Cryogenic quenching of a stainless steel rodlet with various coatings

Cryogenic quenching is frequently encountered in many applications, such as material processing, cryogens transport, biological tissues preservation and so on. It has been known that a thin coating layer with low thermal conductivity on the surface can shorten the quenching time duration. In order to explore the effect of thin coating layer on cryogenic quenching, experimental investigations were conducted by immersing stainless steel rodlets with different coating layers in liquid nitrogen at atmospheric pressure. Two types of coating layers with various thicknesses were prepared on the surface of the rodlet to investigate the effects of the thermo-physical property and thickness of the coating layer on the quenching performance, as compared to the electro-polished surface. It is shown that a thin coating layer on the rodlet can enhance the quenching heat transfer performance and reduce the total cooling time duration. To analyze the surface heat transfer characteristics, an inverse heat conduction equation with the consideration of variable thermo-physical properties and thermal contact resistance was numerically solved to obtain the temporal evolutions of surface temperature and heat flux on the coating layer in the present study. The reason for the quenching enhancement can be attributed to the shortening of film boiling regime of cryogenic quenching on the surface by the coating layer which allows the improvement of Lei-denfrost point (LFP) temperature, indicating the boiling mode changing to transition boiling at a higher temperature. Furthermore, a theoretical model was applied to analyze the mechanism of the improvement of the LFP temperature, and it is found that large thermal resistance and small superficial effusivity can allow the local surface temperature to drop rapidly when the surface is cooled instantaneously by liquid nitrogen during film boiling regime, which makes local surface temperature not high enough to maintain the stability of local vapor film, even though the average surface temperature is still high. (C) 2020 Elsevier Ltd. All rights reserved.