Fig. 4. Percent loss (abrasion) at room temperature of systems A and B is shown as compactability increased.
once this interface has changed from the heat, its properties change, leading to increased sand friability. Tis often results in the creation of casting defects. In addition, the current friability test does not consider pressure created when pouring from height. Head pressure always comes from the molten metal when it is poured into the mold. Finally, the friability test does not
represent the ratio of metal to sand. If the ratio is wrong, the sand could be overheated, taking the moisture out of the sand and potentially altering the clay. Tese changes will cause the sand to become more friable. Terefore, the current friability test of two specimens being rubbed together for one minute at room temperature does not accurately depict what is happening in a casting situation. Te lack of information and realism in this test provide a rationale to improve this green sand test. Termal Erosion Testing: Te
thermal erosion test is similar to the friability test in purpose, but the information acquired is different. It measures the bulk surface abrasion characteristics of a specimen at ele-
Fig 5. Percent loss (abrasion) at elevated temperature of systems A and B is shown as compactability increased.
vated temperature. It records the time, temperature and amount of abraded sand. Te discoloration of the sand due to elevated temperatures—a sign of dead clay related to heat damage—is typically observed after the test. Figure 3 shows a green sand sample tested at various temperatures. Te room temperature sample remains black, the green sand tested at 572F (300C) has a gray color, and the sample tested at 1,292F (700C) appears light gray. When thermal erosion data is examined as compactability increases for the same level of clay in the green sand system, the percent green sand loss decreases at room temperature (Fig. 4). However, more losses in green sand system B (100% Southern bentonite) occurred for every com- pactability level tested but especially at the lower compactability levels. When green sand systems A and B
were tested at an elevated temperature the same general trend was seen as tested at room temperature. As com- pactability was increased, both systems showed less loss. However, the major difference was that a greater percent
loss occurred at all the compactability levels but the losses were less drastic with green sand system A (Western bentonite) (Fig. 5). Modified Cone Jolt Test: Tis test measures bulk brittleness and is related to difficulty in pulling deep pockets in a pattern. Te modified cone jolt toughness test is directly related to compactability and measures the sand’s ability to absorb energy. A computer and data acquisi- tion system is used for controlling, monitoring and plotting graphs of jolts versus displacement of a specimen. A standard AFS cone jolt specimen is placed between the base and the cone. When the test is initi- ated, a solenoid is cycled to automat- ically pick up and drop the specimen, while a linear voltage displacement transducer measures longitudinal displacement. Te data acquisition system automatically logs and plots the jolts versus displacement curves. Te test stops automatically when the specimen splits or displaces 0.05 in. (1.25 mm) vertically. As compactability increased, the
Fig 6. Ultimate strength (number of jolts) of systems A and B increased as compactability increased.
38 | MODERN CASTING September 2015
Fig. 7. Toughness (area under jolt displacement curves) of systems A and B increased as compactability increased.
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