a sufficient distance from both the upper and the lower faces of the bars and thus the temperature measured is not affected by the temperature conditions of the bar faces. The measure- ment was conducted using type PtRh10-Pt thermocouples, dia. 0.35 mm (0.013 in), in silica glass tubes of dia. 4 mm (0.157 in). Temperatures were always measured in one of the two castings of dia. 20 mm (0.78 in), 30 mm (1.18 in), and 50 mm (1.96 in) while the other castings of these diam- eters were used to prepare specimens for structure analysis and for tests of mechanical properties.
The shell mould temperature is measured using jacketed thermocouples of the type of “K” NiCr-Ni, dia. 2 mm, po- sitioned in the isotherm direction in pre-moulded openings in the shell mould, at a distance of 60 mm (2.36 in) from the upper face of the bars. The openings are always made af- ter the third layer of shell moulds, i.e., approximately 2 mm (0.078 in) from the casting wall. The thermocouples are con- nected to the measuring computer by compensation lines.
Based on theoretical analysis, a considerable temperature dif- ference was expected to exist between the inner and the out- er sides of shell moulds. For this reason a thermocouple was placed on the inner and the outer sides of the shell mould of the dia. 50 mm (1.96 in) bar. A total of 7 locations are measured in one shell mould. The layout of thermocouples is clear from Fig- ure 11. The temperatures measured in the metal are designated M50, M30, and M20, the temperatures in the shell mould S20, S30, S50inner
, and S50outer (“M” metal, “S” shell mould).
about 200°K (360°F). A consequence of this asymmetry is the shift of heat axes of individual castings towards the cluster axis.
and S50inner
Figure 12 gives an example of measuring on a thermally non- insulated mould. What is important from the viewpoint of op- eration is the rapid decrease in the temperature of non-insulated shell mould in the interval between removal from the furnace and pouring, in the given case at a rate of 0.6 to 1.1 K/s. An effect of the rapid cooling of the shell mould prior to pouring is that any change of this interval may lead to a change in the course of the solidification of castings. Measurements confirm a considerable difference between the mould temperatures on the inner side and the outer side (curves S50outer
); it is
Thermal Insulation of Shell Moulds
Measurements were performed on some moulds, the purpose of which was to establish the effect of thermal insulation on the solidification of castings and on the temperature field of shell moulds. Both insulated and non-insulated moulds are used in practice. The thermal effect of insulation cannot usual- ly be determined exactly and it is chosen empirically. Thermal insulation reduces in particular heat transfer by radiation into the surroundings and, partially, also cooling by convection.
The test moulds were insulated by means of the “Sibral” (a 6.5 mm (0.255 in) thick ceramic fiber blanket) in two ways (see Figure 13).
With the whole periphery insulated, the heat removal by ra- diation from the outer side of the shell mould into the sur- roundings is slowed down while the mutual irradiation of individual parts of the mould remains preserved. When indi- vidual castings are insulated, both radiation in the direction of the surroundings and mutual irradiation of castings in the shell mould are reduced. With methods, reduced cooling in- tensity and increased temperature symmetry over the cross- section of castings can be expected.
Insulating the mould results in a significant reduction of the cooling rate of hot shell mould prior to pouring, roughly to one half, i.e., approximately 0.55 to 0.65 K/s. Therefore at the instant of pouring, the mould temperature is on the whole higher and more homogeneous. The differences in the cool- ing rates of thick and thin walls of castings get smaller, and the solidification time gets markedly longer (in this case it is about twice as long). The difference between the shell mould temperatures on the inner and outer sides of the castings gets smaller and the temperature homogeneity along the periphery of castings is thus increased (Figs. 14 and 15). (Within the experiments conducted, no significant difference between the two insulation methods was established as regards the cooling rate of castings or the temperature fields in shell moulds.)
Figure 11. A schematic diagram of a thermocouple layout in a metal and a shell mould.
76
Figure 12. Temperature curves for a non-insulated shell mould.
International Journal of Metalcasting/Spring 10
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