Heat Transfer Control During Quenching

The aim of this article is to discuss the measures to control the dynamic of heat extraction by changing some parameters during quenching. This is possible for workpieces of not too small a cross-section size, because transformation of the microstructure proceeds gradually from the surface to the core only when a particular point attains the temperature A1. Differences between calculation of the temperature dependent heat transfer coefficient (HTC) for laboratory specimens and for real workpieces, taking into account the damping effect, the time lag, and the thermocouple response time, have been discussed. High Pressure Gas Quenching (HPGQ) in vacuum furnaces is especially prone to changing some parameters during quenching. To increase its quenching intensity, in order to attain high enough hardness in the core, the gas pressure and/or its flow velocity can be increased. When this measure is combined with the transient spraying of liquid nitrogen, a new Controllable Heat Extraction (CHE) Technology can be developed. Moreover, if the furnace is provided with a necessary control system and software program, fully automatic control of the heat extraction during quenching, which guarantees repeatable hardness results, is possible.