Understanding and Minimizing Gas Defects in Iron Castings

Subsurface porosity, oxides, and entrained gas produce some of the most aggravating defects encountered in casting production.

These defects are difficult to control and may not be found until castings are machined. A subsurface blow hole in a ductile iron casting just above the solid shell core is illustrated in Figure 1. This hole was found after sectioning at the site that x-rays revealed as a low-density region. Real-time x-ray evaluation showed that the solid-shell core began to evolve gas when impinged by the liquid iron and continued to do so even after the casting cavity was full. (See the real-time X-ray video at the University of Alabama at Birmingham School of Engineering Materials Science and Engineering Dept. site.)

Methods have been developed at the University of Alabama at Birmingham (UAB) to measure the volume and rate of gas evolved from both green sand and bonded sand when molten metal impinges on mold and core materials. The measurements can provide assistance to foundries by measuring the effects of sand permeability, type and amount of binder, additives, and washes on the gas generated and its ability to escape.

Gas evolution from an uncoated epoxy-acrylic (SO2) bonded core is illustrated in Figure 2. The cores were made with an AFS 45-55 silica sand containing 1.63% epoxy-acrylic binder. An average of 650 cm3 of gas was evolved in 100 seconds, with larger amounts from the higher-density cores. The core, having a density of 93.7 lb/ft3, evolved less gas because of the lower organic content.

A list of the results for both coated and uncoated cores is shown in Table 1. (see p.30). The standard deviation of the peak rate of evolution for both coated and uncoated cores was 1.4 cm3/sec, which represented less than 10% scatter in the measured evolution rate.

The gas-evolution rate peaked about 8 seconds after immersion at a rate of 15 cm3/sec, with the more dense cores producing gas at a higher rate. The shape of the gas-evolution rate curves indicate that there are at least two reactions producing gas, perhaps solvent evaporation, and binder decomposition.

Triplicate data on similar cores coated with a commercial, water-based mixed refractory core wash is illustrated in Figure 3. The coated cores evolved more gas, about 830 cm3 compared to 630 cm3 for the uncoated cores, and the peak rate of evolution was about 65% higher at 25.4 cm3/sec compared to 15 cm3/sec from uncoated cores. Clearly, the coating is evolving a lot of gas immediately after iron impact.

Less dense cores absorb more wash solvents than more dense cores, resulting in incomplete drying and higher amounts of evolved gas. The total volume of gas produced from the coated core with the lowest density was about 880 cm3. This value was higher than that of the most-dense washed core, which was approximately 845 cm3. The more open structure of the less-dense core allows deeper penetration by the wash solvents. Thus, core density is an important factor in coating application and permeability.

There are several reasons to put a wash on a core including:
(1) improving the casting surface finish; and,
(2) resisting passage of gas from the core into the casting.

However, some core washes produce gas, and consideration must be given for allowing all of the gas to exit from the core without blowing into the casting.

Eliminating gas defects can be a frustrating task. Moreover, the large number of sand/binder/additive/wash systems available, as well as newly developed ones, just adds to the confusion of discovering the culprit of gas defects. The techniques developed for measuring gas evolution can be a valuable tool to help minimize gas defects.

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