Date

April 1988

Document Type

Dissertation

Degree Name

Ph.D.

Department

Dept. of Materials Science and Engineering

Institution

Oregon Graduate Center

Abstract

Vacuum arc remelting is the most important and dominant remelting process in the production of double vacuum melted super alloys and titanium alloys for the aerospace industry. Also there is an increased demand for larger and better quality vacuum arc melted ingots of these alloys. However, as the size of the ingot increases, the tendency for segregation increases. This is due to deeper molten metal pool, deeper mushy zone and longer local solidification time. These in turn are influenced by the electrode melt rate and the cooling condition of the ingot. The segregation can be minimized either by decreasing the melt rate or by improving the heat transfer from the ingot. The former technique results in decreased productivity and increased energy consumption during melting. Hence, the latter technique is preferred. The heat transfer from the ingot can be increased by introducing a gas of high thermal conductivity like helium in the shrinkage gap between the ingot and crucible. This technique was patented and is being used by superalloys manufacturers, but, no systematic study has been carried out to quantify the effect of helium gas pressure. Hence the present experimental work was under taken to study the effect of helium gas pressure on the heat extraction rate, metal pool depth, solidification structure, mushy zone size and segregation. The experiments were carried out in a laboratory vacuum arc furnace. Temperature measurements were made on the outside surface of 165 mm diameter crucible by imbedding 18 electrically insulated, chromel/alumel thermocouples along the length of the crucible. The heat flux distribution on the outside surface of the crucible was then computed, and an increase in helium gas pressure resulted in an increased heat flux removal by the cooling water. A theoretical model for heat transfer by gas conduction between the ingot and crucible was derived from the first principles and the heat transfer coefficients were calculated from the model at various gas pressures. The effect of helium gas pressure on molten metal pool depth, dendrite arm spacing (DAS), mushy zone size, Laves phase distribution and segregation for Inconel-718 were studied. The increase in helium gas pressure resulted in a decreased molten metal pool depth. For example with 60 mm of helium gas pressure, the pool depth decreased from 200 mm to 127 mm i.e. about 36% decrease. The calculated mushy zone sizes using the DAS measurements showed decrease in mushy zone size with the helium gas cooling. Also the amount Laves phase formed was decreased with gas cooling. Hence, with gas cooling technique between the ingot and crucible, it is possible to minimize segregation in large size ingots.

Identifier

doi:10.6083/M40R9MBR

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