Austempering is one of the effective quenching methods for reducing the deformation and cracking of seamed steel pipes (straight seam steel pipes , spiral steel pipes and
erw steel pipes) and
seamless steel pipe, but how to properly use the process should be based on the material, size, hardness and toughness of the steel pipe. The requirements for deformation, as well as the specific working conditions, etc., are reasonably selected. The following issues to be noted for the isothermal quenching of seamless steel tubes are summarized below for reference by heat treatment operators.
[1] For austempering of steels with low hardenability such as carbon seamless steel tubes, the quenching heating temperature should be appropriately increased, the stability of austenite should be increased, and the high temperature non-Bayesian in the isothermal process should be avoided and prevented. The transformation of the body.
[2] Austempering is limited by the size of steel and steel. In the case of austempering, 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 the isothermal quenching, 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 viewpoint of carbon content, carbon steel of 0.4% to 0.6% is not suitable for austempering, and carbon steel of 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, with reference to the C curve of the steel. The principle is that all austenite is transformed into lower bainite. For the cooling medium, a nitrate salt bath solution is usually used. To ensure the stability of the temperature, a cooling device should be added to the medium to prevent the temperature of the medium from increasing and affecting the quenching quality. In addition, the length of the isothermal time should be based on the end of the seamless steel tube structure transformation. If it is too long, the work efficiency will be reduced, so it should be taken seriously.
[4] Some seamless steel pipes subjected to austempering may not be tempered. However, for partially quenched workpieces, the internal retained austenite will be transformed into martensite during the subsequent air cooling process, so it must be tempered, in order to eliminate the brittleness and stability of the seamless steel tube. The fire temperature should be lower than the isothermal temperature.
