THERMAL PROCESSES WHILE POURING INTO
CERAMIC SHELLS AND THEIR NUMERICAL SIMULATION J. Roučka, M. Kováč and M. Jaroš
Brno University of Technology, Faculty of Mechanical Engineering O. Šikula
Faculty of Civil Engineering, Czech Republic K. Hrbáček
Prvni brněnská strojírna Velká Bíteš, a.s., Czech Republic B. Podhorná
ÚJP Praha, a.s., Czech Republic Copyright © 2010 American Foundry Society Abstract
In the thermal regime of ceramic shell moulds the role of heat transfer by radiation and convection into the surrounding environment is most significant. In shells the walls are mutually irradiated and non-symmetrical heat flows in individual directions. Experimental measuring enabled establishing the heat relations during casting and solidification of a model system of cylindrical castings of different diameters placed on a ring gate. The N155 alloy (Cr-Ni-Co alloy steel) was cast. Thermocouples were used to measure temperature in the castings and in the shell walls. The experimentally established data used was significant in the
Introduction
The method of casting alloys into ceramic shell moulds by the lost wax process is mostly applied in the manufacture of castings of high dimensional precision. In many cases, however, castings are required that are of high internal ho- mogeneity, defined dispersiveness and directional structure orientation. This requirement is frequent in particular when making castings of alloys based on alloyed steels and nickel alloys for power generating machines and for automobile and aircraft industries. The morphology of structural com- ponents is markedly affected by the heat transfer dynamic during the solidification of castings and by the directional orientation of heat flows.
Heat Transfer Analysis when Casting Into Ceramic Shell Moulds
The shell mould is usually made up of several castings ar- ranged on a common gating system, forming a “cluster,” and usually embedding it in sand. The initial distribution of met- al temperatures after the shell mould has been filled is deter-
subsequent numerical simulation. The boundary conditions of heat transfer were calculated using FLUENT software. The actual thermo-physical and boundary conditions of the system were determined via inverse modelling by means of the ProCAST software. Using the CAFE module of the ProCast program the numerical simulation of the alloy structure was performed; simulated structures were compared with the metallographically established structure.
Keywords: shell mould, crystallisation, insulation, numerical simulation
mined by the temperature distribution in the shell mould at the moment of casting and by the method of filling the shell mould. Metal is cooled during casting mainly due to heat accumulation in the shell mould walls.
When the shell mould has been filled with metal, the heat of superheat and latent heat of solidification are removed. The
heat removed from the casting during solidification ∆Qmetal is in part accumulated by the mould ∆Qmould moved into the mould surroundings ∆Qsurr. = ∆Qmould
and in part re- + ∆Qsurr Therefore: ∆Qmetal
When pouring into thin-wall ceramic shells the ratio of the heat accumulated by the mould to the heat removed into the surroundings is essentially different than in the other found- ry technologies. While in the case of pouring into massive moulds made of aggregate materials or into metallic moulds the predominant part of the physical heat of the casting accu- mulates in the mould mass during solidification (Figure 1a), in the case of pouring into shell moulds the removal of heat into the surroundings comes much more into the foreground (Figure 1b).
International Journal of Metalcasting/Spring 10
71
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