Eddy-Current Testing Makes In-Line Inspection Easy
Many manufacturers are moving toward 100% product testing due to the high cost of failure in critical automotive and aerospace components. For example, one soft bearing surface or improperly heat-treated shaft in a large assembled system can lead to huge warranty costs if the part causes a shortened service life.
Instrument testing of surface-hardened parts is critical for checking quality because the results of the treatment are not visible to the naked eye. Traditional hardness testing, such as Brinell, Knoop, Rockwell and Vickers hardness tests, is time consuming, requires that the surface be perpendicular to the direction of the force applied by the tester, and damages the part to some degree. In addition, these tests may be difficult on curved or inside surfaces of complex shapes. Test accuracy also is affected by the surface roughness of the part, which can add surface-preparation time to the test. To optically measure the depth of hardening, the part is sectioned and etched to reveal the hardened case.
In high-volume manufacturing processes, parts can be produced at line rates higher than 60 parts/minute, which makes 100% testing impossible using traditional hardness and case depth measuring techniques. Sample testing can catch errors that are common to a product batch, but cannot catch random defects.
What is eddy-current testing?
Eddy-current testing involves introducing an alternating current (ac) magnetic field into a conductive material (metal) via an electrical coil, generating a low level electron flow (called an eddy current) within the metal. The flowing eddy currents create a secondary magnetic field that opposes the original ac magnetic source. Eddy-current instruments monitor the conditions of the two opposing magnetic fields.
In a typical application, an ac-driven coil is placed next to a known good part, and the eddy-current instrument is "nulled," or balanced, on the known good part. The coil is then placed next to an unknown part, and if the characteristics of the parts are different (due to differences in chemical composition, flaws or heat-treated condition), the instrument indicates different readings. Surface and subsurface material defects also are detectable using eddy-current testing. While different types of coils and inspection procedures are used, the basic eddy-current theory applies.
Differences to look for
Modern eddy-current instruments are sensitive to changes in a part's material properties such as electrical conductivity, resistivity and permeability, as well as differences in part geometry. Changes are detected by the eddy-current coils and are displayed as an impedance vector (Z); impedance vectors for a known good part are stored in the instrument and compared against test parts. Electrical conductivity is a measure of how well electrons move through a material. Because metals are conductors, it theoretically is possible to conduct an eddy-current test on any piece of metal. Permeability is a magnetic property that indicates how a metal will alter a magnetic field moving through it. Not all metals have significant levels of permeability. Metals having high permeability values (carbon steels, for example) restrict the area that can be inspected to the upper skin layer unless they are magnetically saturated. A large proportion of heat-treated metals are ferromagnetic, which makes eddy-current testing less able to detect deeper heat-treatment conditions. Case hardening depth in steel can be determined by measuring the case thickness. The case produces variations in magnetic permeability, therefore, the thicker the case, the greater the change in signal. The inductive reactance of the eddy current coil increases with increasing permeability.
Sensitivity to geometry differences depends on the type of probe being used. Standard terminology for surface scanning applications includes lift-off and edge effect. For encircling coil probe styles, the terminology includes fill factor and end effect. Instrument response to geometry is considered to be a limiting factor in the ability to detect true material changes. This type of response could lead to a poor signal-to-noise (S/N) ratio, so anything that can be done to limit geometry responses from the field of view improves the quality of the test.
Eddy-current testing is accomplished at ac frequencies from 10 Hz to the MHz range. Eddy currents have a limited depth of penetration due to a skin-effect phenomenon. One standard depth of penetration is the point at which eddy currents have diminished to 37% of the original surface value. Low frequencies are used to test carbon steel (which has a high permeability) to achieve increased depth of penetration.
Eddy-current probes house and protect the eddy-current coils, and modern probes use two or more coils per test. Some probes are shown in Fig. 1. A two-coil probe uses a differential pair (a test coil and a reference, or balancing, coil) to maximize the received S/N ratio (Fig. 2). Another probe design uses a differential driver pick-up configuration, where one coil creates the primary eddy current signal and another coil (or coil array) is used to detect small, localized changes in the test sample.
In hardness-testing applications, encircling and pancake coils are most commonly used. Encircling coils (Fig. 3) are designed to go around a part at a specific location (e.g., an automotive spindle), have a small part pass through the coil (e.g., a ball bearing), or to be inserted into a part (e.g., a cylinder ID). Optimum testing requires getting the coils as close to the part as possible to achieve a good fill factor.
A surface, or spot, probe (Fig. 4) is used to inspect larger parts, such as an automobile bumper. In this case, lift off and edge effects become critical test criteria. It is important to keep the probe as perpendicular to the part as possible. Testing near the edges of a part requires precise coil positioning.
The instrument energizes the driver coils, amplifies and processes the signals from the pick-up coils and shows results on a user-friendly display. Displays usually are shown in an x-y format similar to that of an oscilloscope. This captures both the magnitude and phase information of the signal change sensed by the coil. Earlier instruments using meter-type displays only indicated the magnitude of the response and did not provide traceable data values for test comparison.
Instrument set-up includes setting the drive amplitude and frequencies and receiver gain, filter settings and display controls. Both good and bad parts are analyzed and alarm-box labels are placed around clusters of good data points (Fig. 5). If a test sample exceeds the established alarm/accept limits during testing, it is considered to be a reject and triggers the instrument's industrial I/O as programmed. Individual signal conditioning and I/O configurations can be stored and recalled at any time. Increased throughput and performance is achieved by high instrument sampling rates. Data logging allows storing test data as parts are being inspected.
Eddy-current testing lends itself to high-volume manufacturing. Parts tested for proper heat treatment include automotive hubs and spindles, camshafts, gears, roller bearings, ball bearings, etc. Many of these parts are case hardened using induction heating. Induction hardening and tempering processes also are compatible with modern lean manufacturing processes, and, therefore, are easily incorporated into production lines. Eddy-current testing is used to perform 100% in-line hardness and case-depth testing of these parts. Eddy-current testing is a clean process, is operator independent and is able to automatically operate sorters and markers to segregate good and bad components without touching the parts.
The size and geometry of the part dictate how testing is to be accomplished, as they influence eddy-current probe design and in what manner the probe and part interface. The type of testing required also affects probe design and the instrument used for the test.
For medium-size components such as automotive hubs and spindles, custom probes are designed to bring individual coils in proximity to the area under test. Figure 6a shows a multicoil probe designed to test an automotive hub. The coils are positioned to measure the case depth and hardness at very specific locations. Figure 6b shows how the coils are positioned relative to the part. The coils are positioned to test the bearing races where the induction heating has been applied. The stainless steel jacket covering the coils offers protection for use in high-volume production.
All coils are simultaneously driven at multiple different frequencies. This ensures that a set of predetermined unacceptable conditions, such as shallow case depth or misplaced case, delayed quench and short or no heat treatment, is detected. While a single-frequency test may be adequate to identify problems, three or four frequencies typically are more than adequate to identify all required separations. Test frequencies typically range from 100 Hz to 25 kHz. Figure 7 shows the different groupings of heat-treatment conditions. The alarm box was created around the grouping of acceptable (green) responses, rejecting all other conditions.
Symmetrical parts such as ball bearings and small gears can be passed through standard-shaped encircling probes. A single-coil, single-frequency tester is suitable for this application. Automation can be achieved by connecting the instrument to a simple sorting chute or to a complex sorting mechanism. This type of test is easily integrated into a production line or performed in a bench-top application (Fig. 8). For extremely large parts such as auto bumpers, spot probes can be used with a portable hand-held probe for spot checks either in the field or on the production line.
To achieve 100% in-line hardness inspection of a product, it is essential to consistently position the parts in relation to the eddy-current probes. A part also might have to be picked up and positioned so that a probe can be inserted into the part. Fitting the probe to the part is fundamental to avoid the negative effects of liftoff, which can occur if the probe is tilted or raised away from the component. In an operation that runs 24/7, eddy-current probes must be sufficiently robust to last in the production environment. In many instances, probes are covered with stainless steel or ceramic covers to prevent damage to the coils.
Actual eddy-current testing times of 100 milliseconds are easily achieved. Handling and positioning of the component with respect to the probe can add up to a few seconds. This is consistent with most production line processes.
The material-handling system must be configured to handle rejected parts. For example, when the eddy-current system detects an improperly heat-treated part, the I/O on the eddy-current instrument is activated. The I/O can drive industry-standard industrial controllers, which can direct the component to a reject chute, mark the part, or simply stop the line while alerting the operator that a reject has occurred. Often, this prompts the operator to check a process upstream in the manufacturing line, which can greatly reduce part variations enabling corrective action/ adjustment to be taken before creating large and costly amounts of scrap.
As numerous parts might be manufactured on a single assembly line, it is important that the material-handling system be set up to handle different sizes and styles of probes. Modern eddy-current instruments have the ability to store numerous configuration files, which allows an operator to quickly call up the proper eddy-current test for the part being manufactured. Typical inspection stations are shown in Fig. 9.
Modern eddy-current instruments and probes allow manufacturers to perform 100% hardness and case-depth verification for the in-line manufacturing environment. The use of multifrequency instruments allows the best separation of multiple reject conditions. The use of custom probes and advanced instruments make for easy operation and integration into existing production lines.