Fracture toughness properties of machined specimens from cast and forged railroad wheels

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1 Fracture toughness properties of machined specimens from cast and forged railroad wheels S. R. S. Queiroz 1,2, V. A. Guimarães 3, and I. Ferreira 1 Travessa Vileta, 1342, apto: 202, CEP: , syme.souza@ifpa.edu.br 1 FEM, University of Campinas (Unicamp), Campinas, SP, Brazil 2 DCC, Instituto Federal do Pará (IFPA), Belém, PA, Brazil 3 FEG, Universidade Estadual Paulista, Guaratinguetá, SP, Brazil Knowledge of the mechanical behavior of cast and forged railway steel wheels based on their mechanical properties is essential for the design of railway wheels and the improvement of its manufacturing process. Fracture toughness and Charpy Impact tests were performed in three specimens of rectangular section, machined from rim of three railway wheels, two forged (one with degassing of the steel and other without degassing) and a cast. Then fractographys analysis of specimens fractured regions were performed. The fracture toughness values for both whels, forged and cast, are expected values agreement and recommended by AAR (Association of american Railroads) for railway wheels of Class C. The Charpy Impact values show better performance of forged whells in relation to cast wheel. The fracture toughness specimens fractographys aspects exhibit fatigue and fracture typical zones. The Charpy impacts specimens fractographys aspects exhibit fracture and cleavage typical zones. Keywords: Railway wheel; Toughness; Mechanical properties; Fracture toughness properties INTRODUCTION Fracture 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 5521

2 in the same class of material (class C) from AAR (Association of American Railroads) (1). The fracture toughness and (Charpy impact) impact toughness properties (2, 3, 4, 5) were analyzed in samples taken from the rim of the railway wheels. The wheels were divided as follow: a Wheel 1: Cast wheel Wheel 2: Forged wheel with no degassing steel Wheel 3: Forged wheel with degassing steel Fracture toughness and Charpy impact tests were performed in specimens machined from three railway wheels, two forged (one with degassing of the steel and other without desgassing) and a cast (2, 4, 5, 6). The fractured region microstructure of fracture toughness and impact specimens taken from the rim sections were analyzed (7, 8). b MATERIALS AND METHODS b The major problem, which may occur with a wheel in service, is a fracture. This causes generally a derailment. Extreme conditions, mainly after passages with long inclines, show, that the solid wheel is susceptible to fracture as well braking, particularly if continued for a long time, causes high residual tensile stresses. Cracks that lead to failure are often associated with some type of mechanical (notches) or metallurgical (inclusions) damage. High circumferential tensile stresses in a thermal overheated wheel promote crack growth and thus the destruction of the wheel. According to the introduction, the research analyzed wheels from the same chemical composition class of AAR (see table 1). In order to determine the fracture toughness and Charpy impact properties of cast and forged wheels, one wheel per type were destroyed (1, 2 and 3), the specimens were extracted on the same position in all wheels. Fracture toughness and Charpy impact tests were performed in specimens of rectangular section, machined from three railway wheels. Fracture toughness tests were performed on a machine of the type servo-hydraulic MTS, model TestStar II, and load capacity of 10 tons, with load ratio 0.1, speed 0,03 mm/s and temperature of 25º C. Charpy impact tests were performed on the equipment of the impact pendulum type (LOS), model PSW with capacity of the 300J. For the characterization of the material, railway wheels specimens samples used in the two tests were 5522

3 analyzed. The samples were cleaned properly by (soap and water) and (alcohol and ultrasound) process before. scanning electron microscope (SEM). The follow picture (Figure 1.a) show a fracture toughness specimen fixed to the MTS machine during the tests and the Figure 1.b show the fracture surface of fracture toughness specimens three after tests and the Figure 1.c show a Charpy impact specimen fracture surface after tests. The Figure 2 show a typical curve of the load (N) versus crack-tip opening displacement (CTOD - mm) obtained from fracture toughness tests of a cast wheel specimen (A) (10, 11). 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 a b c Figure 1. (a) Fracture toughness specimens fixed to the MTS. (b) Fracture surface of fracture toughness specimens three after tests; (c) Fracture surface of a Charpy impact specimens after tests (10). 5523

4 Figure 2. Typical curve of the load versus crack-tip opening displacement obtained from fracture toughness tests of a cast wheel specimen (10). RESULTS The results obtained during the fracture toughness tests are shown in Table 2, the table shows the average and standard deviation of the tests results and the values recommended by the Association of American Railroad standards (1). The results for both the wheel are consistent with the values recommended by the AAR standards for railway wheels (Class C). (12,13) in their fracture toughness tests obtained similar results. The values average of fracture toughness obtained for the cast wheel is about 8% lower than the values of forged wheels. In a comparative study with cast and forged wheels, were observed superior behavior of forged wheels in relation to the cast when analyzing the values obtained from fracture toughness tests for two types of wheels (5). Table 2. Fracture toughness test results of wheels 1, 2 and 3 Specimens Wheel 1 Wheel 2 Wheel 3 Wheel (Class C) A (MPa m ) B (MPa m ) > 33(MPa m ) C (MPa m ) Average (MPa m ) >40(MPa m ) Standard deviation The results of Charpy impact tests are shown in Table 2, the table shows the average and standard deviation of the tests results and the values recommended by 5524

5 the British Standards. EN (6). (9) in their impact tests obtained similar results. The results of Charpy impact tests show that the R1 wheel values are on average approximately 53% lower than the values of R2 wheel and approximately 63% lower than the values obtained for R3 wheel. The results of Charpy impact tests also show that the values obtained for both the average and minimum value of cast wheel are less than those recommended values by BS EN (6). The R2 wheel values are near to the minimum value established by the standard and the R3 wheel values are compatible to the minimum and average values recommended by the standard. (9) in their impact tests obtained similar results. Table 3. Impact (Charpy) test results of machined specimens from rim. Specimen Wheel 1(J) Wheel 2(J) Wheel 3(J) BS EN 13262(J) A A A B1 6 * 16 B B C Minimum 12 C C D D D Average (J) 5,6 12,0 15,4 17 Standard Deviation 0,9 1,7 2,2 *Invalid test The fractography results showed the regions different of a fracture toughness specimen. The Figure 4.a show the fatigue/cleavage interface and the Figure 4.b show the final fracture region by cleavage, which is comparatively much more faceted. 5525

6 (a) Fatigue/cleavage interface region. (b) Final fracture region by cleavage. Figure 4: Fractography of the C forged wheel fracture toughness specimen of Forged wheel with no degassing steel. The Charpy impact specimens fractography results show in (Figure 5.a) fracture surface and (Figure 5.b) fracture final- cleavage zone typical. a) Fracture surface. b) Fracture final - cleavage typical zones. Figure 5. Fractography of the C forged wheel Charpy impact specimens of forged wheel with degassing steel. CONCLUSION The values of fracture toughness for both the cast wheel and forged wheel are in agreement with the expected values and standard recommended by the AAR for railway wheels (Class C). The average values of fracture toughness obtained for the cast wheel is lower than the values of the forged wheels being in agreement with the expected because forged railway wheels usually have higher fracture toughness compared to cast wheels. The fractographic features of the fracture toughness 5526

7 specimen, exhibit typical fatigue and fracture zones with approximate dimensions. Characteristic behavior of a good level of toughness. The fractography being in agreement with the expected for the two types (castings and forgings) of wheels. The values of Charpy impact tests for forged wheels are in agreement with the expected values and standard recommended by British Standards. EN Forged railway wheels usually have higher impact toughness compared to cast wheels. The fractographic features of the Charpy impact specimen, exhibit typical fracture zones by cleavage. The fractography being in agreement with the expected for the two types (castings and forgings) of wheels. 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 test methods for measurement of fracture toughness.astm E a.USA, p. (3) AMERICAN SOCIETY FOR TESTING AND MATERIALS. ASTM A : Standard test methods and definitions for mechanical testing of steel products. USA, p. (4) AMERICAN SOCIETY FOR TESTING AND MATERIALS. E 23: Standart test methods for Notched Bar Impact Testing of Metallic Materials. USA, p. 5527

8 (5) AMERICAN SOCIETY FOR TESTING AND MATERIALS. E : Standart test methods for linear-elastic plane-strain fracture toughness K ic of metallic materials. USA, p. (6) BRITISH STANDARDS. EN Railway applications Wheelset and Bogies Axles Product requeriments p. (7) FERREIRA, I., Propriedades Mecânicas dos Materiais e Mecanismos de Fratura- parte B. Campinas: FEM, UNICAMP, (8) MINICUCCI, DOMINGOS JOSÉ. Rodas e Eixos Ferroviários Conceitos Básicos. MWL Brasil Rodas & Eixos Ltda p. (9) PARIDA, N.; DAS, S. K.; TARAFDER, S. Failure analysis of railroad wheels. Engineering Failure Analysis 7p (10) 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. (11) Queiroz, S. R. S., Minicucci, D. J., Boas, R. L. V., Guimarães. V. A. G., Ferreira, I. Fracture toughness Properties of Machined Specimens from Cast and Forged Railroad Steel Wheels. XII Brazilian MRS Meeting, Campos do Jordão, SP, Brazil, (12) 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 (13) 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

9 Fracture toughness properties of machined specimens from cast and forged railroad wheels Knowledge of the mechanical behavior of cast and forged railway steel wheels based on their mechanical properties is essential for the design of railway wheels and the improvement of its manufacturing process. Fracture toughness and Charpy Impact tests were performed in three specimens of rectangular section, machined from rim of three railway wheels, two forged (one with degassing of the steel and other without degassing) and a cast. Then fractographys analysis of specimens fractured regions were performed. The fracture toughness values for both whels, forged and cast, are expected values agreement and recommended by AAR (Association of american Railroads) for railway wheels of Class C. The Charpy Impact values show better performance of forged whells in relation to cast wheel. The fracture toughness specimens fractographys aspects exhibit fatigue and fracture typical zones. The Charpy impacts specimens fractographys aspects exhibit fracture and cleavage typical zones. Keywords: Railway wheel; Toughness; Mechanical properties; Fracture toughness properties 5529