Effect of Water Addition on Cooling Power of Molten Salt in Austempering

Abstract

We investigated the addition of water to molten nitrate-nitrite salt and its effect on the cooling power of the salt when used as a quenching medium in austempering. We also investigated and compared the quenching distortion and mechanical properties of carbon steel, austempered using water-added salt, with those of conventionally quenched-and-tempered steel. The cooling power was evaluated using the heat-transfer coefficient obtained from a cooling curve measured using a silver probe. The results showed that the heat-transfer coefficient of molten salt saturated with water was approximately 2.0 to 4.1 times higher than that without water because of the vaporisation heat of the added water and the agitation effect of the vapour bubbles. The variation in distortion after austempered quenching decreased when using water-added salt. The results also showed that carbon steel austempered using water-added salt had a higher toughness than conventional quenched-and-tempered steel.