Using Specialty Sand Blends to Reduce Veining Defects
Research was conducted to evaluate the effect of the addition of specialty aggregates to silica sand. SAIRAM RAVI AND JERRY THIEL, UNIVERSITY OF NORTHERN IOWA (CEDAR FALLS, IOWA)
M
etal casters continu- ally strive to produce the highest quality castings at the lowest
competitive cost. Unfortunately, they have little control of the cost of the required materials. Molding materials such as chromite, zircon and mullite all exhibit low expansion, leading to few casting defects related to expansion and better dimensional accuracy. Tey also have refractory values significantly higher than silica sand. Although some casting applications require the use of 100% specialty sands, a significant amount require only modest improve- ments to the properties of silica sand to yield significant casting quality benefits. Silica sand is the most widely used
aggregate in the metalcasting industry. Its low cost due to its abundance makes it an attractive option to metal cast- ers. However, steel and iron castings in silica-sand molds tend to exhibit defects such as veining and fins. Tis is, in part, due to the large thermal expansion of silica sand. Previous stud- ies into the high temperature proper- ties of silica sand have addressed the technical limitations metalcasters face
while using silica sand molds or cores. As shown in Fig. 1 and Table 1, silica sand undergoes various phase transi- tions while being heated up to high temperatures.
Once past the alpha-beta phase
transition at approximately 1,063F (573C), silica sand experiences a steady contraction till the cristobalite phase transition at 2,678F (1,470C). Sand additives such as iron oxide or engineered sand additives are used in
Fig.1. This shows the thermal expansion of foundry sands.
Table 1. High Temperature Phase Transitions of Silica Sand Phase Transition
Alpha-Quartz to Beta-Quartz Beta-Quartz to Beta-Tridymite Beta-Tridymite to Beta-Cristobalite Beta-Quartz to Beta-Cristobalite
Temperature 1,063F (573C) 1,598F (870C) 2,678F (1,470C) 2,678F (1,470C)
Density Change (Volume) 2.65 to 2.53 (+4.74%) 2.53 to 2.25 (+12.44%) 2.25 to 2.20 (+2.27%) 2.53 to 2.20 (+14.71%)
Linear Change
+1.56% (0.0156 in/in) +3.99% (0.0399 in/in) +0.75% (0.0075 in/in) +4.74% (0.0474 in/in)
February 2017 MODERN CASTING | 31
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52 |
Page 53 |
Page 54 |
Page 55 |
Page 56 |
Page 57 |
Page 58 |
Page 59 |
Page 60