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Abstract
A series of experimental hypereutectoid pearlitic steels were tested under rolling contact sliding conditions using a lubricated twin-disc setup to study the influence of different chemical compositions and heat treatments on rolling contact fatigue life. Tested samples were then characterised using microscopy and synchrotron measurements as a function of depth from the contact surface. Results, analysed through neural networks, indicate that the most influential factor in lengthening the number of cycles to crack initiation of hypereutectoid steels is hardness, attained by increasing the cooling rate from the hot rolling temperature, but adequate alloying additions can enhance it further. The harder, fast-cooled samples displayed less plastic flow at the surface than the softer slow-cooled ones. With regard to chemical composition, silicon was found to strengthen the ferrite thus reducing strain incompatibilities with the cementite, preventing in this way the fragmentation and eventual dissolution of the lamellae. This is beneficial since larger depths of cementite dissolution were found in samples with lower cycles to crack initiation for a given cooling rate (hardness). Samples containing vanadium lasted longer and displayed less plastic deformation at the surface than those without, at a similar hardness.
Highlights
Hypereutectoid rail steels (0.8–1.0 wt% C) are attractive for heavy haul applications as well as mixed traffic networks due to their higher resistance to wear and gross plastic deformation that derives mainly from their higher hardness in comparison to eutectoid C-Mn steel [1]
The regression coefficient for the standardised rail hardness was 1.11 as opposed to −0.28 for the standardised inverse interlamellar spacing denoting a contribution almost four times larger of the rail hardness on the cycles to initiation than that of the inverse interlamellar spacing. Using these coefficients, estimated values of the cycles to crack initiation were obtained and plotted against measured values (Fig. 2c) to provide an approximation of the possible error associated with the values of cycles obtained, knowing that in this study each sample was only tested under rolling contact fatigue (RCF) once due to time limitations
Despite having data which only reflects a narrow window of composition and might not be extrapolated to other grades, the following conclusions can be drawn for the evidence presented:
Summary
Hypereutectoid rail steels (0.8–1.0 wt% C) are attractive for heavy haul applications as well as mixed traffic networks due to their higher resistance to wear and gross plastic deformation that derives mainly from their higher hardness in comparison to eutectoid C-Mn steel [1]. In addition to carbon and silicon, high performance ultra-high carbon steels (UHC) can contain elements such as Mn, Cr, Ti, and V, whilst restricting others like N, P, and S, to achieve improved properties even in the relatively soft as-rolled condition (∼370 HV), challenging the hardness based approach [9]. Such improvements have been attributed to interlamellar spacing refinement, increased cementite volume fraction, solid solution strengthening of ferrite [1,9], and precipitation strengthening through V/Ti carbides [13]. Since the improvements brought about through alloying are additive to the known benefits from higher cooling rates to increase hardness through refinement of interlamellar spacing, these steels can be heat treated to endure more demanding service conditions
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