Estimation of spatially dependent heat flux transients during quenching of inconel probe in molten salt bath

Abstract

Several industrial heat treatment processes, such as martempering and austempering, require a quench bath to be maintained at a temperature ranging between 150°C–600°C. Molten salts, molten alkali, and hot oils are the preferred quenchants for these processes. Molten salts and molten alkali are preferred over hot oil because they possess properties like wide operating temperature range, excellent thermal stability, and tolerance for contaminants. In the present work, the performance of a molten potassium nitrate (KNO<inf>3</inf>) quench bath was analyzed with an Inconel probe that measured 60 mm in height and 12.5 mm in diameter. The probe was heated to 850°C and subsequently quenched in a bath maintained at 450°C. Cooling curves at different locations of the probe were recorded using the K-type thermocouples inserted into the probe. Spatially dependent transient heat flux at the metal/quenchant interface was estimated using inverse heat conduction technique. The existence of two stages of quenching—boiling stage and convection stage—was confirmed by analyzing the heat flux. The heat transfer coefficient was calculated based on heat flux obtained by the inverse method. The nonuniformity in heat transfer along the length of the probe was quantified by calculating the range of surface temperatures at each instance. The hardness distribution in an AISI 4140 steel was predicted using the temperature distribution in the Inconel probe and obtained using inverse method. Uneven distribution of hardness predicted in the probe was attributed to the nonuniform cooling of the probe during quenching. © © 2017 by ASTM International.