Basic Technology Series of Bearings (1) Life Improvement with Newly Developed Materials and Heat-treatment

Transcription

1 Basic Technology Series of Bearings () ife Improvement with Newly Developed Materials and Heat-treatment T. HOSHINO * *Bearing Research & Development Department, Research & Development Center Outlines of Koyo new technologies on materials and heat-treatment to improve bearing life are introduced. Particularly, how to choose the material and heat-treatment method most suitable for longer bearing life to prevent flaking under such condition as in clean or contaminated, and at high temperature, is shown. Bearings Koyo developed for various applications with these new technologies showed excellent performance. Key Words: high cleanliness, contaminant, strength, heat treatment, medium-heat-resistant. Introduction Because high contact stress of several GPa is applied to parts of the raceway and rolling elements of bearings, these parts must be strong enough to stand high stress, must be able to resist rolling fatigue, must be tough enough to resist impact loads, and must be able to maintain their precision with minimal secular distortion. In order to meet these requirements, it is necessary to secure the needed qualities such as hardness by selecting the proper materials and providing them with the most suitable heat treatment. In order to accomplish this, quality of general purpose bearing steel has been improved, heat treatment methods and conditions have been improved, and carburized steel is applied. In order to dramatically extend bearing life, it is indispensable to optimize and raise the level of materials and heat treatment technologies. This paper provides a brief description of recent technological trends and technological developments concerning materials and heat treatment combined to extend the life of bearings. 2. Extended Bearing ife and Characteristics Required of Materials and Heat Treatment Bearings exhibit various types of failure according to conditions and the environment in which they are used. Typical types of failure produced by bearings, the process by which such failure progresses and characteristics required of materials and heat treatment are given in Table. Figure illustrates philosophy concerning and measures for dealing with the types of failure regarded to be most Type of failure Process by which damage progresses Required characteristics Flaking in clean Flaking in contaminated Wear Secular distortion Acoustic life Flaking at high operating temperatures Table Forms of bearing damage and characteristics required of materials and heat treatment Origin of crack Material fatigue Non-metallic inclusion Non-homogeneous structure Carbide Matrix fatigue crack initiation crack propagation flaking Plastic deformation caused by contaminants stress concentration surface crack initiation crack propagation flaking Metal-on-metal contact due to poor lubrication Wear (adhesive wear, abrasive wear) Use with contaminated Dimensional expansion of bearing due to transformation of retained austenite Acoustic deterioration due to effect of surface characteristics (nonmetallic inclusion, roughness, etc.) Material fatigue and life reduction due to hardness reduction at high operating temperatures Improvement of material cleanliness Homogenizing of structure Minimizing of carbide Higher yield point Higher hardness Higher hardness Proper amount of retained austenite Higher hardness Higher carbide content Proper amount of alloy elements Reduction of retained austenite Higher cleanliness Higher carbide ratio Proper amount of alloy elements Enhanced tempering resistance KOYO Engineering Journal English Edition No.59E (200) 9

2 Basic Technology Series of Bearings () ife Improvement with Newly Developed Materials and Heat-treatment important among those listed in Table, namely flaking that occurs in clean, flaking that occurs in contaminated with foreign material such as wear particle, and flaking that occurs when the bearings are used in a hightemperature environment. Clean Extended bearing life High-temperature environment Contaminated life, 6 cycles Oxygen content in steel, ppm Subsurface-origin-flaking Surface-origin-flaking 0 Reduction of non-metallic inclusion Matrix strengthening Resistance to temper softening Measures by materials arge compressive residual stress Finer carbide Proper retained austenite Measures by heat treatment Higher hardness Fig. ife improvement and countermeasures by material and heat treatment 3. Technology for ife Improvement in Clean ubricant 3. Higher Cleanliness of Materials When is clean, it is hypothesized that a significant amount of subsurface-orgin-flaking initiates and propagates from cracks produced by concentration of stress on material defects such as non-metallic inclusion at the subsurface where the maximum shear stress. In this case oxide-type (Al 2 O 3, SiO 2, etc.) and Ti-type (TiN) non-metallic inclusions are known to be harmful ones that shortens bearing life. In order to extend bearing life, therefore, it is effective to reduce the amount of non-metallic inclusion, among which it is most effective to reduce the Ti and oxygen content in the steel. In the case of high-carbon chrome bearing steel, the effect of oxygen content of the steel and non-metallic inclusion on bearing life is as shown in Fig. 2. The figure shows life is extended as oxygen content and non-metallic inclusions are reduced. Thus cleanliness of materials has been improved and incidental rolling fatigue life has been extended by reducing the amount of impurities such as non-metallic inclusion and oxygen through ladle refining, introduction of vacuum degassing and improvement of steel manufacturing conditions. Figure 3 shows transition of cleanliness and rolling fatigue life of. Improvements such as strict selection of raw materials, optimization of the manufacturing process and control of the manufacturing conditions, high refining steel (HRS) with remarkably reduced oxygen content and amount of nonmetallic inclusion has been developed ). As shown in Fig. 4, the life of this HRS bearing is three times that of standard bearings, and has been confirmed as being equal to special remelting steel such as VAR and ESR. The statistics of extremes method 2) has recently been considered as a new method for evaluating non-metallic inclusion, and there have been reports of a favorable correlation between rolling fatigue life and estimated Cleanliness (JIS), % Oxygen content, ppm life, 6 cycles life, 6 cycles Cleanliness (JIS), % Fig. 2 Effect of oxygen content in steel and cleanliness on bearing life Vacuum Drive gas RH (Vacuum degassing) Air melting steel Alloy F (adle refining) Alloy Vacuum degassing steel CC (Continuos casting machine) ow oxygen steel Tundish Mold Electromagnetic stirring '60 '70 '80 ' 2000 (year) Rolling fatigue life '60 '70 '80 ' 2000 (year) Oxygen content in steel '60 '70 '80 ' 2000 (year) Cleanliness Fig. 3 Transition of fatigue life and cleanliness of bearing steel maximum size of inclusion by the statistics of extremes method 3). Adopting evaluation by this method, the manufacturing process and conditions have been improved so amount and size of non-metallic inclusion can be reduced. 92 KOYO Engineering Journal English Edition No.59E (200)

3 Basic Technology Series of Bearings () ife Improvement with Newly Developed Materials and Heat-treatment 40 ESR HRS Crushing load, kn φ 2 φ 20 5 VAR Calculated life ratio Fig. 4 Fatigue life of high refining steel a) Crushing strength GT bearing steel 3. 2 Higher Strengthened Matrix One type of subsurface-origin-flaking that occurs in clean is flaking caused by matrix fatigue in the area where maximum shear stress. In this case, microstructural change is produced in the area where matrix fatigue occurs. The flaking is produced by crack initiation and propagation from microstructural change. The reduction of amount and size of non-metallic inclusion mentioned in the preceding section therefore does not contribute to extending bearing life. A material that enhances matrix strength through optimization of alloy elements was therefore developed. The material developed is greater toughness bearing steel GT steel 4). GT steel is a compound whereby Si and Ni are added to an base. Having succeeded in extending fatigue strength and delaying microstructural change, the material is now ready for practical application. Strength and life of GT steel are given in Figs. 5 and 6. The life of the material is at least six times that of standard. 4. Technology for Extending ife in Contaminated ubricant 4. Mechanism of and Countermeasures for ife Shortened by Contaminants When hard contaminant such as powder from wear get on the rolling surface of the bearing, the actual life may drop approximately from /5 to / of calculated life. As a result of detailed analysis of the rolling surface of bearings recovered from the market to study the mechanism by which flaking is produced, it was learned that flaking can be classified as peeling on the rolling surface caused by abrasive wear, somewhat deeper flaking caused by dents, and flaking caused by a combination of the two. This is illustrated by Fig. 7. How to extend life in this field is an important theme for bearing manufacturers, and several instances have been reported 5), 6). The contents of these reports suggest that carbonitriding has been applied as a commonly shared technology, and it appears that in some cases they were aiming to reduce the effect of contaminants by precipitation of carbide No. of stress, cycles Cyclic stress, MPa b) Rotating bending fatigue strength GT bearing steel 7 8 Fig. 5 Comparison of strength between GT steel and Calculated life ife time, h (or carbonitriding) or be making large quantities of retained austenite. As a result of having conducted basic testing and verification regarding the mechanism by which flaking is caused by contaminants and measures for extending life, it was discovered that enhancing wear resistance of the rolling surface by increasing surface hardness and making it more difficult for contaminants to make dents in the rolling surface are most effective. It was next discovered that keeping retained austenite within a certain range and keeping work 5.2 GT bearing steel Fig. 6 Results of life test for GT steel and in clean s 30 2 KOYO Engineering Journal English Edition No.59E (200) 93

4 Basic Technology Series of Bearings () ife Improvement with Newly Developed Materials and Heat-treatment Appearance Surface layer peeling Rolling element Combination flaking Flaking caused by dents life / calculated life Contaminant size 0~ µ 44~ 63 µ 20 µ or less SH bearing Mechanism Wear caused by small, hard debris Wear caused by small, hard debris and plastic deformation caused by large, hard debris Plastic deformation caused by external force or large, hard debris Fig. 7 Mechanism by which flaking occurs in contaminated hardening of the raised edges of dents formed by contaminants to a minimum are effective for reducing the scatter of bearing life. The contaminant-resistant bearing described in the following section was developed based on the results of this basic research Development of Contaminant-resistant Bearing ) SH bearing Bearing steel such as is usually used after first being heated to and held at the austenitizing temperature, then through hardened by total immersion in cooling oil. It is however extremely difficult to produce a tough bearing able to rest contaminants by this heat treatment method. Thus the SH bearing 7) was developed. The surface of the bearing is hardened by special heat treatment allowing the proper amount of retained austenite. Figure 8 shows the life of the SH bearing in contaminated. As shown by the graphs, it was confirmed that the SH bearing has a longer life than standard bearings. It was also confirmed that the higher the concentration of contaminants, the larger the effect is. 2) Koyo Extra-ife bearing Carburization technology was developed for extending bearing life by hardening the surface of casehardened steel while allowing the proper amount of retained austenite. This technology that realizes such conflict characteristics is what is called "KE-heat treatment". As shown in Fig. 9, the Koyo Extra-ife bearing 8) offers life of at least times that of standard bearings in contaminated. The company is currently working on casehardening technology along the same concept able to extend life even further than currently possible. Details will be provided at another opportunity. life / calculated life Contaminant concentration, % a) Example of ball bearing Contaminant size 0~ µ 44~ 63 µ SH bearing Contaminant concentration, % b) Example of tapered roller bearing Fig. 8 Effect of contaminants on bearing life 5. Technology for Extending ife under High Operating Temperatures Hardness of bearings decreases at high operating temperatures. Bearing life also decreases with hardness reduction, as indicated in Fig.. Similarly the internal clearance can become reduced or creep may occur between the rolling shaft and inner race due to distortion of dimensions or deterioration of tolerance. Heat-resistant high-alloy steel such as M or SKH4 is therefore generally used in temperatures above 2 C, because these steels are high resistance to temper softening and hard at high temperatures. These heat-resistant high-alloy steels are however extremely expensive. In inexpensive material that can be used in semi-high temperatures up to 200 C has therefore been developed. If decrease in bearing life due to hardness reduction is not so much a problem, provided with heat stabilization treatment is used to prevent locking due to creep and clearance reduction due to dimensional expansion of bearing. KUJ7 9) developed for semi-high temperatures is the product of adding Si and Mo to an base and has high resistance to temper softening. This steel has also high resistance to microstructural change. Figure shows resistance to temper softening for KUJ7. Figure 2 shows results of the life test at C and 80 C. 94 KOYO Engineering Journal English Edition No.59E (200)

5 Basic Technology Series of Bearings () ife Improvement with Newly Developed Materials and Heat-treatment 6. Conclusion Hardness Average particle diameter 830HV 27 µ 700HV 25 µ Amount 0.55g/R 0.55g/R 0 ife, h KE bearing 000 ) Test results in Fa @ Fr 20.6 kn Rotating speed min ife, h KE bearing ) Test results in clean Fig. 9 Fatigue life of Koyo Extra ife bearing This paper provides a brief description of the long life bearing developed and trends in technological development of materials and heat treatment for extending the life of bearings. Table 2 gives characteristics and applications for Koyo long life bearings. Please refer to the table to select the bearing that best meets your needs. Extending bearing life is a never-ending theme for bearing manufacturers, but at the same time it is also important to develop bearings that are more compact and economical to meet the needs of industry. Thus technological development that takes into account the mainstays of longer life, higher reliability and lower cost is indispensable for the future. Use of relatively inexpensive existing material must be considered, various types of heat treatment and surface modification technology must be developed and combined to bring out new performance and functions, not to mention development of new materials. ife ratio Hardness, HRC As quenched KUJ Tempering temperature, C Fig. Relationship between tempering temperature and hardness Calculated value for roller bearing KUJ7S S Stress cycles, Hardness, HV a) C test Stress cycles, 5 b) 80 C test Test values for ball values for roller bearing (% life ratio) Hardness coefficient Calculated value for ball bearing Fig. Relationship between hardness and bearing life KUJ7S S Fig. 2 Results of rolling fatigue life test KOYO Engineering Journal English Edition No.59E (200) 95

6 Basic Technology Series of Bearings () ife Improvement with Newly Developed Materials and Heat-treatment Table 2 Characteristics and application of Koyo ong life bearings ong life bearing High refining steel bearing (HRS) Greater toughness steel bearing (GTS) SH bearing KE bearing Medium-heat-resistant steel bearing (KUJ7) References Characteristics and effect Reduction of non-metallic inclusion Equal to special remelting steel Matrix strength enhanced by optimizing of alloy elements High surface hardness Enhances wear resistance High surface hardness Optimized retained austenite content Enhances temper softening resistance ife ratio (compared with standard bearing) In clean In contaminated Application examples Min. 3 times Equal Automobile (general) Min. 6 times Min. 2 times For heavy loads Min. 2 times Min. 2 times Min. 3 times Min. times For use in places where s tend to be contaminated, such as in differential gearboxes and transmissions. Min. 7 times Min. 2 times Continuous casting facility ( C) Turbochargers ) G. Morihara, Y. Fujita, Y. Fujimoto: KOYO Engineering Journal, No. 28 (985) 20. 2) Y. Murakami: Metal Fatigue "Effects of Small Defects and Non-metallic Inclusions" (993) Yokendo. 3) T. Seki, J. Eguchi, A. Muroga, T. Nishikawa, T. Kimura: CAMP-ISIJ Vol. 7 (994) 7. 4) M. Shibata, H. ee: KOYO Engineering Journal, No. 4 (992) 26. 5) Y. Murakami, N. Mitamura, K. Furumura: NSK Technical Journal No. 652 (992) 9. 6) K. Maeda, H. Nakashima, H. Kashimura: NTN TECHNICA REVIEW No. 65 (996) 7. 7) T. Hoshino, M. Goto: SAE Technical Paper Series 8977 (989) 69. 8) K. Toda, T. Mikami: KOYO Engineering Journal, No. 43 (993) 5. 9) A. Ohta, T. M. Johns: KOYO Engineering Journal, No. 5E (998) KOYO Engineering Journal English Edition No.59E (200)