For small and micro precision bearings, the chemical composition of raw materials, original organization, machining level, heat treatment process, and processing equipment may greatly affect the service life of the bearing. Based on experience, Eric Bearing Limited talks about vacuum heat treatment of NSK precision bearings mainly from the aspects of vacuum heat treatment process and equipment.
Overview of the development of vacuum heat treatment
Around 1968, American companies and Japan Vacuum Research Institute developed vacuum quenching oil and water quenching agent. The heat treatment of bearing steel began to use vacuum heat treatment, and vacuum technology has also been rapidly developed in the heat treatment industry. From single-chamber furnace to multi-chamber combined machine group, from general vacuum heat treatment to high-pressure gas quenching, vacuum water quenching, vacuum carburizing, vacuum carbonitriding and multi-component co-infiltration, etc. After decades of hard work, the production quality and level of domestic vacuum furnaces have also been greatly improved, the production cost of the unit used has been reduced, and the application range of vacuum heat treatment has rapidly expanded.
At present, vacuum heat treatment has been applied to the heat treatment of important parts such as various tool steel, stainless steel, high-speed steel, bearing steel, and all carbon steel and alloy steel.
Advantages of vacuum heat treatment
1. Can realize bright heat treatment. When the workpiece is heated at a pressure lower than atmospheric pressure, it will not contact air and will not change its surface. When the furnace is evacuated to 0.1~1Pa, the impurity content in the workpiece can be reduced to 1.5ppm (1ppm=0.0001%), so the workpiece can achieve the purpose of clean and bright quenching.
2. The distortion of the workpiece is small after vacuum quenching. For example, the 9Cr18 radial bearing ring with an outer diameter of 65mm and a wall thickness of 2mm, after vacuum quenching, its ellipticity is reduced to 1/8～1/9 compared with ordinary heat treatment.
3. It has a degassing effect, which improves the mechanical properties and service life of the processed workpiece. Because steel will inhale gas (H2, N2, CO, etc.) during smelting, resulting in tiny pores and hydrogen embrittlement, vacuum technology can remove impure gases such as moisture in the workpiece, so that the surface of the workpiece is clean and bright, and hydrogen is not prone to occur. brittle. And because the surface quality is improved, the wear resistance of the workpiece is improved, and its service life can be increased by 30% to 120%.
4. It can reduce the machining allowance after heat treatment. There is no oxidative decarburization after vacuum heat treatment, and the distortion is very small, so processing can be reduced or even eliminated after heat treatment. NSK 6312 DDUC3 bearings online , pls click here :
5. Decompose oxides. When the vacuum degree is lower than the decomposition pressure of the corresponding oxides of the metal or alloy, the oxides will decompose to form free oxygen to be discharged, which further improves the surface quality of the workpiece and can activate the surface of the workpiece.
6. The operating environment is clean, the operation is automatic and easy, and it is a pollution-free and pollution-free clean heat treatment.
1. WZC—45 double-chamber oil and gas quenching furnace
The heating power of the furnace is 60kW, the size of the furnace heating chamber is 450mm×670mm×350mm, the rated furnace capacity is 120kg, the maximum working temperature is 1300℃, the furnace temperature uniformity is ±5℃, and the ultimate vacuum of the heating chamber is 6.6×10-3～6.6Pa , The pressure rise rate is 0.66Pa/h, and the quenching gas pressure is 8.7×104Pa. This furnace type is suitable for the quenching of small and miniature NSK bearings.
In order to ensure the quenching quality and output of NSK bearing rings, three layers of trays are placed in the quenching basket. According to different types of bearing rings, the output per furnace is 40-70kg. Because the vacuum heating speed is relatively slow, the vacuum heating time is 4-6 times that of the salt furnace and 1.5-2 times that of the air furnace, so it is necessary to preheat to make the temperature of the ferrule on each layer of the tray uniform. Set the preheating temperature at 700℃, the heating time from room temperature to 700℃ is 45min, keep it at 700℃ for 40min, the vacuum degree is 40Pa; then it will be heated to 835～845℃ for 15min and keep for 50～90min (according to the model Corresponding quenching temperature and holding time), the vacuum degree is 0.1Pa. For bearing steel, heating under 900℃, the higher the vacuum degree, the better the degassing effect, and the brighter the surface; but the vacuum degree should not be too high, otherwise it will cause dechromization, product adhesion, and affect the NSK bearing Service life.
2. Quenching cooling medium
For small and miniature NSK bearings, when vacuum quenching is used, the cooling medium needs to be vacuum quenching oil, and the vacuum quenching oil must meet the following conditions:
(1) The vapor pressure is relatively low, generally 10-2～10-4Pa, to ensure that the vacuum quenching oil will not evaporate significantly under the condition of 10-1～10Pa vacuum.
(2) Less impurities and residual carbon, and low acid value. The surface brightness of the parts after vacuum quenching should not be less than 70% of the standard sample.
(3) Low critical pressure and good cooling performance. The hardness of the parts after vacuum quenching should be uniform, reaching atmospheric quenching hardness or higher.
3. Oil bath tempering furnace
The circulating oil bath tempering furnace is to ensure that the quenching stress of the small, micro-precision NSK bearing ring is fully relieved, the dimensional stability is good, the hardness is uniform, and the finished product will not be distorted due to insufficient tempering after assembly. The requirements for tempered oil are high flash point, low oil fume, good thermal oxidation stability, and long service life.