Microstructure alterations in bearing steels exposed to severe plastic deformation
Year: 2020
Type: dissertation
Abstract: White etching cracks (WECs) lead to unpredictable premature bearing failures in the form of white structure flaking (WSF) in many engineering applications, the most predominate one being wind turbines. Although much effort has been devoted to the investigation of WECs, elimination or mitigation of this failure mode in bearings still remains an ongoing challenge. The formation mechanism of WECs is interpreted as follows: These cracks do not only propagate but also move normal to the crack plane through the material. Rubbing of the surfaces of the moving cracks mechanically decomposes the original microstructure and transforms it into white etching area (WEA). Finding counter measures against WECs on the one hand requires a deeper understanding of the microstructure alterations in bearing steels induced by severe plastic deformation. On the other hand it requires investigation of the microstructural characteristics of bearing steels that exhibit resistance to this failure mechanism. Based on these two approaches, this thesis proposes potential methods for the mitigation of WEC failure mode in bearing applications and provides insights into the design of new bearing steels with high WEC resistance. The results provide ideas for the prolongation of the service life of wind turbine gearbox bearings, which corresponds to a lower cost of wind energy, and the cost-effective solutions to WEC premature bearing failures in many other engineering applications.The WEC failure mechanism is frequently observed in almost all commercial high carbon bearing steel grades, the most prominent one being the widely used 100Cr6. As microstructure alterations in WEAs result from local severe plastic deformation which induces carbide decomposition and carbon redistribution, the formation of WECs/WEAs in the 100Cr6 bearings could potentially be slowed down by precipitates that are less prone to mechanical decomposition. Therefore, the mechanism influencing the resistance of cementite to mechanical decomposition process was investigated. The decomposition of spheroidal cementite in the martensitic bearing steel 100Cr6 with 62 Rockwell hardness was triggered by high pressure torsion at 9.5 GPa applied pressure. The hard martensitic matrix and the even harder 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 cementite as indicated by continuously increasing levels of chromium in the matrix. Plastic deformation of cementite via dislocation gliding supposedly accelerates this process as slip steps generated thereby are preferential sites of wear at the matrix/cementite interface. The deformation and decomposition of spheroidal cementite is found to be precipitate size dependent. The larger the size of the cementite is, the easier it will be plastically deformed and decomposed. The precipitate size dependence of the deformation is attributed to the precipitate size dependence of the yield strength of the precipitate and of the local stresses. The higher susceptibility of the larger spheroidal cementite to deformation, together with the higher interfacial wear rates induced by the higher local matrix strain and strain rates, also makes it more prone to decomposition. 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.Different from conventional high carbon bearing steels, the high nitrogen bearing steel X30CrMoN15-1, limited to aerospace applications due to its high costs, is considered to have high WEC resistance as up to now there are no reports about failure by WECs for this steel grade. Rolling contact fatigue tests were carried out on a commercial X30CrMoN15-1 high nitrogen bearing and a commercial 100Cr6 high carbon bearing, whereby the superior WEC resistance of the former over the latter was experimentally verified. To understand the origin of its superior performance, the microstructure of the X30CrMoN15-1 bearing prior to rolling contact fatigue was analyzed by means of scanning electron microscopy, atom probe tomography and nano-indentation, and related to its thermal processing history. The origin of the exceptional WEC resistance of this alloy is rationalized in terms of the mechanical and thermodynamic stability of the precipitates, the different grain boundary segregation behavior between nitrogen and carbon as indicated by the atom probe results, and the cleanliness of the steel. Inspired by this, the design of new bearing steels where the operation of WEC formation mechanism is restricted should rely on enhanced precipitate stability, reduced grain boundary segregation of solutes, and improved steel cleanliness.
Source: RWTH Publications (RWTH Aachen)
Author: Qin, Yu
Institution: RWTH Aachen University
Language: English
Subfield: Mechanical Engineering
Field: Engineering
Domain: Physical Sciences
Open Access status: green
OpenAlex ID: W3085867809