Effect of Bubbling Boiling and Breaking of Steam Film on Heat Transfer Coefficient in Stirring Quenching Process

摘要

Steam film occurring on the boundary layer between quenching steel and quenchant takes large effects on their heat transfer, which often results in uneven microstructure and mechanical properties of quenching steel. In this research, the behavior of steam film in quenching process was investigated with high-speed video camera by visualization method. The behaviors of the boiling bubbles including generation and movement as well as the breaking during stirring quenching process were clarified. The flow velocity fields around the quenching parts at no stirring and stirring quenching were investigated. The heat transfer coefficients were identified from the measured cooling curves of surface temperature. Effects of stirring on breaking the steam film and improving heat transfer were confirmed. Steam film and boiling bubbles occurring on the boundary layer between quenching steel and quenchant take large effects on their heat transfer with coolant . The steam film formed on the machinery parts during quenching steel not only reduces the heat transfer efficiency but also lead to uneven heat transfer which in turn often results in heterogonous microstructure and mechanical properties of quenching steel. The generation of boiling film and its effect on the heat transfer is still open as a strong coupling problem of multi-physics phenomena. As the request on high quality parts such as gears and shaft for automobiles, studying the boiling heat transfer and developing high efficient heat treatment has attracted much attention. In this research, the formation and movement of bubbles in boiling film during quenching process were investigated with high-speed video camera by visualization method. Cooling curves of surface temperature during quenching of steel cylinder were measured and the heat transfer coefficients between quenchant and steel were identified. Effect of mechanical stirring of quenchant oil or water on steam film and heat transfer was studied by experiments and numerical simulation. The improvement on heat transfer by stirring in quenching process is confirmed by this research.