Volume 6, Issue 4 (April 2009)
Correlation between Cooling Curves Obtained with a Silver Probe and Quenching Properties of 5140 Steel Bars
A computational method for the prediction of cooling curves at the center of a steel bar using experimentally measured cooling time-temperature data obtained from a cylindrical silver probe was developed based on nonsteady state heat conduction equations. This method involves: (1) the calculation of the critical size of the steel bar which is defined as the smallest size where no film boiling occurs, and (2) initial state of cooling of the steel bar in the quenchant, and the critical heat transfer coefficient of the quenching medium during nucleate boiling. This method was successfully used to: calculate the cooling curves at different locations within a 5140 steel bar which was quenched in three different quenching media, the hardness distribution, and French’s n and C values. The calculated data were validated against experimentally measured data. Using this method, it was shown that the critical heat transfer coefficient between the quenching medium and the steel bar is less than the critical heat transfer coefficient between the quenching medium and the silver probe. A logarithmic relationship between these two coefficients was established which can be used to simplify the calculations.