pressure). A 20.3-cm head height for cast iron with a density of 0.69 g/cm3
provides a head pressure of 0.0138 MPa (head height metal density). The load used during TDT was 442 g and was calculated to represent a 20.3- cm cast iron head height (contact area of TDT hot surface head pressure). This particular head height was used to represent a head pressure typical of medium size iron castings.
The computer and data acquisition system was switched on for controlling, monitoring and plotting graphs of tempera- ture/time versus distortion. The temperature was controlled using a K-type thermocouple at the hot surface. The test piece was inserted into the pivoting holder (gimble) designed for holding the disc-shaped specimen. The test piece was then au- tomatically raised until direct symmetrical contact was made with the 2.-cm diameter hot surface. This simultaneously en- gages the linear voltage displacement transducer (LVDT) that measures the distortion longitudinally. The data acquisition system automatically logged and plotted the distortion/tem- perature versus time curves. The duration of the TDT was 90 seconds (specified by the AFS 4F Steering Committee); how- ever, this can be varied. During the test, the predetermined load chosen to represent the force of molten metal pressing against the mold/core wall presses on the gimble, in contact with the circumference of the specimen, which presses its top against the 2-cm diameter hot surface. Any downward move- ment of the gimble is recorded as expansion (and appears as upward movement when plotted). Any upward movement of the gimble, due to the specimen becoming plastic and distort- ing, is recorded as plastic distortion (and appears as down- ward movement when plotted).
For longitudinal distortion it is possible to differentiate be- tween expansion (DE) and plastic distortion (DP) separately from the thermal distortion curve (TDC). In this investiga- tion, the authors chose to record the total distortion (TD), simply stated as TD = ΣDE + ΣDP.
Findings:
Thermal distortion data is presented in Tables 4 and 5 ac- cording to sand distribution and coating thickness (coarse, medium, and fine). The TDCs showed undulations that in- dicate thermo-mechanical and thermo-chemical changes in the both the coating layer and binder system at elevated tem- perature. The longitudinal distortion curves all showed an initial expansion (upward movement of TDC) before plastic deformation (downward movement of TDC). Note that a TDC for longitudinal distortion depicts an average for 10 specimens tested (Figures 21 and 22).
The uncoated specimens tested at 1000C showed a TDC that had some expansion followed by plastic deformation for the duration of the test (Figure 21). For refractory coated specimens tested at 1000C, there was greater expansion fol- lowed by lower plastic deformation when compared to the uncoated specimens respective to sand distribution. How- ever, the thicker the coated layer the greater the expansion (Figures 22 and 23). In TDT, stresses are developed in the disc-shaped specimen. More specifically, tensile stresses are developed on the back side and compressive stresses on the heat-affected side. On the uncoated specimens, the heat-af- fected zone revealed white unbound sand where the binder had been completely pyrolyzed but remained intact because
Figure 19. Surface coating at the two different surfactant levels with Coating thickness and penetration. International Journal of Metalcasting/Spring 11 17
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