Comparing Sand Additives in Steel Castings


Adjustments in additives and binders can lead to significant differences in mold properties and quality in steel castings. RALPH SHOWMAN AND ERIC SCHELLER, ASK CHEMICALS, DUBLIN, OHIO


casting, heat moves rapidly from the liquid metal into the surface of the sand and produces steep thermal gra- dients. As a result, the sand grains will expand and the organic binder in the mold will pyrolyze, producing carbon monoxide and other gasses that can cause defects. Sand additives are commonly used in molds and cores to reduce defects like veining, metal penetration and unacceptable surface finish. Histori- cally, iron oxides have been the addi- tives of choice, primarily red iron oxide (hematite, Fe2


W (magnetite, Fe3O4 O3


) and black iron oxide ). However, these


options can negatively impact the cast- ing in terms of cost and quality. Engi- neered sand additives (ESAs) have been developed that are promoted to lower costs, improve veining resistance and/or reduce gas defects, but little comparative data is currently available to the steel casting industry. A recent research project com-


pared additives and their effects on both the mold/core properties and the final casting’s quality, exploring which sand additive should be used for specific casting results and what, if any, tradeoffs can be expected on mold/ core properties. Experiments were conducted to measure the performance of several ESAs and red and black iron oxides. Te impacts of each additive on mold/core properties and casting qual- ity were measured in order to provide some insight on which sand additive should be used to achieve particular casting results.


38 | MODERN CASTING December 2015


hen molten steel is poured into a chem- ically bonded sand mold to produce a


Common Sand Defects Veining: Veining defects result


from the expansion and contraction in silica sand when it comes in contact with molten steel. Sand additives can modify these reactions, but they also can affect negatively mold/core strength, permeability and benchlife. Te challenge has been finding an additive with the best anti-veining qualities with minimal impact on other properties. Metal Penetration: Also known as


burn-in/burn-on in less severe cases, metal penetration occurs when liquid metal fills the small voids between the grains of a sand mold or core. Tis


bonds a layer of sand to the surface of the casting, increasing cleaning room time and cost. Penetration, which can be influ-


increase chemical penetration. Surface Finish: Surface roughness of a steel casting depends on the sand particles of the mold/core. Larger par- ticles will produce a rougher surface than finer grains. Te surface tension of the liquid metal and its ability to smooth over the small imperfec- tions on the mold/core will impact the eventual finish. Sand additives will affect the surface finish similarly to metal penetrations. Fine material will tend to fill in imperfections on the mold/core surface. Carbonaceous additives may positively impact metal surface tension while oxides and fluxes may reduce surface tension. Carbon Pickup: Steel castings can


SiO4


enced by sand additives, can occur by both mechanical and chemical mecha- nisms. An additive with a particle size smaller than the sand grains will tend to fill voids and reduce mechanical pen- etration. Other additives, such as iron oxide and fluxing ESAs, may promote the formation of fayalite (Fe2


) and


Fig. 1. The stepcone was inverted with the small end of the core on top.


pick up surface carbon created by the decomposition of an organic binder. Carbon pickup can be controlled through a number of different process variables, including binder type and percentage. Sand additives also can have an effect. Additives like iron oxide that release oxygen at casting temperatures can remove some of the carbon, while others may promote car- bon pickup if they contain carbon or if they reduce mold/core permeability and retard the escape of the carbona- ceous gasses.


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