Effects of Heat Treatments on Steels for Bearing Applications

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1.Rolling bearings

The characteristics of rolling bearings: easy use and maintenance, reliable operation, good starting performance, and high load-bearing capacity at medium speeds. Compared with sliding bearings, rolling bearings have larger radial dimensions, poorer vibration damping capabilities, shorter service life at high speeds, and louder noise. The structure of rolling bearing: outer ring, inner ring, rolling elements and cage.

Rolling bearing material: Rings and rolling elements are usually made of rolling bearing steel with high strength and good wear resistance.

Application: Generally under medium speed and light load conditions

2.Rolling bearing structure

(1)Inner ring

(2)Outer ring

(3)Rolling elements

(4) Cage

(1), (2), (3) are all rolling bearing steel

(4) For low carbon steel (ball bearings) and non-ferrous metals (roller bearings)

3.Service conditions of rolling bearings

a. High contact stress (up to 3000~35000MPa) and rolling and sliding. The failure mode is wear and contact fatigue.

The surface has high hardness, wear resistance and contact fatigue strength. Material selection: high carbon steel

b. For bearings with larger sizes, the force they bear is also very large, and there will also be a certain impact.

Failure mode: fracture. Large bearings should have higher toughness and material selection: carburized steel

Rolling bearing steel must have high and uniform wear resistance, high elastic limit and contact fatigue strength, sufficient toughness and hardenability, and at the same time have certain corrosion resistance in the atmosphere and lubricants.

4.Heat treatment of bearing steel

(1) Normalizing and spheroidizing annealing

a.Normalizing

Purpose: If the final forging temperature is relatively high and the cooling rate after forging is relatively slow, network carbide defects will appear. This kind of network carbide is not easy to be eliminated during spheroidizing annealing and needs to be eliminated by normalizing process before spheroidizing annealing.

b.Spheroidizing annealing

Purpose: To facilitate cutting and processing of 207~229HBS

The granular pearlite is prepared for vacuum hardening. When heated, cementite is not easily dissolved, and A+ undissolved cementite with a carbon content of about 0.45~0.6% is obtained. The cementite is evenly distributed in the austenite matrix in the form of fine balls. On the surface, the austenite grains are very fine. After vacuum quenching, hidden needles M and undissolved carbides evenly distributed on M are obtained. The hardness, contact fatigue strength and wear resistance are all very high.

(2)Vacuum hardening and vacuum tempering

The heating temperature of vacuum hardening has a great influence on the structure and properties of high carbon steel. When the heating temperature is higher than Ac1, as the heating temperature increases, the following changes will occur:

As the carbon content of austenite continues to increase, the hardness of martensite continues to increase and the toughness decreases.

The retained austenite content continues to increase, resulting in a slight decrease in hardness after hardening.

The grains continue to grow, and after hardening, the structure changes from hidden needle M to coarse lens-like M, and the strength and toughness decrease.

(3)Cold treatment

For precision bearings, the residual A is required to be small, otherwise the size will change due to the conversion of residual A to M during use.

Immediately after vacuum hardening, cool to -79~-80°C and hold for 1~2 hours. After vacuum tempering, the residual A can be reduced to about 5%. After cold treatment, the hardness can be increased by about 2HRC. Multiple vacuum temperings can also provide dimensional stability.

(4) Aging treatment

For precision bearings, aging treatment must be performed after grinding and kept at a temperature lower than the tempering temperature for 5 to 10 hours to eliminate internal stress and stabilize dimensions.

(5) Isothermal quenching

For high-speed railway bearings and bearings used in mining and metallurgical machinery, they bear large loads and require high toughness. In order to improve toughness, isothermal quenching can be used.

5.Production process route of bearing parts

(a) Steel balls and rollers

Bar (spheroidized annealed state) → cold sensitive → rough grinding → vacuum hardening and low temperature tempering → grinding → grinding (polishing)

(b) Micro and small ferrules

Seamless tube (spheroidized annealed state) → machining → vacuum hardening and low temperature tempering → rough grinding → grinding (polishing)

(c) Small, medium, large

Forging → spheroidizing annealing → machining → vacuum hardening and low temperature tempering → rough grinding and grinding (polishing)

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