Quenching method for deformation and cracking of seamless black steel pipe

Posted by zora li on October 10th, 2022

Austempering is one of the effective quenching methods to reduce the deformation and cracking of seamed steel pipes (straight seam steel pipes, spiral steel pipes and ERW steel pipes) and seamless black steel pipes, but how to use the process correctly should be determined according to the material, the size and hardness of the steel pipe. and toughness. Reasonable selection of deformation requirements, as well as specific working conditions. The following is a summary of the problems that need to be paid attention to in isothermal quenching of seamless steel pipes for the reference of heat treatment operators.

[1] For austempering steel with low hardenability such as seamless carbon steel pipe, the quenching heating temperature should be appropriately increased, the stability of austenite should be increased, and the high temperature in the isothermal process should be avoided and prevented. change.

[2] Austempering is limited by steel and steel size. In the case of isothermal quenching, the basic principle is to ensure that the seamless steel pipe does not undergo high temperature transformation during the cooling process. Therefore, the steel grade and size of the seamless steel pipe directly affect the quality of austempering, and the general requirement is carbon steel seamless steel pipe. The effective thickness should be no more than 5mm, and the effective thickness of alloy steel should be below 30mm. From the perspective of carbon content, carbon steel with 0.4% to 0.6% is not suitable for austempering, and carbon steel with more than 0.6% can be austempered.

[3] The temperature and time of austempering should be determined according to the performance requirements of the steel pipe and with reference to the C curve of the steel. The principle is that all austenite is transformed into lower bainite. For the cooling medium, nitrate bath solutions are usually used. In order to ensure the stability of the temperature, a cooling device should be installed in the medium to prevent the temperature of the medium from rising and affecting the quenching quality. In addition, the length of the isothermal time should be determined according to the end time of the seamless steel pipe structural transformation. Too long time will reduce work efficiency and should be paid attention to.

[4] Some austempered seamless steel pipes may not be tempered. However, for the partially quenched workpiece, the retained austenite inside it will be transformed into martensite in the subsequent air cooling process, so it must be tempered to eliminate the brittleness and stability of the seamless steel pipe. The ignition temperature should be lower than the isothermal temperature.

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