| According to the aviation sector and puts forward to new requirements for bearings working in special harsh environment, to keep up with the future development trend of the more high-tech and more precision for aero bearings, and imitating the idea of "composition-structure designing" then developed the high performance Cr-Co-Mo bearing steel which has high strength, high hardness, good corrosion resistance, well toughness and excellent high temperature performance. In the past, researchers focused on the studies of strength-toughness and the mechanisms mostly around a relatively simple system structure, while the characterization of single structure and single scale. However, Cr-Co-Mo bearing steel have more complex structure elements, multi-scale, multiphase and multi-level coupling, which make Cr-Co-Mo bearing steel gains high strength, enough toughness, and improve the other properties as well.Inspired by this concept, this thesis follows the multi-scale analysis method, and considers the characteristics of fatigue resistance which influenced by various factors. Through the systematic studies of each phase, each scale microturctures and all kinds of mechanisms for high-alloy Cr-Co-Mo martensite bearing steel, their effects on strength, toughness and fatigue life was revealed in detail. By means of SEM, EBSD and TEM observations, all of the microstructure evolution had been quantitatively summarized. Meanwhile, the intrinsic mechanical properties and strengthening-toughening mechanisms controlled by the complex multiphase, multi-scale microstructures had been clarified, then provided the strategies to optimize of the comprehensive performance. In addition, based on the project indicators, three comparison groups with slightly different performance were established, then analyzing surface state, stress state and defects in matrix as influential factors which could control the fatigue crack initiation of the Cr-Co-Mo bearing steel. Concluded, fatigue damage control factors and the corresponding critical value could be found out. On account of the combination of theoretical research and practical results, the microstructural controlling strategy has obtained, as well as some innovative research results and engineering application value.The microstructure evolution rule of high-alloy Cr-Co-Mo low carbon martensite bearing steel under different heat treatments has been found out. Cryogenic treatment (CT) under-82℃ for 2h made martensite lattice shrinking causing interstitial carbon atoms segregated near the M/A phase interface, and then spread into the retained austenite in the subsequent process of tempering at 540 ℃, by doing so a small amount of retained austenite with thin film morphology will exist stably between martensite laths. Statistical analysis was carried out on the carbide found that when the solid solution temperature to 1060℃, there was few undissolved M6C carbides with average size of 0.3μm located on high angle grain boundaries to restrain austenite grain from coarsening by pinning grain boundaries at high temperatures. The nano-scaled M2C carbides has an area percentage less than 1%, and the distribution concentrated between 30-40 nm in size during oil-quenching. After CT the area percentage increased to 7%, then the size distribution concentrated in the less than 20 nm range. During tempering, area percentage of M2C further improved to 7.33%, and the size distribution concentrated between 20-30 nm, therefore too small size M2C (< 10 nm) slightly grew up to increase the effective positions to hinder dislocation movement. In martensitic substructures, packet size is decided by the prior austenite grain size. And block is surrounded by high angle grain boundaries (HAGBs), while the misorientation is about 60 ° presenting twin relationship. From crystallography analysis, packet contains three variant groups, which has a specific combination mode between variants.Quantitative analysis found that the effect of solid solution temperatures on nanoscaled M2C precipitation strengthening was not significant, but the impact on grain refining strengthening is obvious. The yield strength of quenching state sample is controlled by block width, by controlling the block width less than 10μm and reducing the M6C volume fraction less than 0.5%can obtain good strength-plasticity matching. Before CT-tempering, the yield strength contribution of M2C is only 7.6%. The M2C less than 40nm reflecting its scale effect, and interacting with the dislocation, has increased the yield strength more than 800 MPa, while the strength contribution ratio increased to 75%. Ultimately, after secondary CT-tempering cycles the yield strength up to 1532 MPa, and the impact energy keep in 52 J.Comparing with conventional alloy steel, the grain size refinement failed to improve the impact toughness effectively. According to Griffith fracture theory, M6C carbide located on high angle grain boundary decreased toughness but was not the determining factor, because of their size is much smaller than the critical size 3μm from the crack initiation. The analyzed of distribution on characteristic grain boundary is found that coincident site lattice boundaries (CSL) directly decided the toughness, and Σ3 is most important in coincidence site lattice (CSL) grain boundaries. Interestingly, the total proportion of low-energy boundaries has a linear mathematical relationship with block width, bigger block width induced increasing number of martensitic variants so that low-energy boundary ratio improving. Moreover, by observing the cleavage crack path, the crack propagation path unit is packet. Quantitative statistics of cleavage platform size confirmed that packet size is in accordance with cleavage platform size. It can be conclude that, packet size is the toughness controlling unit of the tested steel.Q group and ST group have satisfed with the project indicator, which should passed the 10 stress cycle with fatigue strength beyond 600 MPa. It is found that, fatigue fracture is mainly caused by surface defects in R group. While Surface carburizing treatment had not only reduced the surface roughness, but also provided enough surface compressive stress to suppress fatigue crack initiation and propagation effectively. By quantitative calculation, the machining accuracy of critical roughness for test steel is 0.53μm. The ST group is mainly composed of non-metallic inclusions caused fatigue fracture, the critical inclusions size of 5.5μm. In addition, the fatigue strength controlled by the matrix hardness where inclusions located on, deeper inclusions located, smaller size of inclusions and higher matrix hardness value are assistance to maximize fatigue resistance. For the tested steel, controlling inclusion size spacing to the surface within 100μm can effectively improve the fatigue life. Due to the large grain size of Q group prone to persistent slip bands (PSBs) formation, micro-sized bulge generated on the surface, and eventually make fatigue crack owing to high stress concentration areas on either side of the bulge. |
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