Additionally, Phase I was used to determine the effective- ness of established coating control tests used by the paper industry as a means of monitoring the properties of foundry refractory coatings. The paper industry has developed a set of tests to tightly control their coating properties. Phase I succeeded in validating some of the concerns with Baumé as a coating control test to monitor and control the dilution of foundry refractory coatings. Two additional tests [surface tension (pendant method) and Hercules viscosity (leveling index)]1
were identified as applicable for testing foundry re- fractory coatings.
Coatings are supplied to foundries in a concentrated form and typically have separated (sedimentation) within their con- tainer (with oil on water on top and a thick mud mix on the bottom) while waiting to be used. The coating mixture must therefore be thoroughly mixed, stirred, and diluted to the re- quired percent solids prior to use. If foundries do not properly store, stir, and dilute their coatings the inherent variability in- creases the likelihood of casting defects. The improper prepa- ration of the coating prior to use is mostly due to the lack of proper testing and control methods for identifying the coat- ing adjustments required to produce a consistent coating. The Baumé test remains the most widely used test in the foundry industry for controlling the wet coating properties. Previous work by the AFS 4F Mold-Metal Interface Reaction Commit- tee determined that using the Baumé test as the sole method of coating control is inadequate.1, 6
The Baumé test demon-
strated a high rate of variation and inconsistency as the solids content varied in the mixture. The solids content of the mixed coating prior to application on a resin bonded sand specimen was confirmed to be a critical factor in proud layer and pen- etrated layer thickness. Furthermore, cracking of the coated layer due to thermal expansion of the underlying chemically bonded sand is related to % coating solids. These layers are engineered to withstand the extreme temperature and erosive characteristics of molten iron.2
It is widely accepted that the formulation of the refractory coating is the vendor’s responsibility, but controlling the consistency and homogeneity is the responsibility of the
In Phase II, various sand distributions were studied where the surfactant level in the ceramic-graphite refractory coating was varied together with dip time of the PUCB disc shaped specimens. Permeability and mold quality index (MQI) were investigated as nondestructive tests for assessing coating thickness. It should be noted that the permeability and MQI tests are simple to operate and could be easily adapted by any foundry. Furthermore, thermal distortion tests were per- formed to reveal the effects of elevated temperature on the coated sand specimens.
Methodology and Findings—Coating Study
The testing procedure consisted of eight major steps: 1) preparation of disc-shaped specimens, 2) preparation and characterization of the coatings, 3) coating thickness mea- surements, 4) robotic dip coating of specimens, 5) coating thickness measurement, 6) dip time study, 7) dry and wet coating comparison, and 8) permeability and MQI testing. (Note: All specimens were prepared and tested under con- trolled conditions. Ambient conditions were controlled; tem- perature was set at 21°C ± 1°C and relative humidity was set at 50% ± 2%.)
The experimental design was based on two factors at three levels each. The first factor was dip time which was varied at two, five, and eight seconds. The second factor was % sur- factant or surfactant level, which also had three levels; low (0.15%), medium (0.25%) and high (0.35%) on % volume. A minimum of three samples were prepared per dip time and % surfactant level.
Proud Layer
foundry. Even if we assume that the coating was properly stored and prepared for the application, no non-destructive test exists to obtain a consistent coating layer thickness ap- plied to cores and molds. A crude way to quantify the thick- ness of the coating layer is through destructive testing where the core or mold is broken and the thickness of the coat- ing layer is measured under magnification. This destructive technique is not the most desirable method for refractory coating control at the foundry.
Penetrated Interlocking Interfacial Layer
Sand Grain~300µm Figure 1. Schematic of the wet coating deposit. 8 International Journal of Metalcasting/Spring 11
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