Fracture Analysis and Material Improvement of Brake Discs

Transcription

1 458 Fracture Analysis and Material Improvement of Brake Discs Haruo SAKAMOTO and Kenji HIRAKAWA Using a steel material for forging, development of new discs for the next Shinkansen had been started. However, a fracture in a forged disc occurred in a durability test. Subsequently, from the viewpoint of higher fracture toughness, material improvement to an extent sufficient to withstand such a fracture due to tensile residual stress accumulation became the goal of further development. Among the candidate materials, modified AISI 4330 was selected and evaluated in laboratory and field tests. Following the subsequent durability test in the field, modification for bolt sticking and forging formability was conducted. This new brake disc, which proved to be satisfactory for Shinkansen vehicles, is capable of running at high speeds of more than 270 km/h and therefore suitable for current vehicles such as the Nozomi and Tsubasa. Key Words: Brake Disc, Brittle Fracture, Fracture Toughness, FEM, SEM, Stress Intensity Factor 1. Introduction Brake discs are normally made of cast iron. The reason this material is used is generally thought to be because of its low cost, ease of manufacturing, strength, and resistance to thermal loading (1) (4). The Shinkansen has also used cast iron brake discs with special chemical contents of Ni, Cr, and Mo. These chemical contents were determined before the inauguration of the first Shinkansen from the results of numerous studies. However, due to a trend to develop faster Shinkansen vehicles, brake discs with higher strength and stronger resistance to thermal loading are in demand. Among all required functions for railroad products such as brake discs, light weight is of great importance. When the service maximum speed is raised from the initial brake speed of 210 km/h (58.3 m/s) to 300 km/h (83.3 m/s), the absorbed energy per set of brake discs is doubled. Therefore, the disc weight must be doubled if the material and service life is to be maintained to the same degree as previous disc brakes. An increase in disc weight can add more than kg per vehicle. To improve the resistance of cast iron to thermal Received 1st November, 2004 (No ) Kochi University of Technology, Department of Intelligent Mechanical Systems Engineering, Tosayamada-cho, Kochi , Japan. sakamoto.haruo@kochitech.ac.jp Kyushu Polytechnic College, Kokuraminamiku, Kitakyushu-city , Japan cracking, a variety of materials have been investigated, including carbon morphologies such as flaky or spheroid; chemical compositions with Ni, Cr, Mo; and base microstructures. Some steel materials have also been added. Among cast iron materials, the existing cast iron showed the best resistance based on the results of basic studies using thermal shock tests with small cylindrical specimens. However, the existing cast iron material is not sufficient for higher speed trains. Further, the steel material appeared to show better resistance than all of the cast iron materials. Therefore, the decision was made to give further consideration to steel as a brake disc material. A forged steel disc made of conventional plain carbon steel was produced, and a dynamometer test to evaluate the effect of the material with regard to its strength and frictional behavior was conducted. During the durability test, a fracture occurred. After an investigation of what caused the fracture, the material selection was then made based on the fracture toughness value of the material candidates studied. Using the selected material, an improved forged steel disc was manufactured and examined by laboratory and field tests. In this case, the fin on the backside of the sliding surface was considered important and therefore manufactured by forging. The original design of the fractured disc, however, is of the hat-type without any fins. In this report, the fracture analysis of the fractured forged steel disc, the material selection from the steel material candidates, and the product development of forged steel brake discs with fins are reported.

2 459 Table 1 Chemical compositions and mechanical properties of steel disc (a) Thermal checks (b) Fig. 2 Thermal cracks Fracture of steel disc Fig. 1 Brake disc and brake unit 2. Fracture Analysis 2. 1 Brake disc and fracture The forged steel disc was produced with the chemical content shown in Table 1, and the heat treatment was performed by equalizing and tempering after forging. The steel material, AISI 1045, was selected based on considerations of wear. Figure 1 shows the disc configuration and the disc brake unit, which is attached to the side of the wheel. During a durability test, the disc fractured. The origin of the fracture was two thermal cracks, shown in Fig. 2. On the sliding surfaces, thermal checks (tortoiselike cracks) were observed as is shown in Fig. 2. The aspect ratios of the two semi-elliptical thermal cracks were 0.38 and The SEM observation at crack A clarified three stages for the fracture process, which are shown in Fig. 3. As can be seen, the first stage was the transgranular fracture within a depth of mm from the surface. As is noted in Fig. 4 (a) in the heated surface, tempered martensite, which was changed from the original microstructure in Fig. 4 (b), was formed. This implies that the top surface layer suffered from a temperature rise above AC 1. In the second stage, the striation pattern in Fig. 3 is obvious and considered to have been caused by cyclic thermal loading. The final stage was a brittle fracture from the tip of the semi-elliptical thermal cracks Residual stress analysis The cause of the fracture was thought to be tensile residual stress due to thermal loading. The residual stress was analyzed by both FEM computation and an experimental method using strain gages. The model for FEM (c) Cleavage fracture 30 µ (b) Striation 30 µ Fig. 3 (a) Transgranular fracture Fracture surface observation by SEM is an axi-symmetric one, and the conditions for calculation are as follows. The brake initial speed is 58.3 m/s, the wheel load is N, the brake duration is 70 sec., and the atmospheric temperature is 20 C. Further, the material properties of incremental temperature and incremental strain calculation, which are dependent on temperature, were also considered into the calculation. In addition to the FEM calculation, the experimental analysis was conducted using strain gages. It was assumed that the steel disc had the same stress state as when the fracture occurred. The obtained residual stress in the circumferential direction was compared with the result by calculation, as shown in Fig. 5. The distributions in both the calculated and experimentally obtained results do not

3 460 Table 2 Stress intensity factor when failed Fig. 4 Microstructure of fractured disc Fig. 6 Stress applied during a brake heat cycle Fig. 5 Circumferential residual stress distribution precisely coincide. However, the maximum values obtained for both cases were found to be close Stress intensity factor The residual stress obtained above enables the calculation of the stress intensity factor applied at the crack. The calculation uses the axial stress gradient obtained by the FEM calculation. The stress was divided into the two components of tensile and bending, and was inserted in the following equation, K I = (M e σ e + M b σ b ) πa/q (1) where σ e and σ b are the tensile and bending stress components. M e and M b are the magnification factors (5), (6) for each stress. Q (7) is Irwin s shape factor, and a is the crack depth. The calculated stress intensity factor is shown in Table 2. On the other hand, the fracture toughness test was conducted with the same material using 25 mm (1 in.) thickness CT (Compact Tension) specimens. Two specimens were tested for each temperature of 0 C and 30 C, and the average values for each temperature were 75.4 and 67.0 MPam 1/2. These results are close to the stress intensity factors for the cracks shown in Table Thermal crack propagation As is shown in the photograph in Fig. 3, the striation pattern originated in the thermal crack surface. This was caused by cyclic thermal loading. The cyclic thermal stress is repeated from compressive stress at high temperature to tensile residual stress at room temperature, as is shown in Fig. 6. Using the striation spacing obtained from the frac- Fig. 7 Table 3 Relationship between crack depth and stress intensity factor Fracture process and required material characteristics tured surface, K was calculated and compared to K Imax obtained from the residual stress distribution. The relationship between the crack depth and the stress intensity factor is shown in Fig. 7. As is indicated in the figure K, and K Imax distributions were similar in the initial stage of crack propagation. However, as the crack goes deeper, the difference in the two factors becomes larger. This is considered to be due to the residual stress relief that could result from progressive crack growth. The residual stress relief was not considered in the FEM calculation Information obtained by fracture analysis From the results clarified by the fracture analysis of the failed brake disc, the process and characteristics are summarized in Table Fracture Toughness for Disc Material The material of the failed disc was selected from the viewpoint of its wear characteristics. As has been made

4 461 clear, the fracture toughness viewpoint is of great importance. Thus, to improve the fracture toughness of steel disc material, various kinds of steels were examined. The material selection was conducted based on the high fracture toughness and high wear resistance against braking. For high fracture toughness, contamination of vanadium, solution aluminum, molybdenum, and nickel were considered, and for wear resistance, carbon and chromium were considered. From the comparison, the effect of the heat treatment was also examined. The selection was also conducted based on a consideration of thermal crack resistance. Regarding thermal crack resistance, a type of screening using a thermal shock tester, which utilizes heating by an induction heater and cooling by a water spray, was adopted. After such a screening, the material candidates were selected, and they are listed in Table 4. The experiment on fracture toughness followed ASTM E399, and the thickness of the CT specimen was 40 mm, which is the same as that of the failed disc. The result of the fracture toughness testing is shown in Fig. 8. From these results, the material AISI 4330 was modified and became a candidate for improving the toughness. The thermal crack resis- Table 4 Materials tested by fracture toughness testing tance was also confirmed by the thermal shock tester. The resistance of the modified AISI 4330 was the best among the other low alloy steels tested. 4. Production of Forged Brake Disc 4. 1 Disc configuration It is easy to fit brake discs to a conventional railroad truck if the configuration is the same as wheel mountedtype brake discs with fins. However, thin fins are considered to be harder to produce by forging than by casting. Fin configuration is thought to be determined from heat convection characteristics for cooling efficiency after brake heating. Therefore, instead of having a rotor without fins such as the one in Fig. 1, a disc with fins is preferable. However, since it is hard to obtain thin fins by forging, a modification of the fin configuration is necessary. In such a case, the cooling effect also needs to be considered Cooling efficiency by fins If the cooling efficiency is not sufficient for cooling after brake heating during a cool down time between stations, the temperature in a disc due to braking accumulates. The saturated temperature can be calculated, supposing that the brake conditions and brake intervals in a train schedule are constant. Letting E, C, M, α, andt be the energy absorbed by one braking, specific heat, mass of a disc, coefficient of cooling efficiency, and brake interval, the saturated temperature, T max, is derived (8) as T max = E/CM 1 e αt +T 0 (2) where T max is the accumulated maximum temperature, and T 0 is the room temperature. α is expressed as α = ha CM where h is the heat convection, and A is the area for heat convection. Figure 9 shows the calculated and experimental results for the relationship between α and T max. The experiment was conducted using three different discs used Fig. 8 Results of fracture toughness testing Fig. 9 Cooling efficiency and saturated temperature

5 462 for a subway. In the case of the Shinkansen vehicle Kodama, which stops at every station, the figure shows that the temperature can be higher if the cooling efficiency is below The cooling efficiency of the existing cast iron disc is around and is well above the value Production of forged brake discs From the study described above, the following results can be summarized. ( 1 ) Fracture toughness is one of the most important factors for a steel brake disc because of possible tensile residual stress origination and fracture by such stresses. The modified AISI 4330 material was selected based on this requirement. ( 2 ) In order to achieve good cooling efficiency, a forged disc with fins was found to be preferable. The shape of the fins can be determined in such a manner that the cooling efficiency, α, is not substantially reduced. ( 3 ) A ton press machine was employed to produce the forging steel brake disc with fins. After the production, the performance of the forged brake disc with fins was evaluated by brake testing. Table 5 shows the tested brake discs and linings, and Fig. 10 shows the appearance of the discs. The cast iron disc, manufactured as a next generation disc candidate, was made heavier than conventional cast iron discs in order to obtain heat mass sufficient for use at higher brake speeds. The forged disc of modified AISI 4330 was manufactured because the forging of modified AISI 4330, due to its higher alloy contents, is more difficult because of special chemical contents such as Ni, Cr, and Mo. The durability of modified AISI 4330 was compared to that of AISI Evaluation by Dynamometer Brake Testing 5. 1 Thermal crack initiation and propagation Cyclic brake testing was conducted under the conditions of 130 km/h (36.1 m/s) and 260 km/h (72.2 m/s) initial brake speeds. The cyclic numbers are for the initial speed of 130 km/h (36.1 m/s) speed. Additional 50 cycles for 260 km/h (72.2 m/s) followed. Figure 11 shows how thermal cracks originated and propagated during the brake cycles. The cast iron disc shown in Fig. 11 showed thermal crack origination at an early stage, and the forged disc showed only thermal checks whose depths were 1 2 mm within the surface layer. The thermal cracks on the cast iron discs penetrated to the back surface prior to the test end Cooling performance The cooling pattern after braking was measured by free rotation at a speed of 260km/h (72.2m/s), and the result is shown in Fig. 12. The peak temperature of the forged disc during braking became higher than that of the cast iron disc because the weight is less. However, the cooling efficiencyprovedto be thesameasthatofthe cast iron disc Resistance to fracture The forged disc may produce residual stress after braking and subsequent cooling down, and such a possibility of residual stress origination was mentioned in the Fig. 10 Appearance of brake discs Table 5 Tested brake discs and linings Fig. 11 Thermal crack initiation and propagation

6 463 Fig. 12 Temperature rise and cooling pattern Fig. 14 Forged brake disc for high speed trains (a) Stress intensity factor (b) Fracture toughness Fig. 13 Stress intensity factor and fracture toughness section of this paper concerning the fracture analysis. In the case of the cast iron disc, residual stress was not as high due to the stress relief by thermal crack propagation. If the residual tensile stress remains, however, as it did in the case of forged discs, fractures may occur depending on the fracture toughness, crack size, and the amount of the residual stress. Therefore, residual stress after braking was investigated by both experimental and analytical methods. When the residual stress state was obtained, the stress intensity factor was calculated assuming that a semielliptical thermal crack propagates in the unchanged residual stress field. The obtained relationship between the crack depth and the stress intensity factor is shown in Fig. 13 (a). In Fig. 13 (b), the fracture toughness measured for the specimens extracted from the produced disc is compared with the stress intensity factor. Considering that no detrimental effect due to low temperature was observed for the modified AISI 4330 material, it was selected as the preferred disc material Commercialization The forged brake disc was developed, and the performance was evaluated by analysis and experiment. As a further step, the disc was experimentally used in conventional bullet trains. During this trial use, a problem related to bolt sticking appeared. This was caused by thermal deformation around the bolt holes due to the two-piece configuration. Therefore, the two-piece design in the circumferential arrangement was changed to a one-piece design. Furthermore, to avoid thermal deformation in the out-of-plane mode, which deteriorates lining wear, the initial conical-like declination on the sliding surface was considered. The initial breaking couple is expected to reach saturation, and this will form a flat sliding plane. The second impediment to commercialization was productivity. The modified AISI 4330 material is hard to forge due to its high Ni content. Therefore, the forging process needed to be improved. The improvements such as die cooling, hot die forging, special lubrication for forging, and die design modification were conducted. By such devices, one-time forging was developed to produce the fins shown in Fig Discussion 6. 1 Thermal crack initiation and propagation According to Spera and Cox (9), thermal fatigue is defined as low cycle fatigue under a condition in which the temperature changes with time. Thermal crack resistance is, therefore, a matter of thermal fatigue. For research on thermal fatigue, a variety of methods have been proposed. Coffin (10),Manson (11), Northcott (12),Blauel (13), and many other researchers (14) have proposed their original methods for thermal fatigue or thermal shock testing. Besides such universal test equipment, other methods relating to products for cast iron and brake discs have been introduced. The conditions that reproduce the behavior of thermal cracks in the field are of great importance in order to evaluate the thermal crack performance of materials. Such conditions are the stress-strain state, temperature rise, and fracture or damage morphology. However, the above methods have not provided such conditions simultaneously. In this report, thermal crack initiation and propagation was experimentally estimated. However, it was not possible to evaluate such properties by relationships such as ε t N i and da/dn K using thermal strain range, cycles for initiation, crack propagation versus cycle, or stress intensity range. The reason for this is that we were not able

7 464 to evaluate the properties because, for discs in particular, thermal strain and stress are not currently measurable, and thus such measurements remains to be investigated. The results obtained so far indicated that the material characteristics do not largely affect crack initiation for especially thermal checks, as is shown in Fig Fracture prevention From the fracture analysis described above, the fracture toughness of the selected disc material is of great importance with respect to preventing fractures. This is because the thermal crack initiation and propagation is not largely affected by variations in the steel disc material. In other words, thermal crack initiation and propagation seems to be inevitable when the thermal cycles are caused by braking. The material candidates shown in Table 4 were selected by the first screening from the experiment using a thermal shock tester. However, the screening was used as an approximate rough material selection only because the thermal shock tester does not provide the real stress-strain state caused by the brake cycles. In this way, a consideration for safety was included in this research through a philosophy of trying to obtain the material of highest fracture toughness. 7. Concluding Remarks A brake disc for a high speed train like the Nozomi and Tsubasa, etc., was developed based on fracture experience and experimental and analytical studies. The obtained results are as follows. ( 1 ) During the development of the steel brake disc, a fracture in the disc occurred. From the fracture analysis, the process and cause of the disc fracture were clarified, and the material requirements of fracture toughness for avoiding such a fracture were obtained. ( 2 ) From the fracture toughness measurement of the material candidates, modified AISI 4330 was selected for the brake disc for high speed trains. ( 3 ) By studying the effect of fin configuration on cooling, it was confirmed that the fin configuration by forging could obtain the same cooling effect. ( 4 ) The performance of a produced forged brake disc was evaluated by experimentation, and the result was successful. ( 5 ) For commercialization, modifications for bolt sticking and the production process were conducted, and the brake discs are now in commercial use. Acknowledgment The authors greatly acknowledge the helpful support and cooperation of those in the Japan Railway Companies and Sumitomo Metal Industries, Ltd., who were involved in this study. References ( 1 ) Bradbury, F.J. and Schnidt, E., Disc Brakes for Motor Vehicles, Glasers Annalen, Vol.83, No.4 (1959), pp (2) Sauthoff, F. and Schnidt, E., Die Scheibenbremse, für Schienenfahrzeuge und ihre Bremsbeläge, Glaswers Annalen, Vol.83, No.4 (1959), pp ( 3 ) Munkow, H, Die Schsbremsscheibe und Ihre Problems, OET-Eisenbahn Technik, Vol.24, No.5 (1976), pp ( 4 ) Tompkin, J.B., Development of the Disc Brakes with Particular Reference to British Railway Application, K. of the Institute of Loco. Engi., (1969), pp ( 5 ) Bates, R.C., et al., ASTM Spec. Tech. Publ., 513 (1972), p.3. ( 6 ) Shah, R.C. and Kobayashi, A.S., Stress Intensity Factor for an Elliptical Crack Approaching the Surface of a Plate in Bending, ASTM Spec. Tech. Publ., 513 (1972), pp ( 7 ) Irwin, G.R., Analysis of Stress and Strain Near the End of a Crack Traversing a Plate, Trans. ASME, J. Appl. Mech., Vol.4 (1957), pp ( 8 ) Sakamoto, H., Heat Convection and Design of Brake Discs, Journal of Rail and Transit, I Mech E, PART F (2004), pp ( 9 ) Spera, D.A. and Cox, E.C., Description of a Computerized Method for Predicting Thermal Fatigue Life of Metals. Thermal Fatigue of Materials and Components, ASTM, Spec. Tech. Publ., 612 (1976), pp (10) Coffin, I.F., Jr., A Study of the Effects of Cycle Thermal Stresses in a Ductile Metal, Trans. ASME, Vol.76, No.6 (1954), pp (11) Manson, S.S. and Smith, R.W., Quantitative Evaluation of Thermal-Shock Resistance, Trans. ASME, April (1956), pp (12) Northcott, L. and Baron, H.G., The Craze-Cracking of Metals, JISI, Dec. (1956), pp (13) Blauel, J.G., Kalhoff, J.F. and Stahn, B., Model Experiments for Thermal Shock Fracture Behavior, Trans. ASME, Ser. E, Oct. (1974), pp (14) Taira, S., Relation between Thermal Fatigue and Low- Cycle Fatigue at Elevated Temperature. Fatigue at Elevated Temperature, ASTM Spec. Tech. Publ., 520 (1973), pp