NEW BEARING STEEL FOR HIGH-SPEED APPLICATIONS

The requirements for speed suitability and fatigue strength of motor spindle bearings are constantly increasing. These challenges can be met by further developing the spindle bearings, e.g. by using higher-performance bearing steels. In the following, the experimental investigation results of a spindle bearing made of a new raceway steel tested on a high-speed rolling bearing test rig are presented. Spindle bearings of the type 7008 (hybrid execution) were tested in an endurance run at a rotational speed of 46 krpm and 3 kN axial load. The operating behaviour was validated based on the bearing outer ring temperature and the vibration behaviour. Microscope analysis of the raceways after the test shows that the new steel has good resistance to micropitting and surface fatigue. The calculated contact pressures, wear parameter and lifetime for the bearings in the tests show that the performance limits of spindle bearings are significantly higher than initially assumed.

Fracture behaviors at inclusions of very-high-cycle fatigue in newly developed clean bearing steel

Bearing steel is essential for the driving parts of the rotating body, but fisheye crack of inclusions such as TiN, Al2O3 and Cr matrix is formed in the very-high-cycle fatigue (VHCF) regime. Therefore, research on the development of a clean bearing steel that reduces the size of inclusion such as TiN or that does not contain inclusion has been made recently. In this study, the results of long-term attempts by the maker to improve the performance and quality of bearing steel for very long life were quantitatively analyzed from the viewpoint of fracture mechanics. In other words, the phase transformation properties, VHCF characteristics with ultrasonic nanocrystal surface modification (UNSM) treatment and fracture mechanics analysis of this clean bearing are compared quantitatively with those of other common bearing steels from the viewpoint of engineering and industrial applications. The quenching and tempering (QT) specimens of the clean bearing showed fisheye cracks and duplex [Formula: see text]–[Formula: see text] curves in the VHCF regime. The fisheye crack of the clean bearing steel was an internal fatigue crack caused by a small inclusion mainly composed of Al2O3 in the VHCF region. However, fisheye cracking due to TiN and Cr matrix, which was observed in other common bearing steels, did not occur in the clean bearing steel due to the efforts of the developers of new clean bearing.

Corrosion Behavior of Copper Bearing Steels and the Derived In-Situ Coating

Using a period immersion wet/dry cyclic corrosion test, in-situ copper-coated steels prepared by corroding copper-bearing steels were investigated in this study. The steel with a higher copper content (>3%) has a higher initial corrosion rate due to its obvious two-phase microstructure. The corrosion rates of all copper bearing steels tend to be stable after a certain time of corrosion. A copper-rich layer is formed between the matrix and the rust layer, which is due to the diffusion of copper from the rust layer to the metal surface. The copper’s stability under this corrosion condition led to the formation of a thin copper-rich film, which was uncovered after removing the rust by choosing appropriate descaling reagents. The copper coating was generated from the matrix itself during the corrosion process at 25 °C, which provided a new approach for producing in-situ composite materials without any bonding defect. It is found that the corrosion rate, corrosion time, and copper content in steel all affect the formation of copper-rich layer. In addition to the noble copper surface, the electrochemical corrosion test results show that the corrosion resistance of copper-coated steel has been significantly improved.

Study of factors contributing to decrease of carbide heterogeneity in bearing grades of steel

At present to meet requirement to the quality of bearing steels it is necessary of accomplish estimation of carbide heterogeneity by SEP 1520 and provide the level of carbide network, carbide liquation and streaking, not exceeding adjusted limits, required by customer. To estimate possibility to decrease carbide heterogeneity (segregation), factors influencing its value in the rolling production considered. Description of the process of long products of bearing steels production at the rolling mill 370/150 of the OJSC “BMZ - managing company of the holding “BMK” presented. Possibilities of the existing mill equipment considered for 804 accomplishing of technological operations aimed at decrease of carbide heterogeneity in the finished product. The factors studied, contributing to decrease of carbide heterogeneity under conditions of mill 370/150 operation. To determine the temperature modes effect on the level of carbide heterogeneity in bearing steels, in the process of three campaigns phased decrease of the temperature of end of rolling was accomplished. Results of data analysis obtained at the production of long products profiles of 34-50 diameter of bearing steel grades presented. Conclusion on rolled stock production of bearing steel grades with high requirements to carbide heterogeneity value was formed. It was shown that to obtain high-quality long products it is appropriate to accomplish the heating in the furnace before the rolling no less than 600 min at the temperature in the soaking zone 1150-1220°С followed by application of tech­nology of normalizing rolling and control of temperature of the end of rolling 750°С. For profiles with drawing less 25% it is rec­ommended to accomplish additional heat treatment of the finished profile - normalization. Long time heating of workpieces before the rolling at the temperature 1150-1220°С enables to decrease the carbide heterogeneity down to acceptable level (carbide liquation no more than 3 points and carbide streaking no more than 4 points). The value of carbide network is decreasing at decreasing of tem­perature of the end of the rolling and increasing deformation value in the last stands. A stable satisfactory result (CN no more than 5.4) was reached at the temperature of the end of rolling 750°С and drawing more than 25%. It was established that the higher the total drawing, the lower the point of carbide network and liquation in the finished profile, which is caused by crushing of nondissolved carbides to separate fragments.