Effects of inclusion size and stress ratio on the very-high-cycle fatigue behavior of pearlitic steel
Introduction
Shattered rims are typical failure mode of railway wheels, which will cause abrupt wheel fractures, rail damage, or even derailment in rare cases [1], [2], [3]. Fig. 1 shows the typical fracture surface morphology of a shattered rim, wherein the cracks initiated from the interior inclusion after being subjected to more than 107 cycles; hence, this phenomenon is considered a form of very-high-cycle fatigue (VHCF) in engineering applications [4], [5]. With the situation that VHCF of railway wheel occurred in the service trains, the VHCF behavior of the pearlitic steels is rarely reported. A systematic experimental investigation on the VHCF behavior of the wheel steel should be investigated to illuminate the mechanism responsible for shattered rims and support the fatigue design of railway wheels.
The new research topic of VHCF has been explored in steels [6], [7], titanium alloys [8], [9], [10], [11] and aluminum alloys [12], [13]. Two typical kinds of VHCF behavior have been defined: crack initiation from defect-free (Type I) and defect (Type II) sites [14], [15]. In this study, the defects are related to inclusions or pores. When high-strength steels are subjected to VHCF, cracks initiate from interior nonmetallic inclusions, and the fatigue life is greater than 107 cycles. The whole region of crack initiation and early propagation exhibits a fish-eye (FiE) pattern, and the morphology around the inclusion is often relatively rough [16], [17], [18], [19], [20]. Moreover, nanograins (NGs) form in the fine granular area (FGA) after cracks initiate under VHCF.
Pearlitic steel combines good wear and fatigue properties and is widely applied in different components of railway systems, including wheels, rails and axles. As known, these compoents are designed to bear millions of service miles with a fatigue life more than 107 cycles. Martensite [20], [6], [7], Austenite [21], [22], Troostite [23] and Bainite [24], [25] steels exhibit Type II VHCF. The engineering practice raises an urgent issue: does the pearlitic steel present the VHCF behavior?
In this paper, a systematic experimental investigation on the VHCF behavior of pearlitic steel extracted from railway wheels was conducted. First, samples of the pearlitic steel were subjected to rotating bending fatigue tests and ultrasonic fatigue tests, from which relevant S-N curves were obtained. The fracture surfaces were observed by scanning electron microscopy (SEM), and crack initiation from interior inclusion was presented for the specimens of ultrasonic fatigue tests. The effects of inclusion size and stress ratio on the VHCF were evaluated. Finally, focused ion beam (FIB) microscopy and transmission electron microscopy (TEM) analyses were performed to characterize the microstructure of the fracture surface. A thin NG layer formed on the whole fracture surface.
Section snippets
Test material
The material used in this paper is a medium-carbon steel (CL60), which is a typical wheel steel in China. The chemical composition (mass percentage) of this material is 0.63C, 0.27Si, 0.72 Mn, 0.012P, 0.002 S, 0.12 Cr, 0.25 Cu and balance Fe. Fig. 2 shows the microstructure of CL60, which consists of pearlite (dark region) and ferrite (white region). The formation of pro-eutectoid ferrite is attributed to the heat treatment and carbon content. Pro-eutectoid ferrite is softer than pearlite,
S-N curves
Fig. 4 shows the S–N curves for the three groups of specimens subjected to rotating bending fatigue tests and ultrasonic fatigue tests. The S–N curve under rotating bending loading (Fig. 4a) exhibits a horizontal asymptote shape with a clear fatigue limit, and surface crack initiation is observed only when the fatigue life is less than 106 cycles. The S–N curve under ultrasonic axial loading (Fig. 4b) exhibits a continuously descending shape, and the fatigue limit disappears at a fatigue life of 107
Quantitative evaluation of inclusions caused crack initiation
For each specimen, the values of (area)1/2 were calculated for the sizes of the inclusion cluster and FiE region. These results were plotted as a function of the fatigue life, as shown in Fig. 11a and b. The size of the inclusions (area)Inc1/2 was within a range of 50–200 μm, whereas that of the FiE region was within a range of 160–1108 μm. The interior crack initiated from the inclusion cluster. It is worth noting that the (area)Inc1/2 is related to the area sum of the inclusions in the cluster.
Conclusions
This paper investigated the effects of inclusion size and stress ratio on the VHCF behavior of pearlitic steel. The following conclusions can be drawn from this study.
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The S–N curve under rotating bending loading exhibited a horizontal asymptote shape and a clear fatigue limit. In contrast, the S–N curve under ultrasonic axial loading exhibited a continuously descending shape, and the fatigue limit disappeared at a fatigue life of 107 cycles. The fatigue strength at R = −1 under ultrasonic loading
Declaration of Competing Interest
The authors (Tao Cong, Guian Qian, Guanzhen Zhang, Si Wu, Xiangnan Pan, Leiming Du, Xiaolong Liu) declared that they have no conflicts of interest to this work. We declare that we do not have any commercial or associative interest that represents a conflict of interest in connection with the work submitted.
Acknowledgments
The authors would like to acknowledge financial support from the National Natural Science Foundation of China (No. 11802011), the National Key Research and Development Program of China (No. 2017YFB0702004), the Development Project of China Railway (No. J2019J004) and the China Academy of Railway Sciences Corporation Limited (No. 2019YJ097).
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