The production of metal powders by water atomisation is a well established process, which can be used to produce a wide range of particle sizes. A careful control of the particle size distribution is necessary, to atomise powders with a high quality and at a low production cost. Therefore, it is necessary to have a substantial knowledge of the relation between operational parameters and the particle size, to be able to produce water atomised metal powders with consistent and high yields.

The main purpose with this thesis was to increase the knowledge about factors which affect the mass median particle size (d₅₀) for water atomised metal powders. The specific objectives with the study were to develop a theoretical d₅₀ model and to investigate the relation between the particle size and the physical properties of the liquid metal.

Pilot scale experiments for liquid iron showed that alloy additions of carbon and sulphur decreased the d₅₀ value, at a maintained liquid steel temperature before atomisation. Moreover, it was indicated that the reduced particle size at increased %C and %S contents may be related to a decreased viscosity and surface tension of the liquid metal, respectively. An alternative explanation could be that raised superheats at increased carbon contents increased the total available time for atomisation, which may have contributed to a reduction of the d₅₀ value.

The theoretical d₅₀ model developed in this work showed a very good correlation to the current experimental data. The model considers the influence of surface tension, viscosity, melt stream diameter, water pressure, water jet angle and water to metal ratio. This model was further used to analyse how the d₅₀ value was influenced by the viscosity and the surface tension. A reduced viscosity from 4∙9 to 2∙1 mPa s decreased the d₅₀ value with 33%. In addition, the particle size was estimated to decrease with 21% by decreasing the surface tension from 1840 to 900 mN m^-1.