(V) Austempering and Bainitic Transformation Kinetics of AISI 52100

AISI 52100 is a high carbon alloy steel typically used in bearings. The common hardening method for AISI 52100 is austempering, in which the steel is heated to above austenitizing temperature, cooled to just above martensite starting (Ms) temperature in quench media (typically molten salt), holden at that temperature until the transformation to bainite is completed and then cooled further to room temperature. Different austempering temperature and holding time will develop different bainite percentage in the steel and thus different mechanical properties. In the present work, the bainitic transformation kinetics were investigated through experiments and simulation. Molten salt austempering trials of AISI 52100 were conducted at selected austempering temperatures and holding times. The austempered samples were characterized and the bainitic transformation kinetics were analyzed by Avrami equation using measured hardness data. CHTE quench probe was used to measure the cooling rate of molten salt at selected austempering temperatures. The heat transfer coefficient (HTC) was calculated with measured cooling rate and used to simulate the bainitic transformation kinetics via DANTE software. The experimental results were compared with simulation results and they had good agreement.