TESTING 1-2-3
Eliminating Veining Defects
Simulation software and lab testing methods were used to pinpoint why the defect occurs and how to avoid it. A MODERN CASTING STAFF REPORT
C
asting veining or finning defects appear as projec- tions in the form of veins generally occurring per- pendicular to the casting
surface, either singly or in networks. Tey 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
ADDING IT ALL UP Breaking down the latest research is as easy as 1-2-3.
“Causes and Solutions to Veining Defects in Iron and Steel Castings” Jerry Tiel and Sairam Ravi, University of Northern Iowa, Cedar Falls
Background—The veining defect in cores and molds stems from a tensile stress exerted at the mold metal interface by a combination of contracting and expanding sand. 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. Procedure—Step cone iron and steel castings produced in molds consist- ing of various additives were tested and compared with the surface strain predictions of the casting simulation model. Results and Conclusions—Two forces act on the surface of the sand that can either contribute to or reduce the veining defect. Sand additives that provide liquid on the surface of the sand grain and favor formation of tridymite or cris- tobalite and greater expansion of the sand can reduce veining defects.
30 | MODERN CASTING July 2014
Northern Iowa Metal Casting Center recently conducted studies to deter- mine the mechanisms that cause vein- ing 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. Te results were shared in the paper, “Causes and Solutions to Vein- ing Defects in Iron and Steel Cast- ings,” by Jerry Tiel and Sairam Ravi of the University of Northern Iowa, Cedar Falls, Iowa.
Question What sand factors contribute to
veining defects and how can the defect be avoided?
1
Background 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. Tis situation is made possible by the loss of sand volume after reaching 1,063F (573C). Te 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. Tis differentiates the defect from the scab- bing 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. Engi- neered sand additives work by causing
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