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.

Situation

The fuel pump component manufacturer required an in-line solution to provide test results as accurate as those from Rockwell hardness testers when inspecting 100 percent of their powder metallurgy rings and stars. The system had to provide 100-percent inspection and accommodate the hardness inspection of two lines simultaneously, measuring 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 setup 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. At this company, a compact eddy-current testing system from Criterion NDT 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-percent component testing for up to 60 parts per 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 figure 1. Also tested were 22 suspect and failure specimens.

Figure 1: Comparison of Rockwell hardness values to eddy-current hardness values

From this initial test, the correlation between Rockwell hardness and eddy current hardness was very good, showing only about 1 to 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 powder metallurgy 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.