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Application of lower bainite microstructure for bearing steels and acceleration of transformation kinetics

Application of lower bainite microstructure for bearing steels and acceleration of transformation kinetics
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Conventional martensitic SAE52100 steel has been the most commonly used material for bearing applications. Lower bainitic microstructure was suggested as an alternative microstructure to replace the martensitic structure, since it showed an equivalent fatigue life to martensite under normal environments, and offers a superior fatigue life in water-containing environments. There have been many studies about lower bainitic SAE52100 steel, but most of them were attempted without consideration about spheroidization. The purpose in the present research was therefore to examine the bainite transformation kinetics of SAE52100 steel, with a microstructure containing spheroidized cementite in a ferrite matrix as an initial state. The effect of various solutes such as Si, Cr, Al, and Co on the acceleration of bainite was also demonstrated. Then the mechanical properties of resultant bainitic SAE52100 steels were evaluated and compared with the literatures.The reaction kinetics was evaluated using dilatometry and microstructural observations. As a result, bainite transformation for SAE52100 could be completed in 30 min at 250℃, which is much faster than previous studies based on a non-spheroidized starting structures. Moreover, the transformation of Al-, Co-added, and Cr-decreased alloys has been completed in shorter isothermal transformation compared with SAE52100, while the transformation kinetics of Si-added alloy was delayed. Thermodynamic calculations and experimental observations were performed to measure the quantity of cementite during austenitization, and hence the optimum heat treatment process could be designed for each alloy. The mechanical properties were measured by ultrasonic fatigue testing and Rockwell hardness testing. The fatigue property of lower bainitic SAE52100 could be enhanced by changing the concentration of Cr, Al, and Co, without a particular change in hardness. However, the presence of spheroidized cementite particle was adversely affected on the fatigue property of bainitic SAE52100 steel, since the measured results were inferior to the reported results with non-spheroidized microstructures.
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