Vacuum Heat Treatment of Miniature Bearing Rings

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Miniature bearings (referring to the bearing inner diameter <9 mm) should meet the requirements of high precision, high sensitivity, long life and reliable use, and should have high hardness, wear resistance and dimensional stability after heat treatment. Due to its small contact stress (<1960 MPa), it is not easy to produce fatigue spalling, and the main failure mode is wear. At present, the steel grades used in miniature bearings mainly include ZGCrl5, 9Cr18 and W18Cr4V, etc. The heat treatment generally adopts protective atmosphere or vacuum heat treatment furnace.

Heat treatment plays an important role in improving the inherent quality of rolling bearings and prolonging their service life. In the protective atmosphere heating of steel parts, the endothermic atmosphere is a controllable atmosphere that is widely used. Its main advantage is that it is easy to achieve carbon potential control, but the disadvantage is that it is easy to cause oxidation of chromium, and it is easy to be mixed with air at low temperature. Explosion occurs, and it is easy to form carbon black at low temperature. Most stainless steels are not suitable for protection during heating; and some high-purity inert gases in the protective atmosphere usually contain 0.1% (volume fraction) of active gas impurities. This purity Equivalent to a vacuum of 133 Pa, this gas is used for metal heating protection and will still react with the metal surface, so further purification is required. From the (cost) point of view of purification and maintaining purity, it is very difficult to make the residual gas of 1×10- of impurities, and in the vacuum state, the residual gas of 1×10-6 is equivalent to a vacuum degree of 0.133 Pa, this vacuum degree It is easy to achieve by vacuuming, and it is much cheaper than gas purification to the same degree. Therefore, the vacuum heating method is energy-saving, environmentally friendly and safe, and is suitable for heat treatment of various metals, brazing and sintering. Promote faster.

When the metal is heated under a certain degree of vacuum, in addition to avoiding oxidative decarburization and obtaining a bright surface quality, it also has the effects of degreasing, degassing, and surface oxide decomposition. The characteristic of vacuum heating is that it mainly relies on radiation. Heating in vacuum is slower than that in salt bath and atmosphere. Generally, the time of vacuum heating is 6 times that of salt bath, and the part of the processed workpiece facing the heating body is easily heated, and the heating speed is relatively high. Therefore, the furnace loading and placement of the workpiece and the formulation of process parameters during vacuum heating need to be comprehensively considered to ensure uniform heating and cooling of the workpiece and reasonable control of distortion.

A miniature bearing ring is made of 9Cr18Mo steel, and its chemical composition (mass fraction, %) is 0.95~1.10C, 16.00~18.00Cr, 0.40~0.70Mo, ≤0.80Si, ≤0.80Mn, ≤0.035P, ≤0.030 S, the remainder Fe. 9Crl8Mo is a high-carbon and high-chromium martensitic stainless steel, which is developed by adding Mo on the basis of 9Crl8 steel. Therefore, it has higher hardness, wear resistance, tempering resistance and corrosion resistance than 9Crl8 steel. performance 1. The dimensions of the ferrules are the outer diameter of the outer ring D.=7.00 mm, the inner diameter of the outer ring d, =5.40 mm; the outer diameter of the inner ring D.=4.10 mm, the inner diameter of the inner ring d, =2.90 mm, the height of the inner and outer rings is equal to H=2.50 mm. The manufacturing process of the inner and outer rings is: bar CNC finishing → vacuum quenching and tempering → rough grinding → fine grinding → fine grinding → stabilization treatment → demagnetization → assembly, etc. The size of the inner and outer rings is small and the manufacturing process is complicated, and the bearing ring After vacuum heat treatment, it is required to have a bright surface, uniform hardness (55-58 HRC), inner and outer diameter and height distortion ≤0.06 mm. Therefore, heat treatment is a key process in the bearing manufacturing process, and the requirements are high.

The inner/outer ring of the bearing is made of 9Crl8Mo steel, with high quenching temperature (1020~1100 ℃), small size of the ring, wall thickness of 0.6~0.8 mm, and strict distortion requirements; and each incoming material is large, about 60,000 pieces. When formulating the vacuum heat treatment process, in addition to considering factors such as the thermal conductivity of the metal itself, the degree of alloying of the steel and the dissolution of carbides in the austenitic state, it is also necessary to combine the shape, size and quantity of the parts and the characteristics of vacuum heating. Determine the furnace loading method and process parameters. For carbon steel and general alloy structural steel, heating beyond the transformation point can quickly complete the transformation process, and the homogenization of austenite is also easy to carry out, without considering the dissolution of excess alloy carbides; for alloy tool steel, Other high-alloy steels such as high-speed steel and high-chromium die steel are different, and appropriate process parameters need to be formulated according to the dissolution of carbides and the homogenization of solid solutions. Quenching heating and cooling, especially the thermal stress and structural stress generated in the cooling process, will deform the shape and size of the quenched workpiece, resulting in distortion.

In the test, the ferrule is 9Crl8Mo ledeburite steel, which is easy to form uneven carbide segregation and affect the service life of the workpiece. Therefore, the hot working process must be strictly controlled during hot working, and attention should be paid to selecting an appropriate working ratio; when formulating the heat treatment process, it should be Step heating is used to facilitate the full dissolution of carbides and the homogenization of austenite composition. Therefore, in order to ensure the uniformity of the heating and cooling process of the ferrule heat treatment, the furnace loading method of layered hanging and segmented preheating are selected during heating. This not only avoids the uneven hardness of the ferrule caused by stacking furnaces, but also helps to control the distortion; during the quenching and cooling, the oil temperature is controlled at about 60 ℃, and the quenching method of adding oil and gas without stirring is adopted. Appropriately reducing the cooling rate during vacuum quenching, that is, reducing the cooling rate below the martensitic point, can reduce the distortion of the bearing ring caused by structural stress. The vacuum heat treatment process is shown in the figure. The bearing rings are quenched in a WZC60G vacuum quenching furnace and tempered in an oven.

After vacuum heat treatment, the surface of the inner/outer ring of the bearing is bright and silvery white, and the hardness of the test pieces at three different positions in the front, middle and rear of the testing basket are all within 55-58 HRC.

Equipment selection: The VOGQ series vacuum heat treatment furnace produced by SIMUWU is a high-quality product for the vacuum heat treatment process of tooling and molds. Good temperature control accuracy and temperature control uniformity ensure the effective progress of the vacuum heat treatment process. SIMUWU specializes in the manufacture of vacuum furnaces, has more than ten years of relevant experience, and has a good reputation in the field of vacuum furnace manufacturing. The product line includes vacuum air quenching furnace, vacuum oil quenching furnace, vacuum brazing furnace, etc., which are widely sold in developed and developing countries.