ZWZ15 high-grade carburized bearing steel for wind bearing etc
Abstract: The composition, properties and application of newly developed ZWZ 15 high-grade carburized bearing steel are introduced and compared with 20Cr2Ni4 steel. The results show that ZWZ 15 steel has better properties and can well meet the requirements of manufacturing large bearings.
Key words: rolling bearing; Bearing steel; Mechanical properties; heat treatment
One of the important development directions of equipment manufacturing industry is large-scale. With the increasing large-scale of equipment such as rolling mills, ships, wind turbines, nuclear power equipment, chemical equipment and road header, the supporting bearings also show a large-scale trend. At present, the existing carburized bearing steel in China cannot meet the requirements of manufacturing large bearings. For example, when 20Cr2Ni4 carburized bearing steel is used to manufacture the bearing ring of 6 MW wind turbine main shaft, its surface hardness, effective hardening layer depth and core hardness cannot meet the technical requirements of carburizing heat treatment of bearing parts. Therefore, Wafangdian Bearing Group Co., Ltd., Northeast Special Steel Group and Xining Special Steel jointly developed a ZWZ15 high-grade carburized bearing steel with better performance than 20Cr2Ni4. The composition, properties and application of this steel are introduced and compared with 20Cr2Ni4 steel.
1. 20Cr2Ni4 carburized bearing steel
At present, the commonly used carburized bearing steels in China include 20CrNiMo, 20CrNi2Mo, 10CrNi3Mo and 20Cr2Ni4. 20Cr2Ni4 steel has the best hardenability. At present, it is the most widely used and used in China. It has excellent cold and hot working performance and good mechanical properties. After carburizing and quenching heat treatment, it has excellent hardness, wear resistance and good fatigue life, which can well meet the performance requirements of bearing parts. Suitable for machining rollers with a maximum diameter of 160 mm and bearing rings with an effective thickness of 120 mm.
See Table 1 for the chemical composition standard of 20Cr2Ni4 steel and the actual chemical composition of the sample. See Table 2 for the mechanical property standard of the steel and the mechanical properties of the sample. The heat treatment process of the sample in the table is: (870 ± 20) ℃ heating and oil cooling. Then (790 ± 20) ℃ heating and oil cooling. Tempering at 150 ~ 200 ℃ and air cooling.
The phase transformation temperature of 20Cr2Ni4 steel sample is shown in Table 3, the hardenability data is shown in Table 4, and the end quenching curve is shown in Figure 1.
Fig. 1 End quenching curve of 20cr2ni4 and zwz15 steel samples
If the roller with effective diameter greater than 160 mm and the ferrule with effective thickness more than 120 mm are made of 20Cr2Ni4 steel, its surface hardness, effective hardening layer depth and core hardness are difficult to meet the technical requirements of bearing parts. In order to manufacture these large bearing parts, it is necessary to develop a high-grade carburized bearing steel with better performance than 20Cr2Ni4.
2. Development of ZWZ15 steel
2.1 Composition design
The development process of foreign bearing steel is basically to adjust the composition on the basis of the existing mature steel, so that the new steel can obtain the desired performance, reduce the development cost and difficulty of the new steel, and improve the success rate of the development of the new steel. According to this idea, on the basis of 20Cr2Ni4 steel, the chemical composition is appropriately adjusted. The chemical composition of the designed ZWZ15 steel is shown in Table 5.
Table 5 Chemical composition of ZWZ15 steel
The main differences between ZWZ15 and 20Cr2Ni4 steel are:
(1) ZWZ15 steel is added with Mo, and its mass fraction is 0.25% ~ 0.35%. Mo can improve the hardenability and tempering stability of steel, improve carburizing performance, refine carbide particles, and improve fatigue strength and wear resistance. Adding a small amount of Mo can greatly improve the hardenability of the steel, but it has no effect on the cold and hot workability and mechanical properties of the steel. Performance has little impact.
(2) Properly reducing the content of C and improving the fracture toughness of ZWZ 15 steel can make up for the reduction of fracture toughness of ZWZ 15 steel caused by the addition of Mo to a certain extent and make ZWZ 15 steel fracture. The toughness is equivalent to that of 20Cr2Ni4 steel.
(3) At present, in the production of bearing steel in China, the steel composition control technology has been very mature. Appropriately reducing the composition range of C, Ni and Si elements will not increase the production difficulty and production cost of bearing steel, but also improve the consistency of steel composition, process performance and other parameters, and reduce the quality fluctuation of subsequent hot processing and heat treatment.
2.2 Performance parameters
The chemical composition of ZWZ 15 steel used for the test is shown in Table 5. The heat treatment process of the sample is: (870 ± 20) ℃ heating and oil cooling. Then (790 ± 20) ℃ heating and oil cooling. Tempering at 150 ~ 200 ℃ and air cooling. The mechanical properties of the steel after treatment are shown in Table 6.
Table 6 Mechanical properties of ZWZ15 steel
It can be seen from table 2 and table 6 that when the same heat treatment process is adopted, the mechanical properties of ZWZ 15 steel are equivalent to that of 20Cr2Ni4 steel. The transformation temperature of ZWZ 15 steel sample is shown in Table 7. According to table 3 and table 7, the transformation temperature of ZWZ 15 is similar to that of 20Cr2Ni4 steel. Therefore, the same or similar hot working process can be adopted.
Table 7 Transformation temperature of ZWZ15 steel
The hardenability data of ZWZ 15 steel sample is shown in Table 8, and the end quenching curve is shown in Figure 1. It can be seen from table 4 and table 8 that the hardenability of ZWZ 15 steel is much better than that of 20Cr2Ni4 steel. Therefore, ZWZ 15 steel is suitable for manufacturing larger bearing parts.
Table 8 Hardenability data of ZWZ 15 steel (Austenitizing Temperature 870℃)
3. Smelting and processing technology of ZWZ15 steel
3.1 Smelting and rolling
The smelting and rolling process of ZWZ15 is basically the same as that of 20Cr2Ni4 steel, but pay attention to slow cooling after rolling to prevent cracking.
3.2 Forging and heat treatment
The forging process parameters of ZWZ 15 are basically the same as those of 20Cr2Ni4 steel, but pay attention to slow cooling and timely annealing after forging, and its annealing process is also basically the same as that of 20Cr2Ni4 steel.
The carburizing temperature of ZWZ 15 steel is 920 ~ 980 ℃, and the carburizing time is determined according to the carburizing layer depth. After carburizing, it is oil cooled and tempered at high temperature in time. At the same carburizing temperature, the carburizing speed of ZWZ15 steel is basically the same as that of 20Cr2Ni4 steel. However, due to the larger bearing parts made of ZWZ 15 steel, the required carburizing layer depth is often deeper. In order to shorten the carburizing time and reduce the carburizing cost, a higher carburizing temperature should be adopted as far as possible.
High temperature tempering: 650 ℃ × 12 h tempering + air cooling to 60 ℃. Below + 650 ℃ × 12 h tempering.
Secondary quenching: (790 ± 20) ℃, oil cooling; Tempering at 150 ~ 200 ℃ and air cooling.
3.3 Applicable part size range
ZWZ15 advanced carburized bearing steel is suitable for processing rollers with a diameter of 150 ~ 280 mm and ferrules with an effective wall thickness of 120 ~ 230 mm. After the above carburization and quenching, the surface hardness, effective hardened layer depth and core hardness can meet the requirements of JB / T 8881-2001 technical conditions for carburizing heat treatment of rolling bearing parts.
More about WBM Taper roller:
The inner and outer ring raceways are segments of cones and the rollers are tapered so that the conical surfaces of the raceways, and the roller axes, if projected, would all meet at a common point on the main axis of the bearing. This geometry makes the motion of the cones remain coaxial, with no sliding motion between the raceways and the outside diameter of the rollers.
This conical geometry creates a linear contact patch which permits greater loads to be carried than with spherical (ball) bearings, which have point contact. The geometry means that the tangential speeds of the surfaces of each of the rollers are the same as their raceways along the whole length of the contact patch and no differential scrubbing occurs.
The rollers are stabilized and restrained by a flange on the inner ring, against which their large end slides, which stops the rollers from popping out due to the "pumpkin seed effect" of their conical shape.
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