Table 1. Phase Transformation Temperatures of Quartz Phase Transition
alpha-quartz to beta-quartz beta-quartz to beta-tridymite beta-tridymite to beta-cristobalite beta-quartz to beta-cristobalite
two effects. Te first takes advantage of a high temperature phase change that occurs in sand at approximately 1,598F (870C). Four main phases of quartz impact vein- ing (Table 1). Te first phase is alpha quartz, which is stable from room temperature up to around 1,063F (573C). Te second phase is beta quartz. Tis phase of silica sand is less stable than alpha quartz, and there is a decreased viscosity from a solid indicating some surface softening. Tis change occurs regardless of binder type. Losses in volume during this stage can range from 50 to 100% of the original length of the sample. If there is sufficient softening on
the surface of the sand grains to create a liquid, tridymite will be formed. Materials such as sodium, lithium or aluminum can force the phase change that is associated with linear change three times as great as the sands original alpha/beta expansion. One Engineered Sand Additive (ESA) that contains lithium has been shown to force tridymite transformation on sands resulting in high temperature
Temperature 573C (1,063F)
870C (1,598F) 1,470C (2,678F) 1,470C (2,678F)
Density Change (Volume) 2.65 to 2.53 (+4.7%)
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.)
Fig. 1. Transformation temperatures of silica phases are graphed.
tive in iron castings, the tridymite phase change and resulting increase in volume occur at too low of a temperature to prevent veining in steel castings. Looking at the expansion of silica sands which have additions of iron oxide, it can be seen that although they experience soften- ing, they do not change phase from beta quartz to beta tridymite. The loss of volume continues with increased tempera- tures until a change to
increases in volume. Tis resulting increased volume of up to 12% has been shown to reverse surface strain and thereby effectively eliminate vein- ing defects in iron castings. Steel castings are poured at higher temperatures and therefore the sand temperatures at the mold metal interface are higher. As the sands treated with ESA are heated above 1,922F (1,050C), they soften and lose volume (Fig. 1). This loss of volume at higher temperatures mim- ics the strain induced in plain silica sand setting up high surface stresses that cause cracking and veining in steel castings. Although very effec-
the fourth phase of silica. Tis phase consists of the change
from beta quartz to beta cristobalite and is associated with a volume change up to 14.7% occurring around 2,678F (1,470C). Tis increase mimics the effect of ESA at iron casting tempera- tures and reduces veining by reducing the tensile stress induced at the surface of the core or mold through expansion. Along with the cristobalite transfor- mation and associated expansion, the sintering point of the sand decreases. Unfortunately, the reduction of veining with the use of iron oxide may come at the high cost of room temperature tensile strength.
lack of the availability of appropri- ate equipment and methods to allow the thermal expansion of unbonded sand to be measured. To address the problem of measuring the ther-
2
Procedure The lack of data on the
elevated temperature per- formance of unbonded sand is most likely due to the
mal expansion of foundry sands, the University of Northern Iowa’s Metal Casting Center designed and built an updated dilatometer, capable of accurately measuring the thermal expansion characteristics of heterogeneous foundry sands. The dilatometer was designed robustly to provide the temperature range
and degree of accuracy required. Thermal expansion tests were run on bonded sand samples utilizing the university’s high temperature aggregate dilatometer (Fig. 2). An experimental method to
measure surface softening of small solid particles heated to high temperatures uses a dilatometer to
July 2014 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