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.
Post time: Oct-09-2024