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Auto Parts Vacuum Heat Treatment

Bearings can be seen in many parts of the car body, which have special missions to bear loads, reduce friction, and guide moving parts. Vacuum heat treatment is a key process in the manufacturing process of automobile bearings, and its processing quality and raw materials are two important factors affecting the life of bearings. Vacuum heat treatment has an important impact on the microstructure, mechanical properties, surface quality, dimensional and shape accuracy and stability of bearings and other parts under service conditions. Therefore, optimizing the vacuum heat treatment process, selecting suitable vacuum heat treatment parameters, and obtaining The best comprehensive performance adapted to the use condition and failure mode is an important subject of vacuum heat treatment technology.

Application of bearing materials in automobiles

Bearings are composed of four parts: outer ring, inner ring, rolling elements (balls, cylinders, tapered or needle rollers, etc.) Mainly bearing steel.

When selecting the type and model of automotive bearings, it is mainly based on the nature, direction, size, working environment of the actual part of the load, and the requirements for the rigidity, limit speed, life, and accuracy of the bearing, and is generally completed by the designer. Bearings are widely used in vehicles and vary with vehicle types, installation locations and manufacturers.

Bearing materials and performance requirements for auto parts

At present, automobiles are developing towards light weight, comfort, solidity, high power, compact structure, durability, good acceleration performance, and high reliability. In addition, due to the advancement of electronic control technology, automobiles are also developing in the direction of practical performance such as low energy consumption, simplicity and convenience, and strong operability. Therefore, rolling bearings, as important components for automobile support, must adapt to this trend.

During the service of automobile bearings, the unit area of the surface of the rolling elements and the rings has to bear a lot of pressure, which can reach up to 5000N/mm2 after calculation. When the bearings are running, there are rolling and sliding. In addition to being subjected to high frequency, alternating In addition to the contact stress, it is also affected by centrifugal force. The main failure modes of automobile bearings are peeling, pitting, adhesion, strain, fracture, loss of precision, excessive vibration and noise, etc. Therefore, the performance of bearing steel has the following requirements: ① High purity. ② low oxygen content. ③High hardness and wear resistance. ④ Good dimensional stability. ⑤ Adequate compressive strength and deformation resistance. ⑥ Good process performance.

Vacuum heat treatment technology for auto parts bearings

The main role of the main alloying element chromium in the steel for automobile bearings is to improve the hardenability of the steel, so that the parts can obtain a more uniform structure on the entire cross-section after vacuum quenching and vacuum tempering. Chromium can form alloy cementite (Fe, Cr) 3C, refine austenite grains, reduce the overheating sensitivity of steel during heating, improve wear resistance, and enable steel to obtain fine needles or Cryptocrystalline martensite increases the toughness of steel on the basis of high strength, and generally does not need cryogenic treatment for automotive bearings, unless there are special requirements for the dimensional stability of parts and the content of retained austenite.

(1) Production process route of automobile bearing parts

The general production route of steel ball rolling elements is: bar stock→bullet hot pier forming→light ball (filing)→soft grinding→vacuum heat treatment→hard grinding→fine grinding→finishing (polishing).

The general production route of the inner and outer rings is: pipe material (cold rolling) → vacuum annealing → turning → soft grinding → vacuum heat treatment → grinding → finishing.

The general production route of the cage is: solid cage (nylon): blank → turning → pulling, drilling window → surface treatment.

Stamping cage (metal): strip material or sheet material → forming → bottom cutting → punching window → pressing slope → expansion → surface treatment.

The rolling elements, outer rings and inner rings of automobile bearings, and the second and third generation flanges of hub bearings must be subjected to appropriate vacuum heat treatment in order to give full play to the potential of the material itself, obtain the expected performance of the parts and improve the service life of the assembly, mainly There are spheroidizing annealing, overall quenching + low temperature tempering, chemical vacuum heat treatment, induction vacuum heat treatment and other methods.

(2) Overview of vacuum heat treatment of key automotive bearing parts

The following is a brief introduction of vacuum heat treatment equipment, process examples, technical requirements, etc.

Spheroidizing annealing equipment usually uses a protective atmosphere furnace, so that the surface of the parts after vacuum annealing is less and free of oxidation, and can improve the material utilization rate of bearing parts. The heating temperature of the bearing material is 835~850°C, and the spheroidizing temperature is 750~760°C.

After the ball rolling element is forged, use a vacuum annealing furnace. Annealing temperature is 760°C, oxygen potential ≥900, dew point ≤20°C, propane flow rate 0.18±0.02m3/h, furnace pressure 150~300Pa, hardness ≤210HBW, decarburization layer less than or equal to 0.25mm, network carbide ≤CN4.2, the hardness of the steel ball after overall quenching and tempering is required to be above 60HRC.

The inner and outer rings use a vacuum quenching furnace. In order to reduce deformation, use special bearing quenching oil and adjust parameters such as heating temperature, time, and conveyor belt running speed, or use a salt bath for martensite graded quenching. After vacuum quenching, the hardness reaches 63~64.5HRC , after vacuum tempering at 180±10℃, the hardness is 61~63HRC, the taper of the ferrule is about 0.05mm, and the cylindricity is ≤0.15mm.

The second and third generations of automobile wheel bearings have flanges, the material is S55C, and the surface hardness of the vacuum quenching area is required to be above 60HRC. The cooling medium is water-based quenching liquid with adjustable concentration, the pressure of quenching liquid is 0.2~0.6MPa, the hardness after vacuum quenching is 62~65HRC, the depth of hardened layer is 2.2~3.4mm, the roundness is ≤0.1mm, and the temperature is (160±10)℃× (120±5) min overall tempering hardness 60~63HRC, microstructure 3~6.

Summarize

As an important branch of bearings, automobile bearings have the characteristics of being in the automobile industry and assembly operating environment. In addition to improving structural design and good sealing and lubrication, the reliability of automotive bearings and long life can be improved by using high-purity materials and reasonable vacuum heat treatment equipment and processes.