In-Line Heat Treat and Crack Testing of Fasteners Using Eddy Current Non-Destructive Testing 

One improperly heat treated or manufactured component can have a significant warranty and public relations impact for an automotive supplier.  An improperly manufactured bolt that may have the value of pennies could result in a recall costing hundreds of dollars per vehicle and have significant safety implications.  Plus the manufacturer then has to halt production, find the root cause, and implement corrective action procedures. 

Implementing an in-line inspection system can validate the quality of every component shipped.  Besides catching defective parts before they leave the production line, the system can flag improper manufacturing processes catching problems early which can significantly reduce scrap costs.  Eddy current is a widely accepted in-line process inspection solution that can validate the quality of fasteners at production line rates. 

Eddy current is an electromagnetic test that is used to find material defects such as cracks and voids, as well as for verification of proper heat treatment and material structure (hardness and/or case depth) in metal components.  It is a clean, non-contact test that is used to inspect 100% of the products in an assembly line, keeping up with production rates and integrating with production material handling systems.  For these reasons, it is well suited for the testing of fasteners for cracks and proper heat treat.

 

Fastener Crack Testing

Typical fastener OEMs will perform crack testing on the flange of a bolt and on the radius of a bolt where the flange and shaft meet.   Figure 1 shows typical bolts that are tested using eddy current.

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Figure 1 – Bolt Tested Using Eddy Current

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Figure 2 – Bolt Tested Using Eddy Current

To perform a crack test on a metallic component, a small eddy current coil must be passed over the area to be tested.  In the case of a bolt, it is easier to rotate the bolt itself while keeping the eddy current coil stationary.  The bolt rotation mechanism is integrated into a material handling station that is integral to the production line.  Figure 3 shows a production bolt crack testing station with feeder, rotation mechanism and accept and reject chutes.  Figure 4 shows the testing station integrated into the factory line with eddy current test (ECT) station and associated electronics.

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Figure 3 – Eddy Current Bolt Crack Testing Station

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Figure 4 – In-Line View of Testing Station

 

Fastener Heat Treat Testing

Eddy current testing can effectively detect proper heat treat conditions and material structure of a fastener.  Unlike eddy current crack testing, the eddy current heat treat test is run using an encircling coil.  The fastener is dropped through the middle of the coil and is held while the test is run.  Figure 5 shows a typical eddy current encircling coil used for heat treat verification testing.  In this case, it is bolted onto the mechanism in the background which holds the part while under test, and opens to allow the bolt to pass through to the sorting chute after the test has been completed.  A typical testing cycle is one bolt per second, but will vary depending on the bolt size.

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Figure 5 – Eddy Current Heat Treat Testing Encircling Coil

Conclusion

Eddy current is a clean fast inspection technology that can inspect 100% of critical use fasteners in-line with the production process.  Eddy current inspection can be used to test for crack as well as improper heat treat conditions.  Testing times of one second per part are achievable with integral material sorting and handling systems.