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  • 1.
    Persson, Fredrik
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Process.
    A Study of Parameters and Properties Influencing the Size, Morphology and Oxygen Content of Water Atomized Metal Powders2021Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    The production of metal powders by water atomization is a well-established process, which can be used to produce a wide range of particle sizes for different applications. In general, there is a lack of detailed knowledge about what process parameters that affect the powder properties for water atomized metal powders. More specifically, this thesis focuses on the particle size, morphology and oxygen content of water atomized iron powders. A careful control of the particle size distribution is necessary to atomize powders with a high quality and at a low production cost. Demands on the particle morphologies vary depending on the application for the final product. It is important to control both the melt properties and atomizing parameters, to produce powders with an even particle shape and sintered steel components with tight tolerances. The oxidation of the liquid metal should also be as low as possible during the water atomization, to avoid a large amount of harmful oxide forming in the final powder. Pores are generally considered as defects in metal powders. Therefore, the powder porosity should be as low as possible.

    The main objective of this thesis is to obtain a more in-depth knowledge of water atomization of metal powders, by investigating some fundamental parts of the process. The study investigates how the median particle size (d50 value) for iron powders is influenced by the water pressure, the melt stream diameter, the jet angle, the water level in the atomizing tank, changed configurations of the water jets, superheat of the melt, and the carbon and sulfur content in the liquid steel. Similarly, the thesis also investigates factors that influence the particle shape, porosity and oxidation of water atomized iron powders.

    Laboratory and pilot experiments show that the effect on the d50 value was large for the water pressure, medium for the viscosity, surface tension and water to metal ratio, and small for the melt stream diameter. Calculations indicate that the water jet angle has a large effect on the d50 value. In practice, this effect cannot be exploited beyond certain limits caused by instabilities in the atomizing system, which occur if the jet angle is too large.

    The particle size decreases when the carbon and sulfur contents in the liquid iron are increased. This is attributed to decreased viscosities and surface tensions, respectively. An alternative explanation could be that the superheats at increased carbon contents result in a longer time spent in the molten state before the atomization is completed. This may also lead to a decrease in the particle size. Calculations using a developed d50 model estimate that a decreased viscosity from 6.8 mPa s to 4.3 mPa s leads to a reduction in the d50 value by 33%. Similarly, a decreased surface tension from 1840 mN/m to 900 mN m-1 reduces the d50 value by 27%.

    The distribution of oxides in pilot water atomized Fe-Mn-C powders was determined by using optical and scanning electron microscopy, combined with energy dispersive X-ray microanalysis. The oxygen in the atomized powders was mainly present as thin surface oxide layers, which increase in thickness from 10 nm to 50 nm as the particle sizes increase from 10 microns to 750 microns. Manganese oxides were observed to be unevenly distributed at the surface of several particles, when the alloy contained 0.3 wt.% manganese. Experimental data indicate that between 10 - 20% of the manganese was present as oxides in the powders. However, equilibrium calculations at 1550 °C estimate that only 4% of the initial manganese content remained in the steel after a completed atomization.

    The sphericity of the atomized powders decreases as the particle size increases. One feasible explanation is that some larger particles are irregular, since they are formed by collisions of smaller particles. Conversely, smaller particles are formed directly from breakups of the melt and are not the product of collisions between droplets. The sphericity of the size fraction 20-45 microns increases as the carbon content in the iron increases from 0.2 wt.% to 4.2 wt.%. The atomized droplets with larger carbon contents spend a longer time in the molten state, which allows them more time to form a spherical shape during the atomization process. The porosity of iron-carbon powders increases with increasing carbon contents in the melt. Dissociation of steam to hydrogen at the melt surface and precipitation of hydrogen pores in the melt were the most likely mechanisms to cause a pore formation in the powders.

    Keywords:    water atomization; metal powder: particle size; oxygen content; particle shape; porosity; steelmaking

     

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  • 2.
    Persson, Fredrik
    et al.
    Global Process Development, Höganäs AB, Höganäs 263 83, Sweden.
    Eliasson, Anders
    Global Process Development, Höganäs AB, Höganäs 263 83, Sweden.
    Jönsson, Pär
    Global Process Development, Höganäs AB, Höganäs 263 83, Sweden.
    Prediction of particle size for water atomised metal powders: parameter study2012In: Powder Metallurgy, ISSN 0032-5899, E-ISSN 1743-2901, Vol. 55, no 1, p. 45-53Article in journal (Refereed)
    Abstract [en]

    This work aims to investigate how some significant atomising parameters influence the mass median particle size d50 of water atomised metal powders. More specifically, these were water pressure, melt flowrate, water jet angle, liquid metal viscosity and surface tension. Existing models for the prediction of d50 during water atomisation were reviewed. The selected models were fitted and compared with atomising experiments of liquid iron containing 0.5–4.4%C. Experimental results and model calculations were used in a parameter study to investigate how the different parameters influenced d50. The effect on d50 was large for the water pressure, medium for the viscosity and low for the melt flowrate and surface tension. Model calculations indicate that the jet angle has a large effect on d50, which should be verified by additional studies. The model proposed by Bergquist (B. Bergquist: Powder Metall., 1999, 42, 331–343) showed the best agreement with the current experimental data.

  • 3.
    Persson, Fredrik
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Process.
    Hulme-Smith, Christopher
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Process.
    Jönsson, Pär Göran
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Process.
    Particle morphology of water atomised iron-carbon powders2022In: Powder Technology, ISSN 0032-5910, E-ISSN 1873-328X, Vol. 397, p. 116993-, article id 116993Article in journal (Other academic)
    Abstract [en]

    Water atomisation can produce metal powders faster and at lower cost than gas atomisation, but it is well known that the powder particles are irregular and may contain a large number of pores. The current study analyses three iron-carbon alloys with different superheats, produced as powder by water atomisation and compares the particle shapes and porosity in each. The alloy with the most carbon (4.2 wt%) showed the highest circularity (0.72) for 20-40 µm particles, but the lowest (0.59) for 180-210µm particles. This is consistent with collisions between droplets affecting particle shape. The lowest-carbon melt (0.22 wt%) solidified fastest, so underwent fewest collisions and showed similar circularity for all particle sizes. The breakdown of water to form hydrogen and the formation of hydrogen bubbles was the most likely cause of porosity. The findings of this study may be used to inform future water atomisation process design to control particle shape and minimise porosity.

  • 4.
    Persson, Fredrik
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Process Metallurgy.
    Jönsson, Pär G
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Process Metallurgy.
    Eliasson, Anders
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Process Metallurgy.
    Influence of liquid metal properties for water atomised metal powders2012Manuscript (preprint) (Other academic)
    Abstract [en]

    The main focus of the present study was the influence of liquid metal properties on the particle size during water atomisation. Experiments for liquid iron showed that alloy additions of carbon and sulphur decreased the particle size. Moreover, it was indicated that the reduced d50 value at increased %C and %S may be related to a decreased viscosity and surface tension respectively. An alternative mechanism could be that raised superheats at increased carbon contents increased the total available time for atomisation. This may also have decreased the particle size. The influence of surface tension and viscosity on the d50 value was further analysed with a theoretical d50 model proposed in a previous work. 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.

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