Quality Magazine

Quality Test & Inspection: Evaluate Materials with Eddy Current

October 1, 2004
Eddy-current technology has become an important contributor to quality assurance for the automotive industry.

An eddy current system can test a variety of parts and materials. Photo: ibg NDT Systems Corp.


The rapid growth in performance requirements of modern automobiles and trucks has brought about a more stringent demand for quality control of automotive materials and components. Eddy-current inspection can be an effective way to ensure these requirements are met.

Primary applications are nondestructive verification of material properties resulting from heat treatment and alloy processes and detection of surface cracks and flaws resulting from rolling, forming, machining and finishing and heat-treat processes.



Material property testing

Modern eddy current instruments that use digital electronics to nondestructively test material properties are reliable, repeatable, have high resolution, and are easy to implement and maintain. Properly applied, multi-frequency test protocols extend the scope of the eddy current method, increase test reliability and make the method easy to implement.

Determination of optimum setup and calibration procedure is facilitated by a built-in computer. During calibration, parts known to be metallurgically correct are tested at a multitude of frequencies, producing locus curves for the alloy and structural characteristics of the parts. A minimum of 15 to 20 parts are necessary to provide a sufficiently broad statistical base of allowable production variables. At each test frequency, the scatter of readings is displayed and tolerance zones are created to encompass readings at selected frequencies. Size, shape and position of each tolerance zone is established, calculated and drawn by the computer.

In production use, parts are tested at up to eight selected frequencies. Parts are only accepted if the measured values meet each and every tolerance zone. If the part fails to meet just one criterion, it is rejected. The principal advantage of this method is that a variety of metallurgical anomalies may be detected; hardness variations may be detected at 1 kilohertz (kHz), material mix may better be determined at 12.5 kHz, case depth at 20 hertz (Hz) or decarburization at 63 kHz. Most important, unexpected mixed structures from significant heat-treat process errors are detected and sorted from production. Unwanted mixed structures include retained austinite, untempered martensite, bainite, pearlite and ferrite in their various combinations.

Electronic processing technologies have reduced testing time. Typically, a part can be tested at eight different frequencies in less than 100 milliseconds (0.1 second).



Surface flaw and crack testing

Use of the eddy current method to detect cracks, pores and other surface flaws in critical automotive components is increasing because it is reliable, repeatable, easily automated and provides cost savings over magnetic particle and other methods. It is particularly applicable to test finished machined parts, although there are many applications on as-formed parts.

Eddy-current instrumentation for crack detection functions differently than eddy current material properties testing. It is best characterized as high-speed, high-precision surface scanning. A probe that sequentially senses small sections of the surface-a 1 millimeter or less diameter area-must be moved over the surface area with high precision to reliably detect small cracks and flaws. High precision means that the probe orientation to, and distance from, the metal surface being tested must be maintained within prescribed tolerances. Also, the rotation rate of the part being tested and the scan rate of the probe must be monitored (error proofed) so to guarantee that the surface area is completely scanned with no skipped areas.



Testing critical parts at the factory

Eddy current instruments can be effectively introduced into production lines to provide semi-automated or fully automated 100% testing. Test decisions are clear cut. The human factor is eliminated. An example is the testing of safety critical automotive components such as velocity joint spindles.

In this application, forged parts are subject to final inspection for material mix and heat treatment prior to shipment. An eddy-current sorting unit, operating in the preventive multi-frequency method, is used. Parts are manually placed into a fixed coil with centering provided by a wear-proof plastic insert to protect the coil from damage. The presence of the part in the coil is automatically sensed to initiate the test cycle. If the part meets all criteria, it is accepted and a slide gate opens below the coil to allow transfer to the shipping container. If any criteria are not met, the gate remains closed and an audible alarm and red indicator lamp are initiated. The reject part has to be moved by the operator. Testing can be resumed after canceling the alarm.

Measurements are performed at eight frequencies ranging from 25Hz to 25 kHz. Variations in heat treatment and material alloy are detected.

In another example, ball races in hubs are 100% inspected to verify correct hardness, case depth and hardness/

case depth location from induction hardening. The automatic, in-line test also verifies correct tempering of base material and use of correct alloy. Parts fed by conveyor belts into the test system are tested by two inside diameter (I.D.) test coils, one coil for each ball race.

Eddy current is also being used to test cast-iron cylinder liners for cracks. Prior to shipment, the system scans both outside diameter and inside diameter cylindrical surface areas to detect cracks and surface open pores. This is automatically done at a throughput rate of 400 parts per hour.

Another company uses eddy current to inspect tempered nuts. Nuts are high-volume parts in automatic automotive assembly lines. If nuts are of incorrect material or incorrectly tempered, and therefore too brittle, the consequences can be severe. The eddy current system had to test up to 180 nuts per minute to verify correct tempering and the use of the correct alloy.

Parts fed from a bowl feeder into a rotary indexing table are pressed over a flat test coil where the heat treat and alloy are verified. If approved, a sorting gate is activated. If defective, the gate is not activated and the parts go to the reject bin. Additionally, at a separate station, the presence of the thread is checked with a test probe that is inserted into the nut.

These examples demonstrate that modern, digital eddy current instruments specifically designed to test automotive components can be reliably used to reduce production and testing costs, while increasing components integrity and moving toward zero defects for component material properties and for surface flaws and cracks. Q