These findings support previous research that indicated the refractory coatings offered protection at the heat-affected- zone. Previous research indicated that refractory coated specimens show faint cracks that diminished with higher percent solids.2,4
Conclusions and Recommendations
Specific coating preparation methods were identified as culprits in the variability of the coating properties being measured. The results show the shortcomings of the cur- rent practice of using Baumé as a sole control measure. Baumé when performed in a controlled laboratory envi- ronment, tracks well with certain coating properties, but fails to identify the coating property that must be con- trolled during application. It must therefore be used with one or more additional tests, such as Hercules, surface tension, and % solids. If used in its current practice, cor- rections through dilution should be made which may not address the need for surface tension or rheological ad- justments which have been shown through this work to significantly impact the thickness of the proud and sub- surface coating layer.
The modified permeability and MQI tests were successful in tracking small changes in the venting characteristics of the uncoated and coated PUCB disc-shaped sand speci- mens. The changes in surfactant level affected the thick- nesses of the coating layers deposited in the proud layer and in the interlocking interfacial layer of the specimens at the specified dip times. Permeability and MQI test- ing succeeded in detecting and tracking these changes, particularly those changes in the thickness of the proud coating layer and the dry coat weight. The overall depth of refractory coating penetration decreased with increased surface tension, which is the result of a re- duction in the surfactant level. This implies that in a given coating, the dilution parameters are important in refractory coating control since dilution affects the sur- face tension of the coating. The level of surfactant was found to influence wetting time. This subsequently af- fects the thickness of the proud and penetrated coating layers. The difference in thicknesses of coating (proud and penetrated interlocking interfacial layers) depos- ited as a result of surfactant levels and/or dip times did prevent thermal distortion in the PUCB disc-shaped sand specimens.
For a given dip cycle (dip time, depth, and shake) and sand distribution, the three coating properties important to maintaining a consistent coating layer thickness were found to be solids, surface tension, and viscosity. This study also showed that sand distribution significantly affects the thickness of the proud and interlocking in- terfacial layers. Non-destructive coating thickness mea- surements are needed to assist foundries in monitoring and controlling wet and dry coating deposition.
International Journal of Metalcasting/Spring 11
The improper dilution of the wet coating impacts coat- ing solids, viscosity, and surface tension. Lower surface tension increases the depth of coating penetration. As coating penetration increased, the thickness of the proud coating layer decreased. Thermal expansion increases with the thickness of the proud coating layer. The results therefore demonstrate the importance of coating control to thermal distortion. Previous work has shown Baumé to track with coating solids but not surface tension. There- fore, adjustments to coating solids to maintain a desired Baumé, could lead to an erroneous adjustment resulting in more distortion. This study reemphasizes the need for Baumé to be performed in a controlled laboratory envi- ronment with additional control tests.
The refractory coating penetration varied for the chemi- cally bonded sand specimens with respect to sand distri- bution and surfactant level. Further, the graphitic coating transferred heat back into the chemically bonded sand disc that promoted distortion. The thermo-mechanical changes brought forth are in the forms of TDC, mass loss, and cracks on the surface of the test specimens (refrac- tory-coated and uncoated). The refractory coating being studied, prevented and/or reduced thermal distortion on certain underlying sand distributions. The elevated tem- perature and pressure did promote distortion for the re- fractory- coated and uncoated PUCB specimens. The ability to control thermal expansion would result in the
Figure 22. TDC comparing various coating thicknesses.
Figure 23. Specimen sectioned after TDT showing expansion of coated layer at the heat-affected-zone.
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