Mechanism of cementite decomposition in 100Cr6 bearing steels during high pressure torsion

Abstract

Severe plastic deformation leads to cementite decomposition in pearlitic and martensitic alloys, resulting in high-strength nanocrystalline ferrite. This effect can be employed to strengthen pearlitic wires but it can also be associated with material failure by white etching cracks (WECs) that are primarily known to concern bearings or rails. We investigate the decomposition of spheroidal cementite in the martensitic bearing steel 100Cr6 with 62 Rockwell hardness during high pressure torsion at 9.5 GPa applied pressure. The hard martensitic matrix and the even harder spheroidal cementite precipitates behave plastically very differently. The enforced macroscopic plastic deformation is almost entirely carried by the matrix. Plastic material flow of the matrix around the spheroidal cementite leads to wear of the spheroidal cementite as indicated by continuously increasing levels of chromium in the matrix. Plastic deformation of spheroidal cementite via dislocation gliding supposedly accelerates this process as slip steps generated thereby are preferential sites of wear at the matrix/cementite interface. Larger spheroidal cementite precipitates are more prone to plastic deformation and to decomposition than smaller ones. The mechanism likely holds true in general for multiphase materials with large strain difference between phases subjected to high pressure torsion. Although the formation of WECs in 100Cr6 bearings might be slowed down by reducing the size of the spheroidal cementite precipitates, it is unlikely that this could entirely prevent this failure mechanism.

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.