Abstract: The distribution of carbides before quenching and tempering, the microstructure, mechanical properties, contact fatigue life and corrosion resistance of 40Cr15Mo2VN nitrogen-containing stainless steel and G95Cr18 high-carbon chromium stainless steel after quenching and tempering were tested. The properties of 40Cr15Mo2VN steel were compared and evaluated, which provided data support for the application of this steel in bearings.
At present, the distribution of eutectic carbides in domestic stainless bearing steel materials is uneven, which cannot be eliminated during heat treatment, which adversely affects the grinding and superfinishing processes of bearing rings, and cannot meet the requirements of bearing noise, accuracy and resistance under certain working conditions. Corrosion performance requirements [1-2]. For this purpose, a nitrogen-containing stainless bearing steel 40Cr15Mo2VN was developed. The carbide distribution, mechanical properties, contact fatigue life and corrosion resistance of the steel were tested in the following. comparative analysis.
1.1 Test material
The chemical composition of 40Cr15Mo2VN steel and G95Cr18 steel smelted by electric furnace or induction furnace and remelted by electroslag is shown in Table 1. The chemical composition of 40Cr15Mo2VN steel follows the enterprise standard, and the chemical composition of G95Cr18 follows GB/T3086-2008 High Carbon Chromium Stainless Bearing Steel” standard. Nitrogen was determined by an oxygen-nitrogen analyzer, and the remaining elements were determined by spectrometry.
1.2 Test method
The heat treatment process of 40Cr15Mo2VN steel is 1050℃/30min quenching+－73℃/120min cold treatment+177℃/60min tempering; the heat treatment process of G95Cr18 is 1080℃/35min quenching+－55℃/120min cold treatment+150℃/60min tempering . The microstructure, mechanical properties, contact fatigue life and corrosion resistance of the two materials after quenching and tempering were tested, and the properties of the materials were evaluated by comparison.
2 Test results and analysis
2.1 Carbide distribution
Figure 1 and Figure 2 show the carbide distribution of G95Cr18 and 40Cr15Mo2VN steels respectively. It can be seen that there are large blocks of eutectic carbides in G95Cr18 steel, and the carbide distribution is not uniform, and there is a phenomenon of strip precipitation. These carbides are treated during heat treatment. It cannot be completely eliminated in the process. When the bearing is subjected to a large load, it is easy to cause stress concentration at the eutectic carbide and generate a fatigue crack source, thereby damaging the performance and contact fatigue life of the bearing; while the 40Cr15Mo2VN steel is evenly distributed. It has very small granular carbon and nitrogen compounds, similar to the spheroidized annealing structure of high-carbon chromium bearing steel, but does not have the coarse massive eutectic carbides and needle-like eutectic carbides in high-carbon chromium bearing steels.
2.2 Mechanical properties test
40Cr15Mo2VN and G95Cr18 materials are subjected to tensile test and impact test after the heat treatment process described in Section 1.2. The tensile test is performed according to GB/T228-2002 “Room temperature tensile test method for metal materials” Prepare and test samples; impact test According to GB/T229-2007 “Charpy (Pendulum) Impact Test Method for Metal Materials”, the samples were prepared and tested . The results are shown in Table 2.
It can be seen from Table 2 that the mechanical properties of 40Cr15Mo2VN and G95Cr18 steel are equivalent.
2.3 Contact fatigue life test
The samples were prepared and tested according to JB/T10510-2005 “Contact Fatigue Test Method for Rolling Bearing Materials”, and the number of samples was 20. The contact fatigue life test results are shown in Table 3. The results show that the rated fatigue life of 40Cr15Mo2VN steel is better than that of G95Cr18 steel; the fatigue life of bearing steel obeys Weibull distribution, and its slope b represents the dispersion and stability of contact fatigue life, and the larger the b value is , the smaller the fatigue life dispersion of bearing steel, the higher the reliability. It can be seen that the reliability of 40Cr15Mo2VN steel is better than that of G95Cr18 steel.
2.4 Corrosion resistance test
According to GJB150.11-1986 “Military Equipment Environmental Specimen Method Salt Spray Test” standard, samples were prepared and tested to determine the ability of the material to resist the influence of salt spray atmosphere. The test temperature is 35°C, the salt spray deposition rate is 1~2mL/(80cm2·h, (5±1)% salt solution, the pH value is 6.5~7.2, and the sample temperature is stable for 2h before spraying. Continuous spraying for 96h. After the test is completed, the sample is recovered for 48h before identification. The inspection results of the salt spray test are shown in Table 4.
It can be seen from Table 4 that the salt spray corrosion resistance of 40Cr15Mo2VN steel is far better than that of G95Cr18 steel.
2.5 Quenching and tempering structure
After the 40Cr15Mo2VN steel undergoes a heat treatment process of 1050℃/30min quenching +-73℃/120min cold treatment +177℃/60min tempering, the obtained hardness and retained austenite content can meet the needs of the product. In order to study the characteristics of the structure obtained by this process, the microstructure was analyzed by metallographic microscope and transmission electron microscope respectively. Figure 3 shows the metallographic structure of the steel, and Figures 4 and 5 show the transmission electron microscope structure.
It can be seen from Figure 3 that the structure of the steel after quenching and tempering is tempered martensite + a small amount of primary carbides + dispersed secondary carbides. It can be observed from transmission electron microscope that the martensite structure of the steel is a mixed structure of lath martensite and sheet martensite, in which the substructure of lath martensite is mainly high-density dislocations, and there are local twins. . Figure 5 can clearly observe the middle ridges of flaky martensite, and there are many thin lines in flaky martensite, these thin lines are phase transformation twins, so the substructure of flaky martensite is mainly twins.
- Small granular carbon-nitrogen compounds formed by nitrogen and carbon are evenly distributed in 40Cr15Mo2VN steel, but there are no coarse massive eutectic carbides and needle-like eutectic carbides in high-carbon chromium bearing steel; (2) ) The mechanical properties of 40Cr15Mo2VN and G95Cr18 steel are comparable; (3) the rated fatigue life and reliability of 40Cr15Mo2VN steel are better than those of G95Cr18 steel; (4) The corrosion resistance of 40Cr15Mo2VN steel is far better than that of G95Cr18 steel; (5) 40Cr15Mo2VN steel is quenched The tempered structure is tempered martensite + a small amount of primary carbides + dispersed secondary carbides. The coexistence of lath martensite and flake martensite was observed under transmission electron microscope.
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