FATIGUE PROPERTIES OF MACHINED SPECIMENS FROM CAST AND FORGED RAILROAD WHEELS. FEM, Universidade Estadual de Campinas (Unicamp), Campinas, SP, Brazil

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1 FATIGUE PROPERTIES OF MACHINED SPECIMENS FROM CAST AND FORGED RAILROAD WHEELS Queiroz, Syme R. S. 1, Guimarães, V. 2, and Ferreira, I. 3 1,3 FEM, Universidade Estadual de Campinas (Unicamp), Campinas, SP, Brazil 1 DCC, Instituto Federal do Pará (IFPA), Belém, PA, Brazil 3 FEG, Universidade Estadual Paulista, Guaratinguetá, SP, Brazil ABSTRACT The worldwide tendency of railways is the high load per wagon in cargo transport and high speed in the passenger sector. In both situation the wheel is strongly required, especially in respect of fatigue, which have being the mainly cause in accidents having wheel failure or bring the wheel out of service before end of its life. This research shows the comparison between the fatigue properties of forged and cast wheels. The research indicates what type of wheel is more resistant to fatigue taking account the test conditions. Keywords: Wheel, fatigue, mechanical properties, fatigue properties INTRODUCTION The failures caused by fatigue in railway wheels are the mainly cause of wheels removal before the end of its life, as well as, accidents involving derailment of wagons. This research analyzed specimens of forged and cast wheels having similar chemical composition in the same class of material (class C) from AAR (Association of American Railroads) (1). The fatigue properties were analyzed in samples taken from the rim of the wheel (2, 3). The wheels were divided as follow: Wheel 1: Cast wheel Wheel 2: Forged wheel with no degassing steel Wheel 3: Forged wheel with degassing steel 5673

2 Fatigue tests were performed in three point bending specimens of rectangular section, with 7x14x63 mm, machined from three railway wheels, two forged (one with degassing of the steel and other without desgassing) and a cast, by using tests with load ratio 0.1, maximum load of 15kN (specimens from cast wheel) and 18kN (specimens from forged wheels), sine waves, and in neutral environment (2, 3, 7, 9). MATERIALS AND METHODS The phenomenon of fatigue occurs in railway wheels due to the nature of the request component, This phenomenon is the cycle high fatigue. Fatigue can reduce the tensile strength limit of martensitic carbon steels on average 75%, which can lead to fracture of the same. Cracks openings in railway wheels can lead to loss of part of the wheel by scaling or an extension by radial opening with release consequent of accessories exist between the wheel and the axle. Fatigue tests contribute with caution and defects minimizing in cast and forged railway wheels (4, 7, 8). Most fractures of wheels originate from a fatigue crack on the outer circumference of the wheel tread. Cracks that lead to failure are often associated with some type of mechanical or metallurgical damage (4, 5, 6, 7). The figure 1 gives an overview of the damaged wheel and the resulted final crack, which grew radial towards the axle. The crack origin was at the outer surface of the rim and the critical crack size, which led to the fracture, was only 25 mm in length. Figure 1- Damage on wheel because of fatigue crack. In order to determine the fatigue properties of cast and forged wheels, one wheel per type were destroyed (1, 2 and 3), the specimens were extracted on the same 5674

3 position in all wheels. The following specimens were extracted from each wheel type: 12 specimens. The specimens dimension used on this research are showed on follow picture (Figure 2a). The Figures 2b show the localization of each specimen. Figure 2 - (a) Fatigue specimen. (b) Fatigue specimen on wheel rim. According to the introduction, the research analyzed wheels from the same chemical composition class of AAR. The wheel chemical composition is on Table 1: TABLE 1 Wheels chemical composition Chemical Composition Wheel 1 Wheel 2 Wheel 3 Carbon Manganese Phosphorus Sulfur Silicon Nickel Chromium Molybdenum Vanadium Cupper Aluminum Titanium Niobium Gases (ppm) Nitrogen Hydrogen The railroad wheels are manufactured by forging or casting process. The forged wheels have no restriction application, while cast wheels are restricted for freight transportation. The following flow charts describe the difference between these processes: 5675

4 - Cast Wheel Steel Cast - Molding Heat Treatment Shipping - Forged Wheel Steel manufacture Final Inspection Ingot teeming Bore machining Ingot cut operation in blocks Heat treatment Forging + Rolling mill Blocks heating Wheel machining Final Inspection Shipping Tests were performed on a machine of the type servo-hydraulic MTS (Figure 4.a), model TestStar II, and load capacity of 10 tons. Fatigue tests were performed at three points of bending specimens (Figura 4.b) and temperature of 25º C. a b Figure 3 (a) Photo MTS machine used in the fatigue test. (b) Photo of three points of a railway cast wheel specimen during the bending fatigue test. RESULTS The results are basically the same for three wheels in terms of the Weibull distribution. Table 2 shows the results of fatigue tests for the three wheels, the fatigue life (number of cycles) and average degree (failure rate), the average and standard deviation achieved by the numbers of cycles to fatigue. The characteristic 5676

5 lives were 104,454, 117,453 and 98,801 cycles, respectively for cast and forged wheels, and the correlation coefficient of about 0.9 for the three samples. Figures 5 and 6 shows test results (fatigue tests and fractography). Table 2 - Results of fatigue tests of the specimens from the cast and forged wheels Wheels Cast (Wheel 1) Maximun load of fatigue 15kN e R=0,1 Forged (Wheel 2) Maximun load of fatigue 18kN e R=0,1 Specimens Average degree* Failure probability (F) (%) Fatigue life (N) (cycles) 1-3 6, , , , , , , , , , Average Standard deviation , , , , , , , , , , Average Standard deviation , , , Forged , (Wheel 3) , , Maximun load , of fatigue 18kN , e R=0, , , Average Standard deviation * - Average degree according to standard ABNT NBR 6742 (January / 1987) 5677

6 Figure 4 - Results of the fatigue tests of the specimens machined from cast and forged wheels in terms of the Weibull statistical distribution. F is the failure probability and N is the life (number of cicles to fracture). The maximum load is 15kN for cast specimens and 18kN for the forged. NBR 6752, 1987 (a) Specimens 1-7. Fatigue zone. (b) Specimens Fatigue zone. Figure 5. Fractography of specimens 1-7 and 1-11 of the cast wheel. CONCLUSION The results obtained from the Weibull distribution showed a behavior similar to the three wheels. However, as the maximum load in the case of the cast wheel is 20% smaller, it can be concluded that fatigue life of specimens machined from the forged wheels is around 20% higher than specimens from the cast wheel. 5678

7 ACKNOWLEDGMENTS The authors gratefully acknowledge the funding by MWL Brasil, UNICAMP (Universidade Estadual de Campinas), UNESP (Universidade do Estado de São Paulo) and IFPA (Instituto Federal de Educação, Ciência e Tecnologia do Pará). REFERENCES 1. ASSOCIATION OF AMERICAN RAILROAD. Manual of Standards and Recommended Practices Wheels and Axles, "section G", A.A.R. M-107. USA, p. 2. AMERICAN SOCIETY FOR TESTING AND MATERIALS. Standard terminology relating to fatigue and fracture testing. ASTM E USA, p. 3. ASSOCIAÇÃO BRASILEIRA DE NORMAS TÉCNICAS. Weibull distribution applied to fatigue analysis. NBR (in Portuguese). 18p 4. FERREIRA, I., Propriedades Mecânicas dos Materiais e Mecanismos de Fratura- parte B. Campinas: FEM, UNICAMP, MINICUCCI, DOMINGOS JOSÉ. Rodas e Eixos Ferroviários Conceitos Básicos. MWL Brasil Rodas & Eixos Ltda p. 6. PARIDA, N.; DAS, S. K.; TARAFDER, S. Failure analysis of railroad wheels. Engineering Failure Analysis 7p QUEIROZ, S. R. S., p. Propriedades Mecânicas e Micromecanismos de Fratura de Corpos-de-prova Usinados de Rodas Ferroviárias Fundidas e Forjadas. Tese de Doutorado da Faculdade de Engenharia Mecânica da Universidade Estadual de Campinas UNICAMP, Campinas. 8. SIVAPRASAD, S.; TARAFDER, S.; RANGANATH, V. R.; PARIDA, N. Fatigue and fracture behaviour of forged and cast railway wheels. National metallurgical laboratory Jamshedper. India. 6p TARAFDER S., SIVAPRASAD S., RANGANATH V. R. Comparative assessment of fatigue end fracture behaviour of cast and forged railway wheels. National metallurgical laboratory. Jamshedpur. India. National Aerospace Laboratories. Bangalore , 14p

8 FATIGUE PROPERTIES OF MACHINED SPECIMENS FROM CAST AND FORGED RAILROAD WHEELS Queiroz, Syme R. S. 1, Guimarães, V. 2, and Ferreira, I. 3 1,3 FEM, Universidade Estadual de Campinas (Unicamp), Campinas, SP, Brazil 1 DCC, Instituto Federal do Pará (IFPA), Belém, PA, Brazil 3 FEG, Universidade Estadual Paulista, Guaratinguetá, SP, Brazil ABSTRACT The worldwide tendency of railways is the high load per wagon in cargo transport and high speed in the passenger sector. In both situation the wheel is strongly required, especially in respect of fatigue, which have being the mainly cause in accidents having wheel failure or bring the wheel out of service before end of its life. This research shows the comparison between the fatigue properties of forged and cast wheels. The research indicates what type of wheel is more resistant to fatigue taking account the test conditions. Keywords: Wheel, fatigue, mechanical properties, fatigue properties 5680