Casting answers & advice.

Q We use the reduced pressure test (RPT) to measure gas porosity in our aluminum melt. We compare the porosity in the sample against a wall chart that shows different amounts of porosity, but our customers want a more definitive method for verifying gas porosity levels. What else is available?

Background: The RPT has been used for decades to detect and measure hydrogen gas in aluminum alloys. It is a quick and simple shop-floor test. Used with regularly calibrated systems, the test results are easily repeatable and correlate well with casting quality.

RPT uses a sample of molten aluminum that is cooled under reduced pressure, which encourages pore and gas bubble formation so that the solidified sample contains an exaggerated amount of porosity and the pores are clearly visible. Samples typically are sectioned in half with the cut surface sanded or polished to reveal the (exaggerated) interior porosity and then visually examined against a chart that shows samples with varying porosity levels.

The RPT testis sensitive to test parameters and technique. All procedures must be the same each time the test is run to compare results. The quality and temperature of the metal, sample size, time to the reduced atmosphere, pressure used and interpretation of samples must be consistent. Vacuum control is a requisite for consistent results and the equipment should be regularly calibrated.

Recommendations: The chart that is commonly used with RPT assigns a numerical assessment to quantify the porosity level of the melt, As a visual test, this procedure is subjective. A more quantitative evaluation can be obtained by calculating the specific gravity of the sample and correlating that with the dissolved hydrogen porosity content of the melt. The sample is weighed in air, then weighed in water. The weight in air is divided by the weight in air less its weight in water.

Although this test is influenced by water temperature and cleanliness, the test is accurate and provides a more quantifiable result than the visual evaluation. The specific gravity is typically included on the chart and can be easily correlated to the numerical evaluation.

For a review of RPT test procedures, see The Reduced Pressure Test--a Practical Report, AFS, Des Plaines, IL (2001). For a more complete EPT evaluation, see Development OJA Statistically Optimized Test Method For The RPT, L. Parmenter, 1998 AFS Transactions (9818), AFS, Des Plaines, IL (1998).

Q Our aluminum foundry melts in crucible furnaces and has been experiencing poor operating life from our crucibles. When a failed crucible is removed from the furnace, we find that the outside wall is distorted and the glaze looks like it has melted and turned into foam. What causes this decreased life?

Background: In today's foundry, crucibles are regularly used for nonferrous melting. Their manufacturing method is similar to pottery as the raw materials for aluminum melting crucibles--silicon carbide, graphite and a bonding agent--are mixed together in a muller and then formed the same way a potter forms a piece of pottery. The green crucible is baked in a kiln, cooled, a glaze is applied, and then the crucible is put back into a kiln for "firing.

Crucible life in the foundry is determined primarily by melting equipment and procedures, so the individual foundry can often extend the life of crucibles by improving crucible care and eliminating the factors that cause premature failure,

Recommendations: The problem described sounds like a flux attack, Most fluxes used in molten aluminum production contain alkaline salts that, under certain conditions, can attack crucible walls. Crucibles are porous at operating temperatures and the flux can migrate through the crucible wall toward the hottest point- the furnace combustion chamber.

The classic sign of a flux attack is an outside wall that is distorted and a glaze that has an almost foamy look, sometimes greenish in color. To minimize flux attack:

* use the minimum amount of flux required to clean the melt. Overfluxing is the major cause of flux attack on crucibles;

* never put flux in the bottom of an empty crucible or onto unmelted scrap in the crucible. The fluxes melt at a lower temperature than the aluminum and will react directly with the crucible wall;

* control melt and furnace temperature. Higher furnace temperatures accelerate the migration of the flux through the crucible wall;

* keep the sidewalls of the crucible free of slag build-up. This slag contains flux material that is released when heated, quickly penetrating the crucible wall.