What are the precautions during austempering?

The concept of austempering

Austempering refers to the process of isothermalizing the quenched parts above the Ms point temperature and the temperature region above the "nose" of the C curve for a period of time to fully transform austenite into lower bainite, and then take out air-cooled heat treatment. It can be seen that austempering is carried out within a certain temperature range, so the structural stress and thermal stress are relatively small. Austempering is usually used for workpieces with complex shapes and strict requirements for deformation.

Comprehensive analysis of the process and characteristics of austempering, during the heat treatment process, it has the following advantages:

⒈The deformation of the workpiece is small, and there will be no quenching cracks. The reason is that the quenching heating temperature isothermal temperature is small compared with general quenching, so the thermal stress caused by thermal expansion and contraction is small; during the transformation of the structure, the temperature of the inner and outer sections of the workpiece is relatively uniform, basically at the same time The transformation of lower bainite occurs; the specific volume of lower bainite is smaller than that of martensite, and the structural stress during quenching is also smaller.

⒉The workpiece has good toughness, and has higher hardness and strength. Compared with martensite, lower bainite has better toughness, while its strength and hardness are slightly lower; in addition, lower bainite transforms into a semi-diffusion type transformation, the transformation speed is much slower than martensite, and the formation of collisions The probability of microcracks is reduced, so austempering can obtain higher hardness and good toughness.More about:plastic mould steel Suppliers

Austempering is one of the effective quenching methods to reduce deformation and cracking of parts. However, how to correctly adopt this process method should be based on the material, size, hardness, toughness, deformation requirements of the parts, and specific working conditions. The issues that need to be paid attention to during austempering are summarized as follows for the reference of heat treatment operators.

⒈If austempering is performed on steel grades with low hardenability, such as carbon steel, the quenching heating temperature should be appropriately increased. The purpose is to increase the stability of austenite and avoid and prevent the occurrence of high-temperature non-bainite in the isothermal process. change.

⒉Austempering is restricted by the steel grade and the size of the workpiece. When performing austempering, the basic principle is to ensure that the workpiece does not undergo high temperature transformation during the cooling process. Therefore, the steel grade and size of the workpiece directly affect the quality of austempering. The general requirement of www.hongchao-dg.cn is carbon steel. The effective thickness of the workpiece should not be greater than 5mm, and the effective thickness of alloy steel should be less than 30mm. From the perspective of carbon content, carbon steel is not suitable for austempering at 0.4% to 0.6%, and austempering can only be carried out for carbon steels above 0.6%.

⒊The temperature and time of austempering should be determined according to the performance requirements of the workpiece and with reference to the C curve of the steel grade. The principle is that all austenite is transformed into lower bainite. The cooling medium usually adopts nitrate solution to ensure the temperature For stability, a cooling device should be added to the medium to prevent the increase of the medium temperature from affecting the quenching quality. In addition, the length of the isothermal time should be based on the completion of the transformation of the above workpiece organization. If it is too long, it will reduce the efficiency of the operation, so it should be taken seriously.

⒋Some workpieces undergoing austempering may not be tempered. However, for the part of the bainite quenched workpiece, the internal retained austenite will be transformed into martensite in the subsequent air cooling process, so it must be tempered in order to eliminate brittleness and stabilize the size. The fire temperature should be lower than the isothermal temperature.