Fig. 3. Shown are the level averages for the effects on WT/ST. Gas Defects: Gas defects can occur
in steel castings by two mechanisms. If gas pressure at the mold/metal interface is higher than metallostatic pressure, gas can push into the liquid metal to create blow defects. If the mold atmosphere contains gasses that are soluble in the liquid metal, the steel can absorb additional gas like hydrogen or nitrogen that may come out of solution during solidification.
Testing Procedures Testing was conducted using low
carbon steel (ASTM A216 WCB) to produce step-cone castings (Fig. 1). Cores were made using two different sands (W410, AFS GFN of 51 and W460, AFS GFN of 34) and phenolic urethane nobake (PUNB) binder levels of 1% (considered typical) and 2% (to simulate higher LOIs as would be expected with mechanically reclaimed sand). No additives or coatings were used for the baseline cores. Castings were poured at 2,950F
(1,510C). Following cooling and shakeout, the risers and gating were removed and the castings were sec- tioned and sand blasted. Te castings
Fig. 2. Eight different additives were used in the experiments.
Fig. 4. Shown are the level averages for the effects on tensile strengths.
were then rated on a five-point scale for veining, penetration and surface finish (Table 1). Differences between the surface
structures of the casting were imme- diately noticed. Te two castings
produced with the W460 coarser sand had much rougher surfaces and contained burn-in or trapped sand grains. Te two castings produced with 2% binder showed more pearlite in the surface layer, indicating surface carburization. Experiments were conducted to
compare red and black iron oxide to six other materials and engineered sand additives (Fig. 2) at two different levels. • RIO: Red iron oxide or hematite (Fe2O3) with a fine powder.
• BIO: Black iron oxide or magne- tite (Fe3O4) with a relatively fine powder, but coarser than RIO.
• SX: A synthetic ESA consisting primarily of iron oxide, with a grain size similar to sand and a rounded particle shape. Typical addition rates are in a range of 2-8% and it reportedly has low impact on mold/ core strength.
• Additive V2: A mineral ESA with some red iron oxide that is a flux- ing material with a lower impact on mold/core strength than RIO or BIO.
• Additive I900: A mineral ESA with some red iron oxide.
December 2015 MODERN CASTING | 39
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