Multifrequency eddy current testing is being increasingly used for 100% inspection of heat treated parts. Although spot checks and statistical process control methods can detect slow
changes in a process, unpredictable defects occurring for only a short time are likely to go undetected. Only a method like multifrequency eddy current inspection can guarantee that all defective parts will be found. The computer-based nondestructive testing (NDT) instruments use 8 to 32 test frequencies to check hardness, case depth, and heat treat pattern; to detect material mixes and grinding abuse; and to determine whether parts have the required microstructure. Among the method's many applications are forgings, bearing balls and rings, fasteners, and a variety of automobile components, such as brake disks and parts for seat belts, transmissions, and steering systems. Testing of a power steering rack is shown in Fig. 1.

Why multiple frequencies?
In eddy current testing, a coil having primary and secondary (sending and receiving) windings functions as a transformer. The windings are positioned so that they are only loosely coupled. A small voltage is induced in the receiving winding of the "empty" coil by the magnetic field of the primary, sending winding.
When the part to be inspected is placed within the coil, or moves through the coil, the coupling factor between the windings changes. The extent of the change is primarily determined by the electrical conductivity and magnetic permeability (magnetic conductivity) of the material being tested. These two electromagnetic characteristics are strongly influenced by the structure of the part. Multidimensional analysis of signal amplitude and phase at the test frequency can provide a reliable indication of whether the part is "good" or defective.There is a correlation between relative permeability, urei, and magnetic field strength. Different materials (Fig. 2) and different microstructures of the same material have different urei vs. field strength curves.

Single-frequency tester:
For example, if a material mix is suspected between 100Cr6 (AISI 52100) and St70-2 (a C-Mn-Si constructional steel containing 0.55% C), a conventional single-frequency tester would be calibrated to distinguish between the two. However, a material mix between 100Cr6 and C45 (AISI 1043) wouldn't be detected because at the selected test frequency (and magnetic field strength), the permeabilities of C45 and 100Cr6 are roughly the same (see Fig. 2). Most single-frequency instruments operate at 50 Hz, 10 to 50 V. Some units do allow a choice of frequency. Test speed is slow. The instrument is calibrated using both "good" and "not good" (improperly tempered, for example) parts. All possible defect conditions can't be covered. Thus, defects for which the instrument has not been set up to identify will escape detection.

Multiple frequencies:
On the other hand, if the test is run using several different frequencies, reliable separation of "good" 100Cr6 from all other materials is possible. Adding frequency as a variable produces a 3-D diagram (Fig. 3, top), which features a prominent permeability "hill." There's a unique point (permeability value) on the hill for every combination of field strength and frequency. In multi-frequency eddy current testing, diagonal cuts are taken through the hill (Fig. 3, bottom) on the frequency vs. field strength plane. Different materials, structures, and defects have unique permeability hills. However, as previously noted, their permeability values may be equivalent or nearly so for some field strength-frequency combinations, but significantly different for others. Superimposing data for tests at different frequencies reveals any and all differences (defects). The number of different frequencies is important, but so is the distance between the lowest and highest test frequency. The ratio of the low to high frequency should be at least 1:1000, but 1:5000 is ideal. Both conditions must be met to realize the full benefit of multi-frequency testing.

Commercially available multi-frequency testers employ a very broad frequency band: a minimum of eight and a maximum of 32 frequencies, with one tolerance field per frequency, for ibg's eddy liner P, for example. Only "good" parts are used to calibrate the instrument. Their eddy current behavior is stored in memory, and valid tolerance fields are then determined automatically. The part being tested will fail inspection if a single signal falls outside of a tolerance field. Test speed is measured in milliseconds. Virtually all information about the mate rial can be extracted in a very short time — if the defect can be detected using eddy currents, it will be found.

Test instrument:
The eddyliner P's frequency range is 5 Hz to 300 kHz, which covers all applications for materials testing with eddy currents. It can test stationary parts or moving parts, such as those on a conveyor belt. Up to 10 parts per second (36,000 per hour) can be tested. Lower frequencies, especially those below 50 Hz, are used for deep penetration applications, such as assessing the depth of hardening to several millimeters. Higher frequencies are used for surface or near-surface conditions and for materials having low or no magnetic permeability. Depth of penetration is low, which facilitates checking for decarburization or surface hardness, for example.

Limitation:
All eddy current instruments provide only "yes/no" statements. Testing recognizes that a part deviates from a good part, but no reason for the deviation is given. That information must be obtained from rejects using conventional hardness testing, mechanical testing, and/or metallography. (Note: With 100% eddy current inspection it is possible to relegate destructive testing to setting up the hardening process and evaluating rejected parts.)

Application examples
Brief descriptions follow of several applications of multi-frequency eddy current testing using the eddyliner P. Note the compatibility between eddy current testing and induction hardening.

• Austempered seat belt lock parts are inspected for three defect states: unhardened, improperly hardened (out of the specified hardness range), and no uniformly hardened (harder on one side). In the latter case, a defective part might not be detected using conventional hardness testing if only properly hardened areas happened to be checked. Parts are conveyed one by one through the single inspection coil.

• Forged pairs of Cf53 (AISI1050) steel connecting rods are 100% eddy current tested after cooling from forging temperature. Test time is less than 3 seconds per pair. Inspection occurs as parts move through the coils (Fig. 4).

• An eddy current system for inspecting induction hardened shaft and toothed areas of AISI 1141 steel power steering racks is integrated into a two-lane hardening machine (Fig.1). In each lane, one part is tested at two test positions — shaft and toothed area — for hardening pattern, depth of hardening, hardness, and material mix. Eight frequencies, ranging from 25 Hz to 25 kHz, are used at every test position. The eddy current tester is linked to the heat treating system controller, which will shut down the line if several "not good" parts are detected in a given period of time. Cycle time of the tester is much shorter than that of the hardening machine.

• An automatic induction hardening system and eddy current tester are integrated for processing Cf53 steel constant velocity (CV) joints. A multiple-coil head is used to measure hardness and case depth at different locations on the part's shaft and bell. Destructive testing has been reduced to almost zero.

• An eddyliner P and a special handling system test and sort 100Cr6 (AISI 52100) alloy steel bearing balls for hardness and material mix at 36,000 balls per hour (Fig. 4, inset).

• In another bearing ball application, decarburization (depletion of carbides) on portions of the finish machined surface is detected using a test system equipped with encircling
  coils. The decarb, which leads to soft areas on the surface, is often caused by improper annealing of the wire from which balls are made. The tester's coils detect differences in hardness of 100 to 200 HV5. (Good areas have a hardness of 820 to 888 HV5; decarburized areas, 606 to 782 HV5.)

• A similar setup is used to inspect 100Cr6 steel bearing needles and rollers at a rate of 1 to 3 per second, depending on part size. The tester is also used to detect soft (improperly hardened) areas on 100Cr6 bearing rings measuring 5 to 300 mm in diameter. Rings are pushed through rectangular coils. Large-diameter parts are tested at several positions by the instrument.