Case Study - Powder Metal Hardness TestingThe powder metallurgy (PM) process offers manufacturers the ability to produce high volumes of complex parts in a cost effective manner. Many PM components go through heat-treat processes producing parts with increased strength or wear resistance. Traditional static indentation testing, such as Rockwell and Vickers hardness tests, is time consuming, subject to human interpretation, and damages the part to some degree. One major PM fuel pump gear manufacturer has integrated an automated eddy current testing solution from Criterion NDT in their production line to test 100% of their components for proper heat treat conditions. Situation
A major PM fuel pump component manufacturer required an in-line solution to provide test results as accurate as those from Rockwell hardness testers when inspecting their PM rings and stars. The system had to provide 100% inspection and accommodate the hardness inspection of two lines simultaneously; including 15 star sizes, 10 ring sizes and three lobe geometries for each size. The test had to be easy to configure with a minimum of set-up changes. The overall goal was to reduce both warranty and scrap costs. Solution Eddy current testing has been widely used with traditional metal components to verify heat-treat conditions, confirm proper case depth and verify alloy content, in addition to finding component flaws and defects. A compact eddy current testing system was installed directly downstream of the heat-treat furnace. The system consists of a multi-channel, multi-frequency eddy current instrument, two eddy current hardness testing coils (one for stars and one for rings) and two sorting stations (one per lane). This provides 100% component testing for up to 60 parts/minute with the ability to physically reject out of tolerance components. Two tests were conducted to develop a correlation between a measured Rockwell hardness (HrB) and the eddy current hardness values. First, 38 sets of 10 samples (380 samples) were tested with eddy current. Next, all 380 samples were sent through a normal Rockwell hardness tester. The eddy current hardness values and actual Rockwell hardness readings are compared in Fig. 3. Twenty-two suspect and failure specimens were tested as well.
From this initial test the correlation between Rockwell hardness and eddy current hardness was very good, showing only about 1~3 HrB points variation. Eddy current readings (solid blue line) tended to show a more consistent value when compared to the Rockwell readings. This was readily shown with the results from a sample production test. During this test, nearly 35,000 samples were tested with eddy current. Random samples were taken and had Rockwell hardness checks completed to verify the results. The eddy current coils inspect more of the component’s surface area to compute a more accurate result. The Rockwell hardness readings were only accurate to ±1.0 HrB while the eddy current testing provided more consistent results. Benefits The eddy current testing solution allowed the PM manufacturer to achieve their quality inspection goals while maintaining production throughput and reducing labor. Because the eddy current instrument captures and stores all of the testing data, it could allow them to identify manufacturing problems helping with their continuous improvement processes. To 100% inspect and insure that all parts are above the specifications, I believe there is no other way to provide zero defect products than to use an eddy current test system. Manufacturing Project Engineer About Criterion NDT USA based Criterion NDT specializes in providing engineered eddy current application solutions for the non-destructive testing of critical or essential components. For more information please visit http://www.criterionndt.com/
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