PRODUCT CATEGORIES

Vacuum Heat Treatment Furnace

Vacuum Sintering Furnace

Vacuum Brazing Furnace

The Difference Between Austempering and Graded Quenching

Austempering: After heating the steel to make it austenitized and homogenized, it is quickly cooled to a certain temperature in a given bainite transformation temperature zone, and maintained at that temperature for a certain period of time to make it undergo isothermal transformation to form shellfish The microstructure is then removed from the heat treatment process where it is cooled in the air. Austempering is generally carried out in a salt bath or metal bath (lead bath) at a temperature between 300 and 500 ℃, also known as bainite quenching.

Since the bainite transformation is incomplete, there is still a small amount of martensite formed in the air cooling process after the actual isothermal transformation, so the actual austempered structure should be a complex phase of bainite + a small amount of martensite + a small amount of retained austenite Organization, this is a toughened organization. The isothermal time should include the cooling time from the quenching temperature to the isothermal temperature, the soaking time and the bainite transformation time. There is no need to temper after austempering. Austempering is mainly used for alloy steel whose C curve is far away from the longitudinal axis.

Graded quenching: the austenitized and homogenized workpiece is quenched into a constant temperature quenching agent (such as low temperature salt bath, alkali solution or oil bath, etc.) whose temperature is slightly higher (or slightly lower than if necessary) Ms point For a certain period of time, the temperature of the core and surface of the workpiece tends to be uniform, but the microstructure remains austenite (or contains a small amount of martensite) and then takes out air cooling, and completes the martensite transformation under slow cooling (air cooling) conditions. Therefore, the quenching stress produced by graded quenching is very small, which can ensure the quality of quenching and reduce the tendency of deformation and cracking.

The key to grading quenching is the correct selection of quenching temperature, grading temperature and grading time. The austenitizing temperature of graded quenching is 10~20℃ higher than that of ordinary quenching, and the graded temperature depends on the hardenability of the steel, technical requirements, shape and size of the workpiece and other factors. In general, high hardenability steel can choose MS+ (30~40℃). In order to increase the depth of hardened layer, MS- (30~40℃) temperature can be used for larger workpieces. For complex shapes and deformations High-alloy steel tools with strict requirements can be graded twice or more. The grading temperature should be selected in the stable temperature zone of supercooled austenite according to the C curve of steel to prevent non-martensitic transformation. The step quenching aims to avoid non-martensitic transformation and to obtain martensitic structure in the end. Experiments show that the cooling rate of stage quenching is lower than that of water, which is only suitable for small workpieces.

The properties of the graded quenched steel are roughly the same as those of ordinary quenching, but because the cooling was once stopped when it is close to the Ms point, the austenite will age and stabilize during this period of time, which will increase the amount of retained austenite after quenching, which will cause the quenched The properties of steel are slightly reduced.