Metal casters continually 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. They 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 improvements 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 casters. However, steel and iron castings in silica-sand molds tend to exhibit defects such as veining and fins. This is, in part, due to the large thermal expansion of silica sand. Previous studies into the high temperature properties 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 transitions 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 the metalcasting industry to either induce a tridymite transition which leads to a secondary expansion or induce the cristobalite transition at a lower temperature which causes a large secondary expansion. These additives caused large changes in the volume of bonded sand. Certain additives promote the sintering of the surface of the core and forms a partially melted surface. This increases the rigidity of the surface due to the increase in the viscosity of the sintered surface. The increase in viscosity at higher temperatures leads to higher strengths on the surface of the core, thereby resulting in reduced core distortion.
Previous research has shown the veining defect in cores and molds is a result of a tensile stress exerted at the mold metal interface by a combination of contracting sand and subsurface expanding sand. This tensile stress is created by the loss of volume observed in silica sand after the alpha-beta phase transition at 1,063 F (573C). This leads to a network of cracks formed due to the high thermal expansion of silica sand. These cracks then are filled by the liquid metal, which forms veins on the surface of the casting.
Specialty aggregates have a lower thermal expansion when compared to silica sand, which will prevent cracks being formed on the surface of cores and reduce veining defects. A major limitation is their high cost. For most applications, a small improvement in the high temperature physical properties of silica sand will be more viable for good casting quality.
Research was conducted to evaluate the effect of the addition of specialty aggregates to silica sand. The addition of two types of zircon and chromite were studied. The zircon sand utilized was a special domestic zircon sand which exhibited unique chemical properties not seen in other zircon sands. The use of this special zircon sand emulated the thermal expansion effects of commercial engineered sand additives while improving the sands refractory value. High temperature physical properties of the various blends were evaluated and test step-cone castings were poured and analyzed for most blends.
Discussion of Results
Figure 3 shows the thermal expansion results for the baseline silica sand sample. The sample can be observed to have a steady expansion leading to the alpha-beta phase transition at 1,063F (573C). A peak expansion of 0.01228 in./in. was observed at this temperature. After this transition, a steady contraction is seen leading to the cristobalite phase transition at higher temperature, where a secondary expansion can be observed. The beta quartz-beta cristobalite phase transition occurs at approximately 2,642F (1,450C).
A graphical representation of the temperature profile of a step-cone core at 5% solid fraction can be seen in Fig. 4. The temperature of the core surface at the thicker metal sections is already at 2,372-2,462F (1,300-1,350C). At this temperature range, the baseline silica sand sample is contracting after the alpha-beta phase transition. However, the subsurface, is still at a temperature range of 752-1,112F (400-600C) where the silica sand is expanding leading to the alpha-beta phase transition. This combination of contracting sand on the surface with expanding sand directly beneath the surface creates tensile failures that fill with liquid metal to form the defect classified as veining.
Figure 5 shows the expansion results for the silica with zircon blends samples. The peak expansion for silica with 10% zircon is similar to baseline silica sand. However, from 20% zircon onwards, a reduction in the alpha-beta phase transition peak expansion can be seen with silica with 40% zircon having the lowest peak of 0.005 in./in., which is lower than baseline silica by 56%. Another trend that can be observed from the thermal expansion of the blends is that the cristobalite phase transition induced at a lower temperature when compared to the baseline silica sand sample. At the 2,372-2,462F (1,300-1,350C) core surface temperature, the silica and zircon blend samples are already going through a secondary expansion. Though a steep contraction can be seen after the alpha-beta phase transition at 1,063F (573C), the large secondary expansion at a lower temperature negates the strain on the surface of the core at higher temperatures as both the surface and sub-surface of the core are expanding, thereby reducing the network of cracks on the surface of the core.
Silica with 7.5% chromite has an expansion profile similar to the zircon blends, with slightly higher temperature of cristobalite transition. The secondary expansion seen after the cristobalite transition is also lower. Silica with 10% chromite shows sudden steep contraction at 2,012F (1,100C) while silica with 30% chromite shows a small secondary expansion at 1,652F (900C) before a steep contraction at the same temperature. This steep contraction for the 10% and 30% chromite samples and the relatively smaller secondary expansion after the cristobalite transition may lead to the sand grains fusing to each other in steel castings.
Surface Viscosity and Specific Heat Capacity Results
The sintering temperature and the peak viscosity at sintering temperature for each sample are shown in Table 2, along with the associated specific heat capacity at 2,192F (1,200C). Baseline silica has a sinter temperature of 2,619.3F (1,437.4C) with a peak viscosity of 5.030 x 108 Pa.s (5.03 x 1011 cP). The sinter temperature of both the zircon and chromite blends decreases with increasing amounts of the aggregates. However, with the zircon blends, the peak viscosity increases with increasing amounts, while the peak viscosity in the case of the chromite blends decreases with increasing amounts.
Casting Quality Analysis
The baseline silica casting obtained is shown in Fig. 6. The casting exhibits several veins along the surface, which is typical of silica sand castings. No penetration defects are visible. More veins are formed along the thicker sections of the casting, where the metal takes longer to solidify. This would enable the cores to reach higher temperatures while the metal is still in its liquid form.
Silica with 10% zircon does not display any veining defects. Though the alpha-beta transition peak expansion for silica with 10% zircon is similar to baseline silica, the early inducement of the cristobalite transition, the secondary expansion and higher viscosity at sintering temperature leads to lower strain on the surface of the core, thereby reducing the veining defect.
However, silica with 20%, 30% and 40% zircon display slight veining and penetration defects at the thicker casting sections. Silica with 7.5% chromite (Fig. 7) displays no veining or penetration defects. Silica with 7.5% chromite displays a lower peak expansion at the alpha-beta phase transition temperature when compared to silica sand. Also, the cristobalite phase transition is induced approximately 374F (190C) lower and the peak viscosity is about twice as much as baseline silica. This would prevent failure on the surface of the core.
However, in the silica with 10% chromite and silica with 30% chromite castings a large extent of fused sand on the castings which increases with increasing amounts of chromite. Looking at the expansion and viscosity, we can see that silica with higher amounts of chromite undergoes a sudden contraction at around (2,012F (1,100C) unlike the steady contraction seen in silica with 7.5% chromite. Also, the peak viscosity decreases and the sinter temperatures drop by as much as approximately 212F (100C) for the higher chromite content samples.
Table 3 displays the veining ranking for baseline silica and the various blends. Silica with 10% and 30% chromite could not be evaluated due to the large extent of fused sand on the castings. A lower content of the specialty aggregates display better performance when compared to the higher content.
Baseline silica has a high veining index, as expected. Silica with 10% zircon and 7.5% chromite both display no indications of veining defects.
The quality of metal castings relates to the high temperature performance of the refractory aggregates used. This high temperature performance is determined by the thermal volume stability and resistance to high temperature softening.
Silica sand with its low cost and abundant availability does have limitations not easily overcome. Its high rate of expansion through phase transformations results in casting defects. The use of molds and cores produced entirely from specialty sands like chromite and zircon are expensive and in many applications do not warrant the increased cost. The use of sand blends of inexpensive silica sand with higher cost specialty sands has been shown to have applications where the quality of the casting can be improved without the associated high cost of 100% specialty sand cores. As little as 10% of specialty sands can improve the quality of the final casting by reducing the extent of veining defects, according to the research results. The effect of blending silica sand and specialty sands highly depends on the thermal input of the metal and the mass of the mold that determines the heating rate of the mold and associated cooling rate of the casting.
The chemical reaction between the base sand and the specialty sand must be accurately determined, as was the case of silica and chromite sand blends. Higher heat inputs in the larger metal sections caused the mixture to fuse, leading to casting defects.
Lower percentages of chromite sand improved casting quality in the test casting. This illustrates the importance of comparing the high temperature physical properties of the sand blend with the specific casting application before practicing the technology. ■
This article is based on the paper “The Use of Specialty Sand Blends to Reduce Veining Defects in Steel Castings” (16-060) which was presented at CastExpo16. Click here to see it as it appears in Global Casting.