - The quenching crack of seamless steel pipe causes and preventive measures

Figure.7 Surface quenching crack morphology of medium carbon Cr Mo microalloyed steel pipe and Figure.8 Carburized structure around quenching crack of medium carbon Cr Mo microalloyed steel pipe

According to the production process of seamless steel pipes, it is speculated that the processes which may cause the increase of C content on the surface of the steel pipe are:

  • ① Casting powder containing much carbon is bonded with surfaces of steel solid bars, which penetrates the matrix in the high-temperature heating process of the ring furnace, resulting in recarburization on partial surfaces after rolling.
  • ② The surface of the steel pipe bonds with high-carbon foreign matter such as oil stains and wood chips before entering the furnace for heat treatment. After high-temperature heat treatment, the C content of surfaces is higher than that of the substrate.

1.4 Crack sensitive steel types of quenching cracks

Certain high-grade seamless steel pipes have a high content of alloying elements and high strength of the pipe body, so the strength factor of the stress field is relatively high. They are steel types that are sensitive to cracks. The surface or internal micro defects of pipes are prone to expand under the action of stress, forming cracks. The morphology and structure of quenching cracks on surfaces of S135 steel grade alloy structural pipes are shown in Figures 9 to 10. The occurrence probability of quenching cracks of this type of seamless steel pipe is significantly higher than that of other steel types. Because this steel type has more Cr and Mo alloy elements, high pipe strength, poor plastic deformation of the pipe microstructure, deformation storage energy can only be released by forming new surface cracks. It belongs to pipes that have a high risk of quenching cracks.
causes and preventive measures of quenching cracks in seamless steel pipes part two 2 - The quenching crack of seamless steel pipe causes and preventive measures 
Figure.9 The morphology of quenching cracks on surfaces of S135 steel-grade alloy structural pipes

2. Preventive measures

Quenching crack is a kind of defect caused by stress cracking. It is caused when the internal stress of the material exceeds the fracture strength. Its reasons are very complicated. The internal stress is formed in the quenching process and has tensile stress properties. The internal cause of quenching cracks is the excessive stress of the material during the martensitic transformation in the quenching process, which is closely related to the chemical composition of the steel, mainly the C content and the alloying element content. Generally, the higher the C content is, the easier it is to produce cracks. It is generally believed that no quenching cracks will occur when the C content is smaller than 0.2%. At the same time, external factors such as cooling methods of quenching and surface quality of steel pipes may also cause quenching cracks.
causes and preventive measures of quenching cracks in seamless steel pipes part two 3 - The quenching crack of seamless steel pipe causes and preventive measures 
Figure.10 The microstructure morphology of quenching cracks on surfaces of S135 steel grade alloy structural pipes
According to a variety of factors that may cause quenching cracks, the following preventive measures can be taken:

(1) Preventive measures for quenching cracks caused by surface defects

For quenching cracks caused by local stress concentration due to surface defects, it is necessary to improve the surface quality of the rolled pipe body and reduce the mutations of macroscopic defects and shapes of materials.

(2) Preventive measures for stress cracking type quenching cracks

Reducing the cooling rate can reduce the residual stress to a certain extent, that is, reduce the structural stress generated by the martensitic transformation, and the inner and outer wall phase transformation processes are carried out in a gradient. Under the premise of ensuring that the cooling rate (50 to 60℃/s) of all martensite structures is obtained, the cooling water flow should be appropriately reduced, and the internal spray plus delayed external cooling method should be adopted. Theoretically, the delay time of external spray cooling being relative to that of the internal spray cooling is the time required for the martensitic transformation of the inner wall of the steel pipe from beginning to the end, which makes the circumference of the pipe produce compressive stress. When the residual compressive stress is generated in the circumferential direction of the pipe body, quenching cracks can be greatly reduced or eliminated.

(3) Preventive measures for surface carburized quenching cracks

Measures for surface carburized quenching cracks are as the following: Properly increase the viscosity of the casting powder; reduce the slag consumption; prevent the fluctuation of the steel liquid; at the same time, because of the increase of the slag viscosity, the diffusion speed of the carbon in the molten slag layer to the molten steel will be greatly slowed down. 
Adding an appropriate amount of oxidant such as MnO2 to the casting powder can promote the oxidation of carbon in the casting powder and effectively inhibit the carbon content in the enriched carbon layer and molten slag layer, or use carbon-free casting powder.

(4) Preventive measures for quenching cracks caused by steel types being sensitive to cracks

Properly adjust the steel composition; reduce the content of C element; refine the grains and improve the crack propagation resistance. The mass fractions of C and Mn should be strictly controlled for water quenching steel grades. When w(C) plus w(Mn)/3 is equal to or higher than 0.9%, there is a risk of cracking for the water quenching, and the oil quenching process should be adopted. For steel grades containing high C and Mn content, reducing the quenching temperature and cooling rate will help prevent quenching cracks in the steel pipe.

3. Conclusion

Through macroscopic and microscopic analysis as well as experimental research on the quenching cracks on the sampled pipe, the type and cause of quenching cracks are determined, and the following improvement measures are proposed:

  • (1) The surface defects of the pipe body caused by the rolling process are prone to stress concentration after quenching, which becomes the cause of quenching cracks. Therefore, optimize the rolling process parameters to improve the surface quality of the pipe and reduce the macroscopic defects and shape mutations of the material.
  • (2) Unreasonable quenching cooling methods will lead to great residual tensile stresses in the pipe body and increase the risk of quenching cracks. Residual compressive stress will generate after the pipe body is quenched by adjusting the sequences of internal spraying and external spraying and flow rates of cooling water of the pipe body, which is an effective measure to reduce the residual stress and eliminate quenching cracks.
  • (3)The molten steel is not stable in the crystallizer. The carbon-rich layer formed by casting powder in the melting process contacting the molten steel and causing recarburization of casting blank is one of the reasons for the carbon increase of the casting blank. Adopt carbon free casting powder and control such as drawing speeds, vibration frequency of the crystallizer to prevent fluctuations of steel liquids and maintain a stable thickness of the liquid slag layer.
  • (4) Properly adjust the steel’s composition that is sensitive to cracks, and reasonably select the cooling medium and cooling rate according to the mass fractions of C and Mn.

Source: China 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]

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