The present study involved the assessment of cooling severity of molten NaNO 3 –KNO 3 mixtures which are widely used as quench media for austempering and martempering operations. An Inconel probe instrumented with thermocouples was quenched in molten NaNO 3 –KNO 3 binary mixtures of varying concentration maintained at different quench bath temperatures. The temperature data acquired at various locations in the Inconel probe during quenching was used to calculate the spatially dependent transient heat flux at the metal–quenchant interface. Two critical points corresponding to peak heat extraction rates during the nucleate boiling stage and transition from boiling to convection stage were identified for each quench medium. The variation of average heat flux and average surface temperature corresponding to these critical points was mapped with variation in bath temperature and composition of the quench medium. AISI 4140 steel probes were quenched in these quench media maintained at 300 and 350 °C. The average hardness values measured in steel probes agreed with the cooling performance of these quench media determined using Inconel probe. The degree of uniformity in heat transfer as indicated by the spatial variation of normalized heat energy decreased with the increase in the concentration of KNO 3 in the quench medium. A mechanism of boiling heat transfer during quenching based on thermochemical decomposition of the salt was proposed.