Many companies carry out inspection spot checks during manufacturing to ensure that product quality is within the limits of established acceptability and that it is repeatable. The variation in product quality usually can be predicted or estimated by means of statistical analysis, and, therefore, it may be sufficient to conduct a certain number of spot checks to conclude the general nature of the quality from the test data. However, problems can arise in this quality control method if during manufacturing some conditions occur that are not subject to standard statistical distribution.
Simple methods like hardness testing cannot be used for reliable determination of the error, and spot checks are not reliable since an error can occur between spot checks. Generally, spot checks are only capable of detecting slow changes in the process. Unpredictable errors occurring for a short time are unlikely to be detected by spot checks. Modern process monitoring systems on induction hardening equipment allow detection of some of these errors, which can be isolated or in combination with others, while they develop. However, if several minor deviations accumulate, the process monitoring system may not immediately respond, while out-of-spec parts are being produced.
Eddy current testing offers an effective quality control tool in various applications including the induction hardening process. Eddy current preventive multifrequency testing (PMFT) is used to verify conditions such as case depth, structure and hardness pattern in induction hardened components including CV joints, gears, transmission shafts, hubs, spindles, cylinder liners, bearing parts and steering racks. Possible sources of induction hardening errors include incorrect austenitizing temperature and/or time and inadequate quench.
Eddy current testing offers a reliable method to monitor 100% of parts. Computer-based multifrequency eddy current test stations having 8 to 32 testing frequencies and up to 32 test positions greatly increases the efficiency of this testing method to monitor structure, hardening and case depth to detect unexpected out-of-spec parts, as well as to detect material mix.
Eddy current testing
A test installation basically consists of a coil with a sending and a receiving winding. The two windings are only loosely coupled. In the empty coil, a low voltage is induced in the receiving winding by the magnetic field of the sending winding. As a test part approaches the coil system, the coupling factor between the sending and the receiving winding changes, which is mainly determined by electrical and magnetic conductivity (permeability) of the test part. These two electromagnetic properties are strongly influenced by the microstructure of the test part. For example, the permeability of a part that is too hard is different than for an annealed part.
Permeability is the key variable of ferromagnetic materials for eddy current testing. Permeability actually means relative permeability, a number without any dimension, which indicates how much better a certain material conducts electric flux lines compared with conductance in air (air has the value of 1). A steel that can be magnetized has a value between ten and a few thousand. The correlation between permeability and field strength is not all linear. For a very small field strength, permeability is low (starting permeability), and increases with increasing field strength to a maximum value, after which it decreases to a smaller value. Every structure and every material has a characteristic permeability, which means parts with different heat treatments and different microstructures exhibit different permeability. Figure 1 shows the different permeability curves for different materials. For example, if testing is conducted at frequencies where C45 (AISI 1045) and 100 Cr6 (AISI 52100) have the same permeability, the two materials cannot be separated or distinguished. This could happen if material mix between 100 Cr6 (52100) and St70 (AISI 1070) is expected and the eddy current instrument is optimally set to cause this result.
Why use PMFT?
Eddy current testing offers the most economical and reliable method for 100% inspection. Earlier monofrequency systems are still used, but are limited to detecting large errors in heat treatment. Monofrequency systems commonly use a group of IN-SPEC and OUT-OF-SPEC (e.g., improper case depth) parts to set the instrument, not taking into consideration many influencing factors. It is virtually impossible to have master parts containing all possible defects to set the instrument. In addition, artificial defects are not as effective as real defects, and it is very difficult to simulate improper heat treatment to create such defects.
Further, monofrequency test instruments use 50 Hz for excitation of eddy currents. Some instruments allow switching from one frequency to another, but only a single frequency is used for testing. Test speed is slow because electronics used for evaluation are rather slow, and evaluation of test results usually is only one dimensional.
By comparison, modern eddy current test instruments based on the PMFT method operate in a completely different manner. A large number of test frequencies is used now because different defects cause different signals in eddy current test instruments, and only IN-SPEC parts are used to set the test instrument. Apart from the large number of frequencies used, it is important that a broad frequency range is covered; that is, the ratio between the lowest and the highest test frequency should be 1:1000 or higher to guarantee reliable testing. The use of advanced electronics components has reduced test time dramatically (Fig. 2). From a time point of view, it does not matter whether two or eight frequencies are used for testing (because testing is done faster), and it is now possible to test preventively. This means that all eddy current-distinguishable material information can be "read' using the eddy current instrument.
Another advantage is the multidimensional evaluation of modern test systems. A separate tolerance field is generated for every test frequency (Fig. 3). Only when all tolerance fields are satisfied can one assume that the part is IN SPEC. If a part is not IN SPEC in only one tolerance field, the part is classified as OUT Of SPEC. Any change in the low, middle and upper frequency ranges are displayed clearly.
The cost for an eddy current NDT system, apart from the criteria for test reliability mentioned above, influences the decision of which method will be used; e.g., nondestructive eddy current vs. sampling and destructive testing. Factors to consider for eddy current include the price of a test station, test instrument, auxiliaries, etc. Factors for sampling and destructive testing include costs for skilled testing staff, various nonproductive times (e.g., interruption of manufacturing for verification, supervision and sample tracking, scrap due to both destruction of parts and delayed adjustments in the manufacturing process and customer complaint.
Cost for equipment and staff can easily be estimated, while cost for organization and scrap are difficult to calculate. The eddy current test instrument (eddyvisor®P) and PMFT make it possible to lower the cost of series testing significantly. Destructive tests basically are used to make induction hardening machine adjustments, to occasionally examine hardened parts and to test OUT-OF-SPEC parts. With respect to series testing, reduction of test scrap and time savings are dramatic.
Two representative examples illustrate the benefits of automated eddy current testing via PMFT).
Induction-hardened steering racks. Figure 4 shows the eddy current test system used to check power steering racks. The system is integrated in the induction hardening line to automatically check two parts separately at two positions each (tooth and shaft areas) for correct hardness and hardening depth after induction hardening and annealing for hardening pattern, hardening depth and hardness. Testing is done at every position using eight test frequencies ranging from 25 Hz to 25 kHz, and OUT-OF-SPEC parts can be sorted out. Cycle time of the test instrument is much less than cycle time of the hardening machine. The test control panel is contained in a dust-proof steel cabinet (Fig. 5)
Induction-hardened CV joints. Figure 6 shows a cross section of an induction-hardened CV joint indicating the positions on the stem and races inside the bell that are automatically eddy current tested for hardness, case depth, core hardness and hardness runout on the stem. Test cycle time is 15 sec/part. Parts are lifted into the test position (Fig. 7) by the ID coil, which tests the bell ID, and into two test coils that test the two shaft positions (Fig. 8). At test position, each part is tested at three locations: on the shaft for hardness runout at the end; on the shaft for hardness, case depth and core hardness at the spline; and in the bell ID for correct hardness of the races. All three tests take less than 1 second. IN-SPEC and OUT-OF-SPEC parts are separated onto different conveyors. A graphic display of the test makes operation easy (Fig. 9).