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3. Bainite austempering
3.1 Microstructure and mechanical properties of bainite quenching
After high-carbon chromium bearing steel is quenched by lower bainite, its structure consists of lower bainite, martensite and residual carbide. The bainite is an irregularly intersecting strip, and the strip is a carbon supersaturated α structure, and a granular or short rod-shaped carbide with a long axis of 55 to 60° is distributed thereon, and the spatial shape is a convex lens. The substructure is dislocation entanglement, and no twin substructure is found. The number and morphology of bainite vary depending on the process conditions. As the quenching temperature increases, the bainite strip becomes longer; the isothermal temperature increases, the bainite strip becomes wider, the carbide particles become larger, and the angle of intersection between the bainite strips becomes smaller, tending to be parallel Arranged to form a structure similar to upper bainite; bainite transformation is a process related to isothermal transformation time, and the amount of bainite after austempering increases with the extension of isothermal time [5,19].
The lower bainite structure of high carbon chromium bearing steel can increase the proportional limit, yield strength, flexural strength and section shrinkage of steel, and has higher impact toughness, fracture toughness and size than quenched martensite structure. Stability, surface stress state is compressive stress.
The high threshold value ΔKth and the low crack growth rate da/dN indicate that the bainite structure is not susceptible to cracking, and existing cracks or newly-initiated cracks are not easily extended [2, 19, 20].
It is generally believed that the wear resistance and contact fatigue properties of the full bainite or the horse/shell composite are lower than that of the quenched low temperature tempered martensite, which is similar to the wear resistance and contact fatigue properties of the martensite structure of similar temperature tempering. Or slightly higher. However, under poor lubrication conditions (such as coal slurry or water), the whole BL structure shows obvious superiority, and has higher contact fatigue life than the low temperature tempered M structure, such as L10 of full BL tissue during water lubrication. =168h, tempered M organization L10 = 52h [21].
3.2 Production application
3.2.5 Application effect
The outstanding characteristics of BL organization are impact toughness, fracture toughness, wear resistance, dimensional stability, and surface residual stress is compressive stress. Therefore, it is suitable for assembling bearings with large interference and poor service conditions, such as railways, rolling mills, cranes, etc., which are subjected to large impact loads, mine transportation machinery or mine loading and unloading systems with poor lubrication conditions, and bearings for coal mines. The high-carbon chromium bearing steel BL austempering process has been successfully applied in railway and rolling mill bearings, and achieved good results.
(1) Expanded the application range of GCr15 steel. Generally, the effective wall thickness of the ferrule is less than 12mm when quenching the GCr15 steel M. However, due to the strong cooling ability of the nitrate salt during the quenching of the BL, if the stirring, stringing, watering and other measures are adopted, the ferrule The effective wall thickness can be expanded to about 28mm.
(2) Stable hardness and good uniformity: Since the BL transformation is a slow process, generally GCr15 steel takes 4h, GCr18Mo steel takes 5h, the ferrule is isothermally long in the nitrate salt, and the surface core structure changes almost simultaneously, so the hardness Stable and uniformity is good. Generally, the hardness of GCr15 steel after quenching is 59-61HRC, and the uniformity is ≤1HRC. Unlike M quenching, the wall thickness of the ferrule is slightly larger, and there are problems such as low hardness, soft point and poor uniformity.
(3) Reducing quenching and grinding cracks: In the production of railway and rolling mill bearings, due to the large size and heavy weight of the ferrule, the M structure is brittle during oil quenching. In order to obtain high hardness after quenching, strong cooling measures are often taken, resulting in Quenching microcracks; due to the tensile stress on the surface after M quenching, the superposition of grinding stress during grinding increases the overall stress level, which is easy to form grinding cracks and cause batch waste. When BL is quenched, the BL structure is much better than the M structure, and the surface is formed with a compressive stress of up to -400 ~ -500MPa, which greatly reduces the tendency of quenching cracks [19]; the surface compressive stress is offset during the grinding process. Partial grinding stress reduces the overall stress level and greatly reduces grinding cracks.
(4) Increased service life of bearings: For railways and rolling mill bearings subjected to large impact loads, the main failure modes after M quenching are: the inner sleeve is cracked during assembly, and the outer ring is blocked by the impact during the use. The ring is broken, and the austempered bearing has good impact toughness and surface compressive stress, no matter whether the inner sleeve is cracked during assembly, or the casing rib is lost during use, the tendency of the inner sleeve to be broken is greatly reduced, and the roller can be reduced. Edge stress concentration. Therefore, after austempering, the average life and reliability after M quenching are improved.
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