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The powder metallurgy (PM) process offers manufacturers the ability to produce high volumes of complex parts in a cost effective manner. Many powder metallurgy components go through heat-treat processes producing parts with increased strength or wear resistance. To ensure the quality of the heat treat process, powder metallurgy components are traditionally tested using static indentation tests, such as Rockwell and Vickers hardness tests. Such tests, however, are time consuming, subject to human interpretation, and damage the part to some degree. That’s why one major powder metallurgy fuel-pump gear manufacturer has integrated an automated eddy current testing solution into their production line to test 100 percent of their components for proper heat treat conditions.
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Caveats for Eddy Current Automating Hardness Testing
While I am sure this method is a good way to do high production hardness tests in a cost effective manner, any user should be aware of the following;
Eddy current testing used here is a process control method. This is not a HRB(W) Rockwell test as defined by ASTM E18 in indentation hardness testing. It is a comparison test. In all cases of comparison tests, the user must ensure that both tests correlate well. This correlation may or may not be true on two different materials tested. In this case with the powder metallurgy, both Eddy Current and HRB(W) tests must be conducted prior to production for that particular product.
The chart does show good correlation though I am not sure the Eddy Current is more accurate or consistent, since it is not the same test. The sensitivity of movement can be different on other methods and scales.
Most parts have a product specification. If a part must be certified to a particular test ( in this case HRB(W)), all parties should be aware when an alternate or comparable test is done such as Eddy Current. Documentation can be done to demonstrate reliable results. It is probably more important that the Eddy Current test ensures that process is good and that the part will not fail in use.
Automated Hardness (Material Structure) Verification
I concur with the previous statement. Eddy current testing is a "non-destructive" comparison inspection method and should be used in conjunction with traditional Rockwell tests for heat treatment verification. It is an augmentation to the quality control process by providing a method to evaluate 100% of the products manufactured "in-line".
When properly setup, eddy current is very sensitive to material structure variations resulting from heat treat process disruptions. This can dramatically reduce the risk of having a large quarantine issue with products that are found to be suspect using Rockwell testing, by detecting and physically sorting them out immediately.
In addition to improving shipped product quality, eddy current testing can provide a real cost savings in both time and money by reducing the amount of scrap product typically generated with traditional destructive inspection methods, especially in facilities with high volume manufacturing.
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