For small and miniature 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. This article mainly talks about my personal views from the aspects of vacuum heat treatment process and equipment. Please criticize and correct any improprieties.
Overview of the development of vacuum heat treatment
Around 1968, American companies and Japan Vacuum Research Institute developed vacuum quenching oil and aqueous 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 a single-chamber furnace to a 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 steels, stainless steels, high-speed steels, bearing steels and all carbon steels and alloy steels.
Advantages of vacuum heat treatment
(1) Bright heat treatment can be realized. When the workpiece is heated at a pressure lower than atmospheric pressure, it does not contact air and does 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 achieves 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 be used to extract impure gases such as moisture in the workpiece, so that the surface of the workpiece is clean and bright, and hydrogen is not easily generated. crisp. 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) Machining allowance after heat treatment can be reduced. 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.
(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 be decomposed to form free oxygen to be discharged, so that the surface quality of the workpiece is further improved, and the surface of the workpiece can be activated.
(6) The operating environment is clean, the operation is automatic and simple, and it is a pollution-free and pollution-free clean heat treatment.
Production case
The main failure modes of small and miniature precision bearings are wear and fatigue damage. Therefore, the workpiece needs to have uniform hardness, high wear resistance, elastic limit and contact fatigue strength, sufficient hardenability and toughness, and small distortion.
The small and miniature bearings produced by our company are made of domestic GCr15, the outer ring diameter is 4-50mm, the thickness is 0.2-1.5mm, the inner ring diameter is 3~45mm, the thickness is 0.5~1.5mm, the quenching hardness is ≥63HRC, and the metallographic structure conforms to JB /T 1255-2001 “High-carbon chromium bearing steel rolling bearing heat treatment technical conditions” specified in the second level of cryptocrystalline or fine crystalline martensite, evenly distributed fine residual carbides and a small amount of retained austenite. The hardness difference of the same furnace product is ≤1HRC, and the distortion is ≤0.01mm. For precision bearings, its quality stability is very important, among which heat treatment plays a key role. For this reason, the determination of process equipment and technology is particularly important.
For this reason, we chose the WZC-45 horizontal double-chamber oil and gas quenching vacuum furnace produced by Beijing Institute of Mechanical and Electrical Technology, and the vacuum quenching oil and tempering oil selected KR368 vacuum quenching oil and KR 535 tempering of Nanjing Kerun Industrial Media Co., Ltd. Oil. Since its use in 2005, the products supplied by the two units are of good quality, stable performance, and high quality after-sales service.
(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 bearings.
In order to ensure the quenching quality and output of bearing rings, three layers of trays are placed in the quenching material 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. Therefore, preheating is required to make the temperature of the ferrule on each layer of the material tray uniform. Set the preheating temperature at 700℃, the heating time from room temperature to 700℃ is 45min, keep it at 700℃ for 40min, and 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 and adhesion of the product, which will affect the use of the bearing. life.
(2) Quenching cooling medium
For small and miniature bearings, when vacuum quenching is used, the cooling medium needs to use vacuum quenching oil, and the vacuum quenching oil must meet the following conditions:
①The vapor pressure is low, generally 10-2～10-4Pa, to ensure that the vacuum quenching oil will not volatilize 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.
③Low critical pressure and good cooling performance. The hardness of the parts after vacuum quenching should be uniform, reaching atmospheric quenching hardness or higher.
④Good thermal stability, that is, good anti-aging performance and long service life.
Through market research, our company chose KR368 vacuum quenching oil produced by Nanjing Kerun Industrial Media. This vacuum quenching oil can meet the technical requirements of the product, has good brightness, easy cleaning, low oil consumption, easy to vacuum, and The characteristics of thermal oxidation and good temperature, the hardness of the quenched workpiece is uniform, and the distortion is small. Over the years, the oil quality has been stable and the service life is long. Since 2005, the entire tank has not been replaced with new quenching oil. Under normal consumption, only new oil is normally added.
Our company strictly abides by the operating procedures, degassing when the new oil is put into use, and the temperature of the vacuum oil is controlled at 70～80℃. At the end of the heating and preparation for quenching, the heating gate must be closed immediately when the material basket enters the quenching chamber from the heating chamber, and the heating chamber shall be continuously evacuated to ensure that the vacuum degree of the heating chamber is always higher than that of the quenching chamber, so that there is a gap between the two chambers. Good heat insulation and sealing condition. During quenching, the oil tank should be filled with 4～6.7×104Pa high-purity nitrogen (99.999%) to maintain the oil surface pressure, inhibit the volatilization of quenching oil, and reduce the concentration of oil mist in the quenching chamber. After the bearing ring is completely cold, drain the residual oil in the oil outlet, evacuate the quenching chamber to remove the oil mist, open the furnace door and take out the workpiece. After the furnace is shut down, the heating chamber should be kept in a vacuum state (3.99×104Pa) to prevent air or moisture Enter. When the temperature of the heating chamber is less than 300°C, the quenching chamber is vacuumed and degassed; when the temperature of the heating chamber drops below 250°C, the water, gas and power supply can be cut off.
(3) Oil bath tempering furnace
The circulating oil bath tempering furnace is to ensure that the quenching stress of small and miniature precision bearing rings 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.
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