Surface tension of steel at high temperatures

Surface tension is a material property that is needed to describe fluid behaviour, which impacts industrial processes, in which molten material is created, such as thermal cutting, welding and Additive Manufacturing. In particular when using metals, the material properties at high temperatures are often not known. This is partly because of limited possibilities to measure those properties, limitations of temperature measurement methods and a lack of theoretical models that describe the circumstances at such high temperatures sufficiently. When using beam heat sources, such as a laser beam, temperatures far above the melting temperature are reached. Therefore, it is mandatory to know the material properties at such high temperatures in order to describe the material behaviour in models and gain understanding of the occurring effects. Therefore, in this work, an experimental surface wave evaluation method is suggested in combination with thermal measurements in order to derive surface tension values of steel at higher temperatures than reported in literature. The evaluation of gravity-capillary waves in high-speed video recordings shows a steeper decrease of surface tension values than the extrapolation of literature values would predict, while the surface tension values seem not to decrease further above boiling temperature. Using a simplified molecular dynamic model based on pair correlation, a similar tendency of surface values was observed, which indicates that the surface tension is an effect requiring at least two atomic layers. The observed and calculated decreasing trend of the surface tension indicates an exponential relation between surface tension and temperature.