Bearing factory is how to control the content of the paralympic?

Bearing factory is how to control the content of the paralympic? 2018 - 11 - 24 kang wave, Gu Yuxin, Luoyang Bearing Science & Technology Co. , Ltd. , the sun in zhejiang He Hongyu high-speed Bearing Co. , Ltd, Luoyang government's Bearing Co. , Ltd. High carbon chromium Bearing steel Bearing parts of the martensite after quenching tempering, the microstructure consists of martensite, retained austenite and uniform distribution of small carbides. Retained austenite content in the GCr15 steel bearing parts, generally about 15%, and GCr15SiMn retained austenite content in the steel bearing parts of more than 20%. Retained austenite is metastable phase, high residual austenite content will result in changes of workpiece size in use process, lower precision, greatly shorten the service life of bearing. As the user requirements for product quality is higher and higher, a lot of bearing manufacturers not only strict requirements, the organization's hardness and other targets of the retained austenite content also strict control, bearing manufacturer abroad on this aspect is particularly hard, general requirements of residual austenite content is below 3%. In dealing with small size, with thin wall thickness of GCr15 steel bearing roller, cold treatment or cryogenic treatment can basically meet the requirements; But when dealing with GCr15SiMn steel bearing roller is difficult to meet the requirements. In order to satisfy the customer's requirements, it is necessary for us to develop a more effective way to reduce the content of retained austenite. 1. The causes of residual austenite bearing steel in quenching process, temperature from the Ms point began to austenite to martensite transformation, has been below the Mf point temperature austenite to martensite transformation completed, Mf points are below room temperature of bearing steel is commonly used, but less than Mf martensitic transformation will not continue. Austenite can't completely change, will be part of residual austenite does not change. This is because the martensite transformation completed experience on austenite around continue to shift effect, martensite volume expansion in turns, produce very big compressive stress, the transformation of residual austenite in, to overcome the compressive stress to do work, thus hinders the further transformation of retained austenite. 2. Test material and process test for GCr15SiMn, specifications & phi; 30mm× 30 mm, its chemical composition as shown in table 1. Technical requirements: 61 ~ 65 HRC hardness, residual austenite content & le; 3%, the rest requirements according to JB/T12552014 execution. Test using love association Lin salt solution multi-purpose furnace, quenching process with 820 ℃ heat preservation 75 min after salt bath quenching, salt bath temperature is 170 ℃, the isothermal time 50 min. Then adopt different tempering temperature and cold treatment process, process curve, and the specific process, such as the appended drawings and are shown in table 2. 3. Results and analysis ( 1) Conventional tempering and second high temperature tempering with 170 ℃ tempering after quenching, and again to 200 ℃, 220 ℃ and 240 ℃ tempering respectively, such as in table 2 1 ~ 4 process. Then on structure, hardness and residual austenite content testing, conventional and secondary after high temperature tempering test results as shown in table 3. Can be found that after ordinary tempered, organization, hardness can meet the technical requirements, but retained austenite content reached 16%. With second high temperature tempering can significantly reduce the content of retained austenite, as temperatures rise, the residual austenite content gradually reduce, when the tempering temperature to 240 ℃, the residual austenite content is basic in 0. Less than 9%. According to research, high temperature tempering can reduce the residual austenite content for two reasons: on the one hand, high temperature tempering can promote the precipitation of carbon and alloy elements in austenite, thus improve the martensitic transformation of Mf points, so as to promote the retained austenite to martensite transformation; On the other hand, high temperature tempering will eliminate prior austenite to martensite transformation stress, reduce the austenite to martensite transformation effect. But the high temperature tempering can reduce the hardness of workpiece, can be found from table 3, 240 ℃ tempering hardness will reduce about 2 HRC. （ 2) Cold treatment or cryogenic treatment + conventional tempering the cold treatment is to use the cooler workpiece cooling to the - Around 70 ℃; Adopt or cryogenic treatment, the workpiece with liquid nitrogen cooled to - About 196 ℃, such workpiece residual austenite experience significantly reduced, cryogenic treatment effect is obvious, many on this research. Using 5 ~ 8 process, test results as shown in table 4. Using this method when handling GCr15 steel workpiece residual austenite content is basic to control below 3%, but when handling GCr15SiMn material, but it is difficult to meet the requirements, general control in 4% ~ 6%, 2 HRC hardness increases. （ 3) By cold treatment + high tempering combining the advantages and disadvantages of the above two kinds of processing way, in guarantee the retained austenite content and hardness are under the premise of qualified, can adopt the way of cold treatment + high tempering control residual austenite content, specific technology such as in table 2, 9, 10, process. Test results as shown in table 5, can be found that the number 10 technology to control the residual austenite content and hardness in the range of technical indicators. 4. Conclusion with the method of cold treatment plus high temperature tempering, can be overcome by cold treatment alone or high temperature tempering faults, reduce the retained austenite content in achieving high hardness under the premise of guarantee the retained austenite content is reduced to below 3%. Source: the metal processing ( Hot working) Magazine,