Causes and preventive measures of quenching cracks in seamless steel pipes

1. Surface defect-induced quenching cracks
When a certain unit rolls 26CrMo4s alloy structural pipes, small quenching cracks often appear on the inner wall. Photos of the polished micromorphology show that there are many surface defects such as pits and warping with a depth of no more than 0.2mm on the inner wall of the pipe. These defective parts produce stress concentration under the action of quenching stress and become the cause of quenching cracks. Surface defect-induced quenching cracks mostly occur in small-diameter, thin-walled steel pipes. On the one hand, the rolling elongation of small-diameter steel pipes is large, and original defects such as pits and scratches are prone to appear on the inner and outer surfaces of the pipe. At the same time, during the quenching and cooling process of thin-walled steel pipes, the surface tensile stress is generated at the defect location. The stress concentration effect is more significant, so defect-induced quenching cracks are prone to occur.

2. Stress cracking type quenching cracks
Stress cracking is a common type of quenching crack. It is a crack defect caused by the surface tensile stress exceeding the material strength during the quenching cooling process. The surface of the pipe body with stress-cracking quenching cracks is smooth and flat, without original defects, and the microstructure is uniform and fine. The cracks are caused by excessive surface tensile stress; the stress cracking quenching cracks are completely perpendicular to the surface of the pipe body, The extension in the direction of wall thickness also shows that this type of crack is entirely caused by excessive surface tensile stress.

3. Surface carburization-type quenching cracks
When using medium carbon Cr-Mo micro-alloy steel with a C content of about 0.30% to produce seamless steel pipes, quenching cracks often occur locally on the outer surface of the pipe. Microscopic analysis results show that the structure around the quenching crack has carburization, and the depth of the carburization layer is 0.5-2.0mm. The reason for the formation of this quenching crack is that there is local carburization on the outer surface of the tube, which leads to excessive stress in the carburized part during the quenching process, thus forming quenching cracks. According to the seamless steel pipe production process, it is speculated that the process that may cause the increase in C content on the surface of the steel pipe is: that the steelmaking high-carbon protective slag adheres to the surface of the silt pipe blank and penetrates the matrix during the high-temperature heating process of the annular furnace, resulting in localization of the surface of the capillary tube after the rolling is completed. Carburization; before the steel pipe enters the heat treatment furnace, high-carbon foreign matter such as oil stains and sawdust adheres to the surface. After high-temperature heat treatment, the C content on the surface is higher than that of the matrix.

4. Quenching cracks in crack-sensitive steel types

Some high-grade seamless steel pipes have high alloying element content and high pipe body strength, so the stress field intensity factor is high. They are crack-sensitive steel types. Microscopic defects on the surface or inside of the pipe are very easy to expand under the action of stress, thus Crack defects form. Surface quenching crack morphology and structure of S135 steel grade alloy structural pipe. The probability of quenching crack defects of this type of seamless steel pipe is significantly higher than that of other steel types. Because this steel type contains more Cr and Mo alloy elements, the pipe strength is higher, the pipe microstructure has a poor ability to coordinate plastic deformation, and the deformation storage capacity is poor. The release can only occur through the formation of new surface cracks, so it is a pipe with a high risk of quenching cracks.

Preventive measures for quenching cracks in seamless steel pipes

Quenching cracks are a defect caused by stress cracking. They are caused when the internal stress of the material exceeds the fracture strength. The cause of the occurrence is very complicated. The internal stress is formed during the quenching process and has the nature of tensile stress. The internal cause of quenching cracks is that the stress is too large when the material undergoes martensitic transformation during the quenching process. It is closely related to the chemical composition of the steel, mainly the C content and the alloy element content. Generally, the higher the C content, the easier it is to crack. It is generally believed that no quenching cracks will occur when w (C) is 0.2%. At the same time, external factors such as quenching cooling methods and steel pipe surface quality may also cause quenching cracks.

According to the various factors that may cause quenching cracks, the following preventive measures can be taken:

1. Preventive measures for surface defect-induced quenching cracks. For quenching cracks caused by local stress concentration induced by surface defects, it is necessary to improve the surface quality of the rolled tube body and reduce the macro defects and shape mutations of the material.

2. Preventive measures for stress-cracking quenching cracks. Reducing the cooling rate can reduce the residual stress to a certain extent, that is, reduce the organizational stress generated by the martensitic phase transformation and the phase transformation process of the inner and outer walls is carried out in a gradient. Under the premise of ensuring the cooling rate (50-60℃/s) of the entire martensitic structure, appropriately reduce the cooling water flow rate, and adopt the internal spray + external shower (delayed) cooling method. In theory, the delay time of external shower cooling relative to internal spray cooling is the time required for the martensitic phase transformation of the inner wall of the steel pipe to start and complete, so that compressive stress is generated around the pipe body. When 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 carburization-type quenching cracks. Appropriately increase the viscosity of the protective slag, reduce the slag consumption, and thicken the liquid slag layer to prevent the fluctuation of the molten steel surface; at the same time, due to the increase in slag viscosity, the diffusion rate of carbon in the slag layer to the molten steel will be greatly slowed down; adding an appropriate amount of oxidant (such as MnO2, etc.) to the protective slag can promote the oxidation of carbon in the protective slag and effectively inhibit the carbon content in the carbon-enriched layer and the slag layer; or use carbon-free protective slag.

4. Preventive measures for quenching cracks in crack-sensitive steels. Appropriately adjust the composition of the steel grade, reduce the C element content, refine the grains, and improve the resistance to crack propagation. The mass fraction of C and Mn should be strictly controlled for water-quenched steels. There is a risk of cracking when using water-quenching technology, and oil-quenching technology should be used. For high-C and high-Mn steels, reducing the quenching temperature and cooling rate is conducive to preventing the occurrence of quenching cracks in steel pipes.