Microstructural Decay in High-Strength Bearing Steels under Rolling Contact Fatigue

Sammanfattning: The work presented in this thesis aims to enhance our understanding of material decay in high-strength steels used for bearing applications. The primary objective is to investigate the microstructural changes in two high-strength steels: 52100 steels, a popular bearing steel, and Hybrid 60, a relatively new bearing steel designed for long fatigue life and use at elevated temperatures. In addition, the investigations were also carried out on a low-carbon low-alloy steel (Hardox 400). The evolution of the microstructure of these materials under rolling contact fatigue (RCF) conditions was investigated in detail.The results revealed distinct microstructural alterations in the region of maximum shear stress beneath the raceway surface, observed in all three steels under investigation. These alterations include the presence of ferrite microbands, dissolution of carbides and precipitates, and the formation of nano-ferrite grains. The decayed regions exhibit differences in mechanical properties compared to the virgin material. In all materials, the presence of both ferrite microbands and nano-ferrite is associated with the rearrangement of dislocations into low-energy configurations, induced by stress-induced cyclic flow during RCF.Hybrid 60 exhibits a lower area fraction of material decay after the same number of stress cycles (\(1.0 \times 10^8\)) compared to 52100 steel and Hardox 400. This difference can be ascribed to the highly effective dislocation pinning provided by the precipitates and their thermodynamic stability in Hybrid 60, which reduce the likelihood of the formation of dislocation substructure in the stressed region, thereby enhancing its resistance to softening.In contrast to 52100 and Hardox steel where cementite precipitates and  \(\varepsilon\)-Fe\(_2\)C carbides are present, the carbon content in 52100 steel plays a more significant role in influencing the dislocation movement under cyclic loading. A higher carbon content results in enhanced solid solution hardening and improved resistance to RCF in 52100 steel. The high carbon content in 52100 steel makes it harder for dislocations to move under the applied cyclic load, increasing resistance to deformation and microstructural change during RCF compared to Hardox 400. In the case of Hybrid 60 steel, dislocation movement is constrained by the formation of secondary carbides and NiAl intermetallic precipitates. The material's resistance to the formation of dislocation cells and ferrite bands is intricately linked to its ability to withstand the dissolution of precipitates through dislocation shearing. These findings highlight the crucial role of alloy carbides in preventing material deterioration. Despite lower levels of interstitial carbon, the alloyed steel (Hybrid 60) exhibits enhanced durability when subject to RCF in comparison with 52100 steel.