Wear behavior of bainitic and pearlitic microstructures from microalloyed railway wheel steel
Introduction
The search for steel with better mechanical properties and lower wear rates for railway wheel or rail applications has been the aim of investigations for the last 40 years [[1], [2], [3], [4], [5]]. Pearlitic microstructure has been extensively used in rails and wheels, and the wear and rolling contact fatigue (RCF) behavior have been studied exhaustively by many researchers [[6], [7], [8]]. Their main objective has been to reduce interlamellar spacing and the size of pearlite colonies by combining thermal and thermomechanical treatment routes and by adding microalloying elements with affinity to form carbides, such as niobium, vanadium, and titanium [[9], [10], [11], [12]]. However, the increase of mechanical strength of pearlitic structures is reaching the limit, making it essential to seek alternative microstructures for wheel and rail applications [13,14]. Hence, research with bainitic steel for wheels and rails has recently been conducted [[15], [16], [17], [18]], demonstrating that the microstructure application did not come to an agreement on the wear performance of bainitic and pearlitic steel. Different compositions, hardness, the bainite type, and the presence of other microstructures are some of the variables that can lead to confusing conclusions [19].
The bainitic microstructure showed a higher wear resistance than pearlite in some works (Table 1) [15]. In the literature, comparing the performance of pearlite and bainite becomes difficult as the chemical compositions of the steel used for each structure are different. Godefroid et al. [20] explained that changing the chemical composition could alter the crack growth behavior in steel for railway applications.
The development of new types of steel for railway wheel applications requires knowledge of the mechanical properties and microstructural characteristics [17,25]. In addition, the estimation of the behavior of railway wheels in operation is fundamental [15]. As a result, laboratory tests are commonly applied to simulate operating conditions on a reduced scale, allowing for the evaluation and characterization of new materials that may reduce operating costs [26]. However, considering the fact that laboratory tests have become more complex, the precision of contact geometry representation with different loading conditions needs refining. Consequently, the twin-disc test happens to offer one of the best solutions for the evaluation of these variables and has been widely used to study the fatigue and wear properties of wheel and rail steel [26].
The twin-disc wear test evaluates the wear resistance of the materials through the mass loss of the components and RCF by analyzing the superficial or subsurface cracks [27]. This way, in the present research, a twin-disc tribometer assessed the RCF and the wear of the bainitic and pearlitic microstructures obtained by isothermal treatment from forged microalloyed railway wheel steel.
Section snippets
Materials end methods
The specimens were manufactured from forged microalloyed railway wheel steel, and its chemical composition (Table 2) was measured by optical spectrometry (ARL 3460 OES, Thermo Scientific).
The isothermal heat treatment consisted of austenitization at 900 °C, in a muffle furnace; then quickly cooled (20 °C/s) to 350 °C, and kept in the tin bath at 350 °C for 50 min, with subsequent air cooling to obtain a lower bainitic microstructure. The pearlitic microstructure followed the same procedure,
Results and discussion
The microstructure obtained from isothermal heat treatment at 600 °C exhibited proeutectoid ferrite, lamellar, and degenerated pearlite (Fig. 5a), whereas at 350 °C it showed upper and lower bainite (Fig. 5b). Thin particles of cementite in lower bainite [31] can be seen in Fig. 6. The mechanical properties are presented in Table 4.
Fig. 7 presents the mass loss average during the wear test for pearlitic and bainitic microstructures. The total mass losses were 537 and 992 mg for bainitic and
Conclusions
For pearlitic and bainitic microstructures obtained from a forged railway wheel steel (0.7C/0.4Si/0.8Mn/0.22Mo + Nb) and tested for 100,000 cycles in a twin-disc tribometer without debris at a dry environment, it was concluded that:
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The mass loss of the bainitic microstructure was approximately 45 % lower than the pearlitic microstructure due to two factors: the bainite presented higher hardness and greater capacity to absorb plastic deformation in volume.
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The roughness measurements (Ra) after
CRediT authorship contribution statement
A.B. Rezende: Conceptualization, Methodology, Validation, Formal analysis, Investigation, Data curation, Writing - original draft, Writing - review & editing, Visualization. S.T. Fonseca: Methodology, Validation, Writing - review & editing, Supervision, Project administration, Conceptualization. F.M. Fernandes: Formal analysis, Investigation, Data curation, Writing - original draft, Visualization. R.S. Miranda: Investigation, Data curation, Writing - original draft. F.A.F. Grijalba: Resources,
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
The authors thank the Brazilian Nanotechnology National Laboratory (LNNano), Metals Characterization and Processing Laboratory (CNPEM/MCTIC), Vale S.A, and the Brazilian National Council for Scientific and Technological Development (CNPQ).
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