Mechanism of cementite decomposition in 100Cr6 bearing steels during high pressure torsion
Graphical abstract
Introduction
Cementite decomposition in steels is a ubiquitous phenomenon that concerns many technologically crucial applications. It occurs on pearlitic rails during dynamic wheel/rail contacts [1], when subjecting hardened steels to machining processes such as hard turning [2], [3] and high speed drilling [4], during high pressure torsion (HPT) of pearlitic steels [5], [6], [7] and cold drawing of pearlitic wires [8], [9], [10], [11], during ball milling [12,13] or ball drop tests [13,14] of pearlitic and spheroidite steels, as well as in the subsurface of bearings during rolling contact fatigue (RCF) [15], [16], [17], [18], [19], [20]. Cementite decomposition can lead to material strengthening as reported in the case of cold-drawn pearlitic wires [8] but it can also be associated with material failures like in the case of white etching cracks (WECs) in bearings [15], [16], [17], [18], [19], [20]. A better understanding of the underlying processes is the key to further exploit the positive effects of this phenomenon as well as to avoid the negative ones.
Here we focus on cementite decomposition in bearing steels where WECs and the associated microstructural alteration named white etching areas (WEAs) have drawn much attention in the past decades [19,20]. WECs and WEAs are usually observed beneath the bearing raceway after exposure to cyclic rolling with high contact loads. They lead to unpredictable premature bearing failures in the form of white structure flaking (WSF) at as early as 5–10% of the L101 bearing life [19]. The term WEA stems from the white and homogeneous appearance of the altered microstructure under an optical microscope after metallographic preparation and etching. The formation of WEAs involves decomposition of carbides and evolution of the originally martensitic/bainitic matrix into nanocrystalline ferrite [[16], [17], [18],21]. According to atom probe studies by Li et al. [22], solute C atoms arising from the decomposition of the carbides that originally sited at the location of the WEA are enriched at the grain boundaries of the nano-ferrite. The segregation of C atoms at grain boundaries effectively stabilizes the nano-ferrite structure [23,24]. Although much effort has been devoted to the investigation of WECs/WEAs, elimination or mitigation of white etching matter in bearing applications still remains an ongoing challenge [25], [26], [27], [28], [29], [30], [31], [32].
Recently the formation mechanism of WECs/WEAs in bearings was explained [15,31,33,34]. Once a crack is initiated at a defect, such as an inclusion, the crack faces rub on each other during RCF and thereby induces localized severe plastic deformation (SPD). This mechanically decomposes the original cementite-containing microstructure near the crack faces and transforms it into nanocrystalline ferrite. In addition to conventional crack propagation, WECs continuously move normal to the crack plane through the material, leaving behind volumes of WEAs. The WEC failure mechanism is not reserved to any one type of bearing steel but frequently observed in almost all commercial high C bearing steel grades [19], the most prominent one being the widely used 100Cr6 [16,19,35,36]. In contrast, the high N bearing steel X30CrMoN15-1 is considered as a sustainable solution to prevent WSF given the fact that up to now there are no reports about failure by WECs for this steel grade [31]. The superior WEC resistance of the X30CrMoN15-1 over the conventional 100Cr6 is partially attributed to the higher mechanical stability of the precipitates therein [37], as it is supposedly a severe obstacle for the above-mentioned mechanism to operate. Accordingly, the formation of WECs/WEAs in the 100Cr6 bearings could also potentially be slowed down by precipitates that are less prone to mechanical decomposition. This motivates the further investigation of the mechanism influencing the resistance of cementite to mechanical decomposition processes.
To study the decomposition of cementite by SPD in the bearing steel 100Cr6 under controlled laboratory conditions we employ HPT, where a thin-disk sample is placed between the upper and lower anvils of the facility and subjected to a compressive force and concurrent torsional straining [38]. It permits the SPD of high-strength or relatively brittle materials at ambient temperature where SPD is oftentimes not possible [39]. HPT experiments have been conducted before on 100Cr6, but only in the soft-annealed condition at an applied pressure of 6 GPa [40]. Here we use a novel HPT experimental setup that allows to apply quasi-hydrostatic pressures of 9.5 GPa which enables us to investigate the microstructural alteration of 100Cr6 in the hardened conditions (62 HRC) as used in bearing applications. The experiment is expected to throw additional light on the mechanism of cementite decomposition during SPD of bearing steels and help guide the development of practical methods for mitigating WECs in bearing applications.
Section snippets
Experimental
The material used in this investigation was extracted from a commercial through-hardened 100Cr6 (AISI 52100) double row cylindrical roller bearing. The chemical composition measured using wet-chemical analysis is listed in Table 1. The alloy received a standardized heat treatment specific for bearing applications of this steel type [41,42]: After cementite spheroidization the bearing was annealed in the austenite/cementite two phase regime at 840 °C for ~20 min. This led to an austenitic matrix
Microstructural evolution during high pressure torsion
The as-received 100Cr6 bearing steel has a microstructure of spheroidized cementite precipitates dispersed in a plate martensitic matrix, as revealed by the BSE image and EBSD maps in Fig. 1(a). The size of the spheroidal cementite ranges from ~200 nm to ~1.5 μm in diameter and the volume fraction is ~4.2%. The spacing between neighboring spheroidal cementite is in a range of hundreds of nanometers to several micrometers. The martensitic plates have a thickness of ~160 nm to ~1.2 μm. In the
Plastic flow of matrix around spheroidal cementite
We observe an interesting flow behavior of the deformed matrix around the spheroidal cementite precipitates (see Fig. 4(b) and (c)). The layered nanograins of the deformed matrix is almost everywhere parallel to the shear direction in spheroidal cementite-free regions. However, in proximity to the spheroidal cementite significant local deviations are observed. Here, the layered nanograins are curved and adapt to the shape of the precipitate. This morphology reminds of the flow of fluids around
Conclusions
We have investigated the decomposition of cementite in a commercial through-hardened 100Cr6 bearing steel during high pressure torsion (P = 9.5 GPa, r = 3 mm, N = 0.75 revolutions, max. equivalent strain ɛ = 9.3). The investigation was done at the periphery of the HPT-processed disk. The following conclusions are drawn:
- 1.
Under high compressive stresses and concurrent torsional straining, the plate martensitic matrix evolves into layered nanograins. The hard matrix phase plastically flows around
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 are grateful to the German Federal Ministry of Education and Research [Bundesministerium für Bildung und Forschung (BMBF)] for funding this research (through grant 03SF0535). We would like to thank P. Kutlesa from Erich-Schmid-Institut für Materialwissenschaft for conducting the HPT tests.
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