the specimen was in compression against the hot surface. However, cracks and fractures became evident after blowing away the loose grains of sand at 0.07 MPa (10 psi) air pres- sure (Figure 24). These tensile cracks propagated from the back side and behind the heat-affected zone.2
These findings
were confirmed in casting trials. Physical Changes
Prior to TDT, each specimen was weighed. Following TDT the surface of the specimen was blown with 0.07 MPa air pressure to remove any loose sand grains. The specimens then were weighed again, and the percent change in mass was recorded (Tables 4 and 5). Next, the specimens were visually examined for signs of thermally induced cracking of the surface, loss of sand where contact was made with the hot surface, and any other discolorations (Tables 4 and 5). If the refractory coating broke down, it could indicate the tendency to produce cuts and washes, erosion/inclusion type defects. In interpreting this data, it is critical to identify the compo- nents causing the change in mass. The percent change in mass was calculated based upon the difference between the weight before and after as a percent of the weight before. All percent change in mass values represents the percent- age of weight lost (Tables 4 and 5).
Observations from the heat-affected zone on the surface of tested specimens revealed that the uncoated specimens had visible sand loss- es and crack propagation. For all uncoated specimens tested, the hot surface/specimen interface showed a crater with black discol- oration due to binder degradation. In addi- tion, sand binder losses were evident at the hot surface/specimen interface where binder bridges pyrolyzed and sand grains broke loose. The loose sand at the hot surface/speci- men interface was white. Expansion cracks were macroscopically evident on the uncoat- ed specimens and only evident on refractory coated specimens for the finer sand distribu- tion (710) because the coating layer was thin (Figure 24). This would indicate that a refrac- tory coating has to be engineered to offer the necessary thermal resistance.
For the refractory-coated specimens on the medium (520) and coarse (430) sand dis- tributions, no cracks appeared on the coat- ings, but the opposite side had discoloration similar to uncoated specimens. These re- fractory- coated specimens remained intact and had little change in mass after TDT and 0.07 MPa air pressure. See Figure 24 for examples of typical refractory- coated and uncoated specimens before and after TDT.
18
Figure 20. Thermal distortion tester.
Figure 21. TDC for uncoated specimens. Table 4. Thermo-Mechanical Properties of Uncoated PUCB
Table 5. Thermo-Mechanical Properties According to Total Coating Thickness
International Journal of Metalcasting/Spring 11
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