Retained austenite content in bearing steel and heat treatment

Retained austenite content in bearing steel 2019 - relationship with heat treatment 07 - 03 of ( Wuxi hui precision machinery co. , LTD. ) 1. Relationship between retained austenite content and heat treatment of retained austenite content in the analysis of bearing steel, the mechanical properties of the bearings have a decisive effect, dimensional stability, and bearing material tensile strength, impact toughness, the fatigue strength and corresponding relationship. Bearing ring in oil is cooled, it is tempering treatment directly, or to continue cooling after quenching and tempering treatment again, hardness, and microstructure of bearing ring have greater impact. As is known to all, martensitic transformation is in Ms - Mz point. Because of the chromium bearing steel ( Gcr15） Mz is at low temperature, if the normal heat treatment process, Quenching and low temperature tempering) Processing, the content of retained austenite after quenching is higher, generally retained austenite in more than 15%, the retained austenite in low temperature tempering, also not easy to decompose. Therefore, the dimension stability requirement of high precision bearing is unfavorable. Low temperature ( Below room temperature) Composition of retained austenite is unstable, but due to austenite in low temperature isothermal transformation is relatively slow, so, when cooled to working temperature below, will produce residual austenite slow shift, therefore, should be stable microstructure, reduce the deformation, must through the appropriate cold processing, in order to reduce the content of retained austenite size stabilization. In addition, in order to in line with international standards, the new high chromium bearing steel rolling bearing parts heat treatment technical conditions '(JB/T1255 - 2001 '( Modified version) And will propose to increase the amount of retained austenite detection project. Therefore, in order to meet customer needs, the residual austenite content is controlled at about 1%, we specially made of bearing steel ( Gcr15） : & Phi; 65毫米× 55mm× 32 mm bearing bushing and the same products abroad, in wuxi south and star jay vacuum heat treatment co. , LTD. , multi-purpose furnace and vacuum furnace heat treatment ( Quenching + cryogenic + tempering) Process, the contrast test. 2. Test sample number: 1 # wuxi heat treatment co. , LTD. (south Use multi-purpose furnace heat treatment) 。 2 # sample abroad ( Using vacuum furnace heat treatment) 。 3 # wuxi star jay heat treatment co. , LTD. ( Use multi-purpose furnace heat treatment) 。 4 # wuxi star jay vacuum heat treatment co. , LTD. Technology ( Using vacuum furnace heat treatment) 。 Hardness value contrast see table. 2 # sample average of abroad: 695. 6 hv1, namely between the 60 ~ 61 HRC. 4 # star jay average heat treatment is: 689. 6 hv1, namely between the 60 ~ 61 HRC. The hardness value is very close to, adopt vacuum heat treatment. Hardness testing table ( HV1） Corrosion process by 4% nitric acid alcohol solution, 2 # sample microstructure is cloud, foreign organization, belongs to the normal quenching + tempering at low temperature, the residual austenite mass is obvious. 4 # wuxi star jay heat treatment company technology. Its microstructure and residual austenite mass similar to 2 # sample abroad. By Shanghai quality supervision and inspection on metal materials & gamma; Phase content and X-ray stress analysis report, 2 # and 4 # was very close to the average value of retained austenite, 0 respectively. 1% and 0. 4%. Fully meet customer in about 1% of the retained austenite control requirements. Hardness value of 2 # and 4 #, metallographic organization was very close to the average value of retained austenite, their heat treatment process adopts vacuum furnace heating + cryogenic + of low temperature tempering heat treatment, and to reduce the bearing steel, the content of retained austenite in small deformation, good dimensional stability, good metallurgical structure, should use vacuum heat treatment, quenching heating temperature should be lower limit, should be in commonly - cold temperature Above 100 ℃, the residual austenite size can be significantly reduced. 3. Conclusion in order to reduce the content of retained austenite in the bearing steel, bearing parts must be vacuum furnace heating mode, USES the floor temperature quenching, cold temperature should be beyond traditional general bearing steel cold - processing temperature Range of 70 ~ 80 ℃, should use liquid nitrogen freezing, temperature in - 100 - - Between 130 ℃, the time 3 ~ 4 h, can achieve satisfactory results. Source: the metal processing ( Hot working) 》