Abstract

The present study involved the assessment of cooling severity of molten NaNO₃–KNO₃ mixtures which are widely used as quench media for austempering and martempering operations. An Inconel probe instrumented with thermocouples was quenched in molten NaNO₃–KNO₃ 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 KNO₃ in the quench medium. A mechanism of boiling heat transfer during quenching based on thermochemical decomposition of the salt was proposed.

中文翻译：

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NaNO 3 -KNO 3熔融盐混合物淬火过程中成分和浴温对传热的影响

摘要

本研究涉及评估熔融的NaNO ₃ -KNO ₃混合物的冷却强度，该混合物被广泛用作奥氏体和回火操作的淬火介质。将装有热电偶的Inconel探针在熔融的NaNO ₃ -KNO _3中淬火不同浓度的二元混合物保持在不同的淬火浴温度下。在淬火过程中，Inconel探针在各个位置获得的温度数据用于计算在金属-淬火剂界面处随空间变化的瞬态热通量。对于每种淬火介质，确定了与在核沸腾阶段以及从沸腾到对流阶段的峰值吸热速率相对应的两个临界点。将与这些临界点相对应的平均热通量和平均表面温度的变化与熔池温度和淬火介质组成的变化进行映射。在保持在300和350°C的淬火介质中淬火AISI 4140钢探针。在钢制探针中测得的平均硬度值与使用Inconel探针测定的这些淬火介质的冷却性能一致。归一化热能的空间变化表示的传热均匀度随KNO浓度的增加而降低₃在淬火介质中。提出了基于盐的热化学分解的淬火沸腾传热机理。