Assessment of Cooling Performance of High Temperature Quenchants for Industrial Heat Treatment

Abstract

The present work provides a significant insight into cooling performance of molten salt mixtures and hot oils used for austempering/martempering of steel parts. The analysis is based on the assessment of spatially dependent transient heat flux at the metal-quenchant interface using an inverse heat transfer method. The results were used for quantification of the cooling performance as well as cooling uniformity of quench baths maintained at different bath temperatures. Cooling performance of hot oil and molten NaNO2 eutectic mixture at bath temperature of 150°C was studied. The heat extraction mechanism was completely different, and the study revealed that the molten salt offered higher cooling rate and more uniform cooling as compared to hot oil quench media. For molten KNO3- NaNO3 eutectic mixture, a linear regression model to predict critical average heat flux parameters during cooling of standard Inconel probe at a given bath temperature was developed. In the case of molten NaNO3-KNO3 mixtures, a contour map was drawn to predict average peak heat flux, heat flux at the start of convective cooling stage and corresponding average surface temperatures with varying quench bath composition and temperature. The bath composition significantly affected their cooling performance and uniformity of cooling. The increase in NaNO3 content of the salt bath enhanced its cooling performance and cooling uniformity. A mechanism of boiling heat transfer based on endothermic thermochemical decomposition of the salt is proposed. A low melting KNO3-LiNO3-NaNO3 eutectic salt mixture and neem oil were investigated to assess their suitability as high temperature quench media. Inconel probe experiments involving this salt mixture suggested lower hardness. However, the wettability study on steel indicated an extended boiling stage yielding hardness comparable to that obtained with conventional molten salt mixtures. The cooling performance and uniformity of neem oil was higher compared to hot oils. A simulation study to model the effect of diameter, heat transfer coefficient, bath temperature and residence time on hardness distribution during martempering of AISI 4140 steel cylinders was conducted. An artificial neural network model to predict hardness distribution in AISI 4140 cylinder was proposed.