measure the rate of deformation of a granular material and calculate the viscosity. The calculated viscosity is useful in describing the sintering characteristics of a granular material. An extension to the this method was developed to better understand the surface softening of sand particles. The surface viscosity measurement is based on the compaction of the sand particles while under a com- pressive load and constant heating rate. Sand particles, being porous, initially expand with temperature but subsequently contract due to softening and sintering at inter-
that can either contribute to or reduce the veining defect. Te linear expansion of bonded silica sand causes the volume of sand to increase sharply until 1,063F (573C), where it changes phase from alpha quartz to beta quartz. Upon further heat- ing, the sand loses its volume due to loss of binder volume and softening and rearrangement at the surface of the sand grains. Tis loss of volume at temperatures above 1,063F is the main cause of veining defects. As the temperature of the mold or core sur- face increases, the length and volume of the sand decrease. Te cooler sand directly beneath the surface increases in volume as it passes through the alpha to beta quartz transformation. Te combination of contracting sand on the surface with expanding sand directly beneath the surface creates tensile failures that fill with liquid metal forming the defect classified as veining. Sand additives that reduce veining defects provide liquid on the surface of the sand grain and favor formation of tridymite or cristobalite and greater expansion of the sand. Tis secondary expansion reduces the negative strain at the surface of the core and prevents tensile failure and the associated cracks. Trough
3 32 | MODERN CASTING July 2014
granular contact points. This behav- ior results from surface softening and deformation where the com- pressive load is concentrated. Since veining defects are common
in silica sand cores and/or molds, a casting simulation software model was created based on the expansion and contraction of silica sand as it was heated up to a high temperature to simulate surface strain. Step cone iron and steel cast-
ings produced in molds consisting of various additives were tested and compared with the predictions of the casting simulation software.
Fig. 2. Shown is the general layout of the dilatometer used in the study.
Results and Conclusions
Results from the tests
showed that two forces act on the surface of the sand
the fluxing action of sand additives, the surfaces of the particles adhere to each other, increasing the tensile strength of the sand on the surface of the core or mold. Tis increase in strength reduces the tensile failure on the sand’s surface and reduces veining defects. Te accepted methods of reducing
veining defects in castings are: Using low expansion aggregates
including chromite, zircon, olivine and ceramics to eliminate the differ- ence of expansion rates of surface and subsurface sections of cores and molds. Tese materials generally exhibit linear thermal expansion and little if any phase transformations. Teir refractory value is higher than silica sand, and therefore they show minimal soften- ing and volume loss. Te strain values at the mold metal interface closely match the subsurface strain values and therefore eliminate any mechanical forces that would cause tensile failures and veins. Blends of low expansion aggregates with silica sand have been successfully used to reduce or elimi- nate veining defects. Using sand additives containing
fluxes, such as iron oxide and lithium- based products. Fluxes decrease the temperature at which the silica starts to soften and provides liquid on the surface of the grains, increasing the reactivity and lowering the transition temperatures for tridymite and cristo- balite. Tese transitions force increases
in volume of the subsurface sand and reduce the vein strain on the surface of the core or mold. Tese fluxes can also sinter the sand together at high temperatures, effectively increasing their resistance to tensile failures as shown by increasing viscosities of bonded sands. Using sand additives containing organic materials such assaccharides or dextrin to provide a slight cushion- ing effect as the sand goes through the alpha/beta transformation, but mainly to act as a carbon source for a high temperature bonding of the sand. Any available oxygen in the mold cavity is depleted shortly after it fills with liquid metal. In the absence of oxygen, the organic materials break down to primarily carbon, which bonds to the surface of the sand grains, increasing the viscosity and tensile strength of the surface sand. Tis increase in tensile strength resists the vein strain and reduces veining defects. Often these materials are blended with fluxes and oxides to increase their effectiveness.
Tis article is based on a paper (14-030) presented at the 2014 Metalcasting Congress.
ONLINE RESOURCE
Find the original research papers at
www.moderncasting.com
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