The good, the bad and the porous.

Q We use the visual comparison method to rate sectioned samples for hydrogen level to evaluate solidified reduced pressure samples. But recently, we have had problems where apparently "good" gas samples still allowed our customer to find unacceptable porosity levels upon machining. What could account for the difference in results?

There are several possible causes for the problems you have identified, and they deal with your sampling processes, the way you are performing the reduced pressure test (RPT) and variations in the samples themselves. Improving your current testing technique or implementing newer testing procedures can eliminate those difficulties.

If your vacuum tester is in good condition and well maintained, an initial source of trouble could be in the sampling practice. Great care should be taken to preheat the sample cup adequately, collect the sample from a clean surface without agitation, transport it to the vacuum tester with minimum loss in temperature and use an insulated pad under the sample cup. Also, the size of the sample should be kept as close as possible to a target weight, as changes will influence the cooling rate and the size of the bubbles in the sample.

A constant vacuum level should be used for the evaluation, and it should be adjusted daily for the current atmospheric pressure. Too often shops will pull down to a constant target vacuum level of 26 or 27 in. of mercury, without adjusting for changes in the ambient pressure. This can introduce errors in actual chamber vacuum levels equivalent to more than 2 in. of mercury and will make the test more or less critical depending upon high- or low-pressure weather systems moving through your area.

After the sample has been collected and cooled, errors can be introduced by not polishing the cut surface of the sample. Depending upon the level of gas you are targeting in your process, a smeared sample surface can hide a lot of small indications that might prove to be a problem for your customer.

A further problem with sectioned samples is that the bubble density is not always the same throughout a typical RPT sample. Multiple cuts through the same sample can reveal rather wide ranges in rating numbers, which may further be compounded by differences in individual reader interpretations.

Better Under Pressure

To combat these difficulties, much of the metalcasting industry is moving toward the use of specific gravity measurements rather than the subjective method of examining cut and polished surfaces. Weighing the sample in both air and water, followed by a simple calculation, develops a quantitative number that may be used for process control of your degassing/gassing operations. Equipment is also available to automate the measurement and generate the specific gravity number in less time than is required to cut, polish and rate sectioned samples.

For additional information on the general subject of reduced pressure testing, see the AFS Special Report "The Reduced Pressure Test--A Practical Approach" in the AFS Library.

AFS Technical Department, Schaumburg, Illinois