Casting answers & advice.

Q A customer took a sample from one of our iron castings and sent it to an independent lab. The report from this lab shows the chemistry to be out of specification, but our own internal quality assurance controls indicate that the chemistry is OK. What could account for this difference?

Answer: There are a number of potential +reasons that could create a discrepancy in test results between laboratories. The following are several possible explanations.

Sample--Verify that the cast sample tested by the independent lab was representative of the casting in question. ASTM E59 "Method of Sampling Steel and Iron for Determination of Chemical Composition" is a reference for good sampling technique.

Specification--If the independent lab's analysis of an element falls close to, but outside of, the alloy chemistry specification limits, this could be due to deviations associated with analytical reproducibility and the heterogeneity of the casting. Many standard specifications provide a greater permissible variation for a product or a check analysis than for the heat or ladle analysis. Check the material specification with which you are working. Also, reference ASTM A751.

Standards--Many chemical analysis methods require the use of calibration standards that are similar to the material being tested. If the outside lab was not given the rough chemistry of the sample, or if the lab does not have appropriate calibration standards, their analysis may be significantly invalid. If in doubt, ask the lab for a description of the method of analysis and the calibration standards used in the analysis.

The information submitted in this column was supplied by the AFS Special Irons Committee (5-D.)

Q Does it matter which side of the filter paper is used during the methylene blue test? Will the test results vary if we are not consistent?

Background: The methylene blue (MB) clay test measures the amount of live clay present in a sample of molding sand. The test is performed by mixing a sand sample with a chemical solution and placing a drop onto a piece of filter paper. The industry-accepted method for performing the MB clay test (as outlined in the AFS Mold & Core Test Handbook-3rd Edition) calls for using Whatman #50 hardness filter paper or equivalent.

This question is raised by a sand laboratory technician. The technician says that one side of the filter paper used for the MB test is smoother than the other and claims that there is a difference in the test results between the "top" or "bottom" side of the filter paper. The technician claims that the test results will vary by 1-2 mL (0.034-0.068 oz).

Answer: Although there may be a slight difference in the smoothness of the filter paper, using one side or the other should not affect the results that are generated. To verify the results, a sample test was run using both sides of the same piece of filter paper. The paper was cut in half, and one half turned over so that tests were performed on each side of the paper.

The test results (Fig. 1) show no detectable difference in the halos generated on either side of the paper. Even if there were a slight difference, such as 1 mL (0.034 oz) as in the question above, this would translate to only a 0.2% difference in MB clay. Few facilities are capable of controlling their sand systems to this degree of accuracy when it comes to MB clay, so this difference would not be significant.

FIGURE 1 OMITTED

The information submitted in this column was supplied by the Cast Metals Institute.