MARKETING MATTERS CAST TIP
Wet Tensile Test Provides Early Warning T
AFS TECHNICAL DEPARTMENT, SCHAUMBURG, ILLINOIS
he green sand system is complex and fluid. A variety of tests are required to define
sand properties and the critical pa- rameters of the system. Typical sand tests include compactibility, moisture, green compression strength, perme- ability, methylene blue clay, AFS clay, grain fineness and loss on ignition. However, some metalcasters have identified sand related casting prob- lems when these tests do not indicate any change in the system. Te wet tensile test is sensitive
to changes in the sand system and can identify more quickly than any other test potential trends that can lead to casting defects, such as scabbing. Many metalcasting facilities use the wet tensile test as an early warning system to inves- tigate changes in the system (Fig. 1). Lower readings correlate to an increase in defects. Used in this manner, the test can be a useful tool to reduce scrap, augment the methylene blue test for clay control and help reduce bond consumption.
Using the Test Te wet tensile test examines
green sand under elevated tem- peratures to replicate the mold- metal interface. Te test creates a
Fig. 1. The percentage of scrap due to surface defects is shown charted against wet ten- sile strength test results in an iron casting facility. When the wet tensile strength is higher, the scrap rates are lower.
condensation zone similar to what is seen in the mold, then measures the strength of the condensation layer in the mold. In the basic test, the sand is heated, moisture is driven back, a condensation layer forms and the fracture point is determined. Except
for some correlation with loss on ignition and pH, wet tensile test re- sults do not always correlate to other sand tests. Following are the test’s basic steps: 1. Preheat a wet tensile tester to the desired temperature and set the heat- ing time.
2. Prepare a sand sample according to the equipment manufacturer’s recom- mended procedure and place it into the specimen tube.
3. Place the specimen tube assembly into the tester and activate the test machine.
4. When the test is complete, examine the break (Fig. 2). Te sample should fail with an even break through the wet layer, just below the top edge of the sample tube. A complete solid plate of sand should lift off the sample.
Fig. 2. A typical 2 x 2 sand test sample used in the wet tensile test is shown. The wet tensile test replicates the moisture and temperature zones that occur in the mold at the interface with the hot metal.
5. If the break is uneven, repeat the test, increasing or decreasing the heating time, until an even break through the wet layer is produced. 6. Record the results.
November 2011 MODERN CASTING | 55
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