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Effect of Heat Treatment Process on Mechanical Properties of 20Cr14Co12Mo5 Bearing Steel

With the development of aerospace technology at home and abroad, bearing materials are facing the needs of high speed, high Hertz stress, temperature resistance, corrosion resistance and weight reduction. The requirements for various aspects of bearing materials are getting higher and higher, especially for strength and toughness. Directly lead to the rapid development of ultra-high-strength bearings and gear steels. The new generation of high-strength, ultra-high-strength bearings and gear steels are designed with the principle of secondary hardening. 20Cr14Co12Mo5 bearing steel is designed with low carbon martensite and secondary hardening mechanism, and has high tensile strength, fracture toughness, corrosion resistance and certain heat resistance. How to heat treatment plays a very important role in the mechanical properties of the test steel. The cryogenic treatment proposed by V.LESKOVSEK of Slovenia slightly increases the hardness after tempering, and the hardness is basically unchanged after the second tempering, which makes the test steel in the second tempering. While the toughness increases, the surface hardness can also meet the requirements. Researchers from the Institute of Structural Materials of the Central Iron and Steel Research Institute studied the influence of the heat treatment process of 20Cr14Co12Mo5 high-performance bearing steel on the structure and strength and toughness properties.

The main components of the 20Cr14Co12Mo5 steel used for the test are shown in Table 1. After VIM+VAR treatment, it is cast into a 100kg steel ingot and forged into a φ20mm bar at high temperature. Use HR150A Rockwell hardness tester to test the hardness, each sample is the average of the hardness values of 5 points; the tensile sample size (mm) is the standard sample of φ5×65, and each data is the average of 3 samples Value; the size of the impact sample (mm) is 10×10×55, U-shaped notch; the size of the fracture toughness sample (mm) is 20×40×180, and the fracture toughness test is carried out on the MTS8.8 million capacity testing machine. The 20Cr14Co12Mo5 bearing steel is quenched at 1050℃, 1080℃, 1100℃, and the tempering temperature is 500℃ after cold treatment, and the tempering time is 3h; after quenching and cold treatment at 1100℃, 490℃×2h is adopted. 490℃×3h, 500℃×3h tempering system.

C	Cr	Co	Mo	Ni	S	P
0.10-0.20	13.6-14.4	11.5-12.4	4.5-5.4	1.8-2.3	≤0.002	≤0.008

Tab 1 Chemical composition of test steel (mass fraction %)

The test results are as follows:

1. Double vacuum ultra-clean smelting has preliminarily developed a kind of 20Cr14Co12Mo5 ultra-high strength, toughness, heat and corrosion resistance bearing steel, through controlled phase transformation, Rm is 1840MPa, Rp0.2 is 1410MPa, A is 17%, Z is 62.5%, Aku is 75J•cm-2, KIc is 108MPa•m1/2 and so on.

2. Through the analysis of the characteristics of the strength and toughness changes of bearing steel, it is found that solid solution strengthening, ultra-fine martensite lath, fine and dispersed strengthening phase Laves phase, M2C, M23C6 and other carbides and a small amount of retained austenite are the bearing An important factor in the strengthening and toughening of steel.

3. Through the analysis of the influence of different heat treatment processes on the hardness and impact toughness, it is found that the cold treatment has a significant impact on the hardness and impact toughness of the steel; the first tempering significantly increases the hardness of the material, but has a small impact on the impact toughness.