At present, the rolling bearing is refrigerated for 30 min (- 196 ℃) in the industrial freezer before assembly on the motorcycle, and then loaded into the motorcycle. This technology has been widely used in motorcycle industry. Before, motorcycles used imported bearings. In order to realize the localization of imported bearings, our factory produced alternative products for motorcycles. However, once the bearing products of conventional heat treatment are frozen, the size will increase and the deformation will affect the assembly of motorcycle. In order to solve the problem of dimension expansion after freezing, we have carried out heat treatment process test and analysis, and adopted the process of increasing tempering time and temperature to meet the special requirements of users.
(1) After the bearing steel is quenched and tempered at 840 ℃ and 165 ℃, about 8% retained austenite remains in the structure, which retains the full potential of austenization when heated to 840 ℃. It can be seen from the thermodynamic principle that these retained austenites are unstable phases. Under certain dynamic conditions, they can spontaneously change to more stable structures with low energy, which is the basic reason for the size increase after deep cooling. Because in deep cooling, the retained austenite with high energy and small specific volume changes into martensite with low energy and large specific volume, which makes the size increase.
(2) In the test process, increase the tempering time and temperature. The amount of retained austenite in the bearing ring is reduced to 4.04%. This is because with the increase of tempering time and tempering temperature, the decomposition amount of retained austenite increases, and the undissolved retained austenite is further aged, which makes the bearing structure treated by the process test more stable than that treated by the original process. In the subsequent freezing assembly process of industrial freezer, the transformation amount of retained austenite is very small, so the bearing size remains stable and can be controlled within 0.003mm.
(1) The transformation of retained austenite to martensite is the basic reason for the increase of bearing in the process of cryogenic assembly.
(2) Increasing tempering temperature and time can effectively reduce the amount of retained austenite and improve the stability of microstructure.
(3) Compared with the original process, the stability of bearing size is greatly improved, which can meet the requirements of industrial freezer assembly process.
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