New Technology And Direction Of Bearing Steel Ball

- Sep 29, 2020-

Bearing steel balls are mainly used to manufacture rolling elements and rings of rolling bearings. Since the bearing should have the characteristics of long life, high precision, low heat generation, high speed, high rigidity, low noise, high wear resistance, etc., the bearing steel is required to have: high hardness, uniform hardness, high elastic limit, high contact fatigue Strength, necessary toughness, certain hardenability, corrosion resistance in atmospheric lubricants. In order to meet the above performance requirements, strict requirements are imposed on the uniformity of the chemical composition of the bearing steel, the content and type of non-metallic inclusions, the size and distribution of carbides, and decarburization. Bearing steel generally develops towards high quality, high performance and multiple varieties. Bearing steels are classified according to their characteristics and application environment: high-carbon chromium bearing steel, carburized bearing steel, high-temperature bearing steel, stainless bearing steel and special special bearing materials.


In order to meet the requirements of high temperature, high speed, high load, corrosion resistance and radiation resistance, it is necessary to develop a series of new bearing steels with special properties. In order to reduce the oxygen content of bearing steel, smelting technologies for bearing steel such as vacuum smelting, electroslag remelting and electron beam remelting have been developed. The smelting of large quantities of bearing steel has been smelted by electric arc furnaces and developed into various types of primary furnaces plus external refining. At present, the bearing steel is produced by the primary smelting furnace with a capacity of more than 60 tons + LF/VD or RH + continuous casting + continuous rolling to achieve high quality, high efficiency, and low energy consumption. In terms of heat treatment process, it has developed from car hearth furnace and bell furnace to continuous controllable atmosphere annealing furnace heat treatment. At present, the longest type of continuous heat treatment furnace is 150m. The spheroidization structure of bearing steel is stable and uniform, the decarburized layer is small, and the energy consumption is low.


Since the 1970s, with economic development and industrial technology progress, the application range of bearings has expanded; and the development of international trade has promoted the internationalization of bearing steel standards and the development and application of new technologies, new processes and new equipment. High, high-quality, low-cost supporting technology and process equipment came into being. Japan and Germany have built high-cleanliness, high-quality bearing steel production lines, which has rapidly increased the output of steel and greatly improved the quality and fatigue life of steel. The oxygen content of bearing steel produced in Japan and Sweden has dropped below 10 ppm. In the late 1980s, the advanced level of Japan's Sanyo Special Steel Company was 5.4ppm, reaching the level of vacuum remelted bearing steel.


The contact fatigue life of the bearing is very sensitive to the uniformity of the steel structure. Improve cleanliness (reduce the content of impurity elements and inclusions in steel), promote the fine and uniform distribution of non-metallic inclusions and carbides in steel, and increase the contact fatigue life of bearing steel. The structure of the bearing steel in use should be the tempered martensite matrix with fine carbide particles evenly distributed. This structure can give the bearing steel the required performance. The main alloying elements in high carbon bearing steel are carbon, chromium, silicon, manganese, vanadium and so on.


How to obtain the spheroidization structure is an important issue in the production of bearing steel, and controlled rolling and cooling is an important production process for advanced bearing steel. Through controlled rolling or rapid cooling after rolling, the network carbide is eliminated, and a suitable preliminary structure is obtained, which can shorten the spheroidizing annealing time of the bearing steel, refine the carbide, and improve the fatigue life. In recent years, Russia and Japan have adopted low-temperature controlled rolling (800℃~850℃). After rolling, air cooling and short-time annealing are adopted, or the spheroidizing annealing process is completely eliminated, and a qualified bearing steel structure can be obtained. 650℃ warm processing of bearing steel is also a new technology. If the eutectoid steel or high carbon steel has a fine grain structure before hot working or can form fine grains during processing, it will exhibit superplasticity at a certain strain rate within the melting temperature range of (0.4 to 0.6). The US Naval Research Institute (NSP) conducted a 650°C warm processing test on 52100 steel and showed that no fracture occurred at a true strain of 2.5 at 650°C. Therefore, it is possible to replace high temperature processing with 650°C temperature processing and combine it with the spheroidizing annealing process, which is of great significance for simplifying equipment and procedures, saving energy, and improving quality.


In terms of heat treatment, progress has been made in improving the quality of spheroidizing annealing, obtaining fine, uniform, and spherical carbides, and shortening the annealing time or eliminating the spheroidizing annealing process. The subsequent 720℃~730℃ recrystallization annealing was changed to 760℃ structure annealing. In this way, a structure with low hardness, good spheroidization, and no network carbide can be obtained, and the key is to ensure that the intermediate drawing reduction rate is ≥14%. This process increases the efficiency of the heat treatment furnace by 25% to 30%. Continuous spheroidizing annealing heat treatment technology is the development direction of bearing steel heat treatment.


Countries are researching and developing new bearing steels, expanding applications and replacing traditional bearing steels. For example, fast carburizing bearing steel, by changing the chemical composition to increase the carburizing speed, in which the carbon content is increased from the traditional 0.08% to 0.20% to about 0.45%, and the carburizing time is shortened from 7 hours to 30 minutes. Developed high-frequency hardening bearing steel, using ordinary medium-carbon steel or medium-carbon manganese, chromium steel to replace ordinary bearing steel through high-frequency heating and quenching, which not only simplifies the production process, reduces the cost, and increases the service life. The fatigue life of GCr465 and SCM465 developed in Japan is 2 to 4 times higher than that of SUJ-2. Due to the increasing use of bearings in environments with high temperature, corrosion, and harsh lubrication conditions, bearing steels such as M50 (CrMo4V) and 440C (9Cr18Mo) used in the past can no longer meet the requirements of use. There is an urgent need to develop good processing performance, low cost, and fatigue. Bearing steels with long life and suitable for different purposes and uses, such as high temperature carburizing steel M50NiL, easy-to-process stainless bearing steel 50X18M and ceramic bearing materials.


Aiming at the weak point of low hardenability of GCr15SiMn steel, my country has developed high hardenability and hardenability bearing steel GCr15SiMo, its hardenability HRC≥60, hardenability J60≥25mm. The contact fatigue life L10 and L50 of GCr15SiMo are respectively 73% and 68% higher than that of GCr15SiMn. Under the same operating conditions, the service life of bearings made of G015SiMo steel is twice that of GCr15SiMo steel. In recent years, my country has also developed GCr4 bearing steel that can save energy, resources and impact resistance. Compared with GCr15, the impact value of GCr4 is increased by 66% to 104%, the fracture toughness is increased by 67%, and the contact fatigue life L10 is increased by 12%. GCr4 steel bearing adopts high temperature heating-surface quenching heat treatment process. Compared with fully hardened GCr15 steel bearings, GCr4 steel bearings have significantly longer life span and can be used for heavy-duty high-speed train bearings.


In the future, bearing steel will mainly develop in two directions: high cleanliness and diversified performance. Improving the cleanliness of bearing steel, especially reducing the oxygen content in the steel, can significantly extend the life of the bearing. The oxygen content is reduced from 28ppm to 5ppm, and the fatigue life can be extended by an order of magnitude. In order to extend the life of bearing steel, people have been working on developing and applying refining technology for many years to reduce the oxygen content in steel. Through unremitting efforts, the minimum oxygen content in bearing steel has been reduced from 28 ppm in the 1960s to 5 ppm in the 1990s. At present, my country can control the minimum oxygen content in bearing steel at about 10 ppm. Changes in the bearing environment require bearing steel to have diversified performance. For example, the increase of equipment speed requires bearing steel for quasi-high temperature (below 200℃) (usually the method of increasing Si content and adding V and Nb on the basis of SUJ2 steel to achieve the purpose of softening resistance and stable size); In order to simplify the process, high-frequency hardening bearing steel and short-time carburizing bearing steel should be developed; in order to meet the needs of aerospace, high-temperature bearing steel should be developed.