Casting veining or finning defects appear as projections in the form of veins generally occurring perpendicular to the casting surface, either singly or in networks. They are not situated along primary parting lines. Veining can occur in any alloy but is primarily seen in either ferrous-based or copper-based metals. Although various studies have examined the defect, the basic causes have been elusive.
Researchers at the University of Northern Iowa Metal Casting Center recently conducted studies to determine the mechanisms that cause veining defects in iron and steel castings. Advanced testing methods along with computer casting simulations were used to evaluate various sand mixtures for the propensity to form casting veins. The results were shared in the paper, “Causes and Solutions to Veining Defects in Iron and Steel Castings,” by Jerry Thiel and Sairam Ravi of the University of Northern Iowa, Cedar Falls, Iowa.
What sand factors contribute to veining defects and how can the defect be avoided?
According to previous research, the veining defect in cores and molds stems from a tensile stress exerted by a combination of contracting sand at the mold metal interface and subsurface expanding sand. This situation is made possible by the loss of sand volume after reaching 1,063F (573C). The stress is created by an imbalance in the temperature at various points in the sand at differing distances from the liquid metal heat source that creates differences in the thermal expansion and related strain.
When the forces exerted on the mold or core’s surface exceed the high temperature strength at the surface, a tensile mode failure cracks the sand and allows liquid metal to enter. This differentiates the defect from the scabbing and buckling defects that exhibit overlap of sand surfaces from increased surface sand volume.
Much of the veining defect research has emphasized the determination and development of sand additives to reduce or eliminate the defect. Engineered sand additives work by causing two effects. The first takes advantage of a high temperature phase change that occurs in sand at approximately 1,598F (870C). Four main phases of quartz impact veining (Table 1). The first phase is alpha quartz, which is stable from room temperature up to around 1,063F (573C). The second phase is beta quartz. This phase of silica sand is less stable than alpha quartz, and there is a decreased viscosity from a solid indicating some surface softening. This 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 increases in volume. This resulting increased volume of up to 12% has been shown to reverse surface strain and thereby effectively eliminate veining defects in iron castings.