Eddy Current Testing of Powder Metal Gears for Proper Heat Treat and Hardness

Eddy current testing is an approved non-destructive testing method for determination of proper heat treat and hardness conditions in gears, bearings, and numerous other automotive components.  Eddy current testing can determine whether a part has been properly heat treated, has the correct harness, and is composed of the correct alloy.  As the powder metallurgy process involves heat treat processes, the use of eddy current to test parts is a good match.

Eddy Current Testing Benefits

Eddy current is an electromagnetic test that is able to examine components at production line rates.  This allows the test to be integrated right into the production line allowing manufacturers to quickly identify improperly manufactured parts and reduce warranty costs.  Since the test is integral to the manufacturing environment, process failures are quickly identified greatly reducing scrap costs.  The eddy current systems are easily integrated with automated material handling systems to allow rapid and accurate sorting.  This is not possible with traditional off-line static indentation testing methods such as Rockwell and Vickers hardness tests.  Plus these tests are time consuming, subject to human interpretation, and may damage the part to some degree.

Eddy Current Powder Metal Gear Testing

A major powder metal fuel pump gear manufacturer required a method to test their powder metal ring and star gear components.  See Figure 1.  The test had to inspect 100% of the parts being manufactured on two separate assembly lines at the same time.  In addition, the test had to accommodate 15 star sizes, 10 ring sizes and three lobe geometries for each size with minimal set-up changes.  
Criterion NDT provided an eddy current test system consisting 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).  See Figure 2.  The system was installed directly downstream from the heat treat furnace.  The test was able to examine 100% of the components at a rate up to 60 parts per minute.  Out of tolerance components were rejected by the material handling systems.

Eddy Current and Rockwell Test Correlation

In order to develop a correlation between the eddy current and Rockwell hardness (HrB) test methods, two tests were conducted.  380 samples (38 sets of 10 samples) were tested with a Rockwell hardness tester and the eddy current test system.  Improperly hardened specimens were included in the test batch.

The initial test results showed a very good correlation between the eddy current and Rockwell hardness tests.  See Figure 3.  These results indicated between 1 and 3 HrB points of variation.  The eddy current test reading shown in blue, were more consistent than the Rockwell test results.  A sample production test of 35,000 parts showed that the eddy current readings were more consistent than the sample Rockwell tests.

Customer Benefits

The powder metal manufacturer was able to meet their quality inspection targets while allowing them to reduce testing labor and maintain production line rates.  The eddy current testing system also allowed them to identify production issues early adding to their continuous process improvements.

Figure 1 - PM Gears

Figure 2 - PM Sorter

Figure 3 - PM Results