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Research on typical bearing vacuum heat treatment technology

Bearings used in automobiles are divided into two categories: sliding bearings and rolling bearings. Sliding bearings are mainly used on engines. Rolling bearings are widely used in various mechanical and electrical assemblies in automobiles. There are two types of rolling bearings, spherical and roller. According to the size and direction of the load, they are divided into radial bearings and thrust bearings. Radial bearings are suitable for bearing radial (heavy straight to the axis center line) loads, and thrust bearings are suitable for bearing axial loads. (parallel to the axis line) load, and there is also a radial thrust bearing that bears both radial and axial forces. In fact, almost all rolling bearings now can bear both radial load and lateral load under certain conditions, but they just focus on one aspect or both. Rolling bearings are mainly divided into ball bearings, roller bearings and needle roller bearings, which are placed in various parts of the car according to their different functions. Since there are many types of bearings, here we will introduce the application of heat treatment technology for such bearings with commonly used typical rolling bearings as representatives.

1.Requirements for use of rolling bearings

There are many types of rolling bearings, but their structure generally consists of an outer sleeve, an inner sleeve, rolling elements (steel balls, rollers, needle rollers) and a cage. Most rolling bearings are used under concentrated, high loads, alternating stress, and high rotational speeds. The main forms of failure are fatigue and wear. The bearing rings and rolling elements of rolling bearings are generally made of high-purity special chromium alloy steel, mainly GCr15 steel and GCr15SiMn steel, of which GCr15 steel uses the largest amount. Vacuum heat treatment plays a vital role in improving the intrinsic quality of rolling bearings and extending their service life.

2.Production process

Taking bearings as an example, the main production process flow is as follows:

(1) Outer ring processing

Raw material (bearing steel bar) blanking → spheroidizing annealing → forging (hot forging or high-speed forging) → ultra-fine processing → rough turning → fine turning → enhanced vacuum heat treatment → shot blasting → double-end rough grinding → centerless grinding Rough grinding → rough grinding of the outer groove → supplementary tempering → cryogenic treatment → double-end fine grinding → centerless grinding and fine grinding → fine grinding of the outer groove → fine grinding of the outer groove → final inspection of parts → marking and cleaning of parts.

(2) Inner ring processing

Raw material (bearing steel bar) blanking → spheroidizing annealing → forging (hot forging or high-speed forging) → ultra-fine processing → rough turning → fine turning → enhanced vacuum heat treatment → shot blasting → double-end rough grinding → centerless grinding Fine grinding → Rough grinding of the inner groove → Rough grinding of the inner hole → Supplementary tempering → Cryogenic treatment → Double end face fine grinding → Fine grinding of the inner groove → Fine grinding of the inner hole → Fine grinding of the inner groove → Parts cleaning.

(3) Assembly (inner ring, outer ring, steel balls, cage)

Fitting → Riveting → Finished product demagnetization and cleaning → Full detection of vibration value → Full inspection of clearance → Full size and appearance inspection → Anti-rust packaging.

3.Vacuum heat treatment process:

(1) After the steel is blanked, the isothermal spheroidizing annealing process is performed before forging. Using the GKT-50 roller-bottom vacuum air-locked continuous annealing production line, after spheroidization, the structure is point-like and spherical pearlite, and the hardness is: 185~207HBW. The goal of refining carbides is achieved after spheroidizing annealing.

(2) Ultra-fine treatment process of blank carbide after forging. In order to reduce energy consumption, the waste heat of forging is used to ensure that the final forging temperature of the bearing workpiece blank is between 850 and 900°C, and then heated to 1050°C for solid solution to dissolve the carbides and homogenize the ingredients, and then quenched into an isothermal bath at 310°C. The lower bainite structure is obtained through isothermal transformation, and then the workpiece after the isothermal transformation is isothermal spheroidized for a short time. The workpiece blank treated in this way has ultra-fine carbides. The shape of the carbides is relatively round and the distribution is relatively uniform. It is also conducive to eliminating or reducing undesirable defects such as band segregation.

4.Carbonitriding process

In view of the characteristics of GCr15 steel and the service conditions of rolling bearings, taking into account the relatively large load usage characteristics of this type of automobile bearings, the carbonitriding process is used for the inner and outer rings:

Specific parameters: heating temperature: 835℃±5℃, the temperature of the last zone drops to 825℃±5℃; carbon potential: 1.3%C, the last zone is 1.2%C; total time from entering the furnace to heating completion: 420min; CH3OH : 2000mL/min; NH3: 2L/min; use graded quenching carbonitriding process kerosene quenching, oil temperature: 110℃±5℃; cleaning: 60～80℃; cold treatment: 5～15℃; tempering: 190 ℃±5℃ (outer ring: 210℃±5℃), vacuum tempering time: 180 min. The test results of the workpiece after the above carbonitriding treatment are: surface hardness: benchmark (61~65HRC), actual performance 63HRC; carburizing depth: benchmark (≥0.25mm), actual performance 0.4mm; metallographic structure: carbide grade 1 , martensite grade 1 (see Figures 3 and 4). In view of the characteristics of the rolling elements, we have also specially developed new carbonitriding equipment, which can make the deep layers of the rolling elements more uniform. We use a carbonitriding process similar to that of the inner ring to achieve a strengthening effect.

5.Conclusion

By applying the pretreatment process of isothermal spheroidizing annealing and ultra-fine treatment, the carbides are refined and homogenized, which greatly improves the band segregation. Afterwards, the inner ring, outer ring and rolling elements are carbonitrided. Strengthened vacuum heat treatment, supplementary tempering to reduce stress during machining, and cryogenic treatment to improve dimensional stability make the engine rolling bearings extremely wear-resistant and durable, improving user safety.