Investigation into Melt Overflow of Alloys

Kalkanli, Ali (1992). Investigation into Melt Overflow of Alloys. PhD thesis The Open University.



The possibility of direct strip casting onto a single rotary chiller has been investigated. A number of alloys including Fe-Cr-Ni, Fe-Ni and Fe-Cr-Al were tested and strips were produced. Strip formation has been modelled theoretically studying two basic consideration: a) Liquid film formation prior to recalescence due to momentum transfer, b) Strip formation in a melt puddle by one dimensional heat transfer across the wheel/strip interface. Experimental modelling of the melt overflow process has been undertaken, to: a) investigate casting instabilities and associated limitations of direct strip casting and, b) reveal the effects of process parameters such as surface tension, viscosity and liquid metal depth on the final characteristics of the strip.

The melt overflow process has been employed for the production of strips, ribbons and fibres with thicknesses ranging between 150-700 μm. A special crucible(2.5 kg capacity) and plunger mechanism was designed and developed for producing up to 30 mm wide directly cast strip from a mini induction furnace. The most efficient pouring channel geometry was determined to minimize the heat losses within the channel and maintain sufficient hot liquid metal flow steadily during the melt overflow. The test material was based on 304 stainless Steel. The critical casting speeds and alloy compositions necessary to produce uniform strips without serrations were determined. Surface tension was modified by the addition of deoxidizing agents and sulphur. Surface tension values were measured by a modified oscillation droplet technique. The lowest value of surface tension in stainless steel was found to be 1.43 N.m-1 at a casting temperature of 1600 °C for sulphur at per cent 0.073. This alloy yielded strip with edge serrations and poor strip quality in contrast to an alloy with a surface tension of 1.98 N.m-1 at a casting temperature of 1580 °C with sulphur content 0.023. Uniform strips were obtained for this case.

The capillary number which is a ratio of viscous forces to surface tension forces in a liquid film were calculated for 304 stainless steel strips as 0.006-0.009 for corresponding thickness values 175-190 μm. High speed photography, video and specialised photographic techniques were utilised to examine Kelvin-Helmholtz instability and the temperature distribution during the residence time of the strip on the wheel.

As cast strip microstructures wore found to consist of dendritic and cellular solidification structures consistent with a cooling rate of 104 K.s-1.

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