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Economic industrial spray drying of ceramic slurries aims for as high as possible solids content. Investigated slurries of up to 80 wt% solids content were analyzed regarding stability while staying processable for granule production via spray drying.
Preliminary stability examinations were carried out on the one hand via zeta potential measurements and on the other hand by optical centrifuge analysis for determination of suitable additive type, quantity and composition while even allowing the detection of potential side effects. The processability of the slurry for spraying has primarily been quantified by viscosity measurements.
Early spray dried granules turned out to have internal voids and/or hard shells leading to defective sinter bodies and low density. Focusing on the root of these voids, the “hollow hard granules”, a controlled destabilization and flocculation was initiated by weakening electrostatic repulsion and approaching the isoelectric point. Destabilization, quantifiable by optical centrifugation, leaded to a change in speed of clarification as well as packing density, influencing movement speed of the phase boundary and the final height of the sediment, respectively. For sufficient destabilization, the solids content needed to be reduced in order to keep the viscosity suitable for the following spray drying procedure.
The versatile controlled destabilization of the ceramic slurry finally leaded to a significantly reduced fraction of hollow granules featuring a sinter body of higher density with smaller pores and a narrower pore size distribution, additionally this destabilization approach has shown to be transferrable with excellent results to zirconia and even ZTA (zirconia toughened alumina) composite materials.
Heat treated 9%Ni steel is considered the most suitable and economic material for construction of large-size liquefied natural gas (LNG) storage tanks which operate at cryogenic temperatures (-196°C). Strength above 700 MPa as well as a minimum impact value of 60 J are required to ensure reliable operation of the LNG tanks at operating temperature. Conventional arc welding processes, including shielded metal arc welding, gas metal arc welding, gas tungsten arc welding and submerged arc welding, are currently used in construction of LNG tanks. Ni based filler wire is the preferred filler metal of choice in LNG tank construction. The main problem with this choice is the lower mechanical properties, particularly tensile strength of the weld metal. To compensate, the wall thickness needs to be excessively thick to ensure the strength of the welded structures. Ni based filler material is expensive and a large quantity is needed to fill the multi-pass weld grooves. These factors significantly add to the cost in the fabrication of LNG storage tanks. For these reasons, exploration of new welding technologies is a priority. A big potential can be seen in laser based welding techniques. Laser beam welding results in much smaller fusion zone with chemical composition and mechanical properties similar to that of the base material. Laser welding is a much faster process and allows for a joint geometry which requires less filler material and fewer welding passes. The advantages of laser welding can help to overcome the problems pointed out above. Trials of autogenous laser welding, laser cold-wire welding and hybrid laser-arc welding conducted on the 9%Ni steel are presented in this paper. Chemical composition of the weld metal as well as effects of welding parameters on the weld formation, microstructure and tensile strength is discussed. Filler wire penetration depth as well as character of its distribution in the narrow laser welds was examined using EPMA - electron probe microanalysis.