May/June_AMP_Digital

FEATURE A D V A N C E D M A T E R I A L S & P R O C E S S E S | M A Y / J U N E 2 0 2 0 6 4 T oday, almost every company carries out spot checks during manufacturing to guarantee product quality. Within the framework of control, one of the most im- portant problems is to ensure repeatability of quality. Quality control faces major problems within this area, starting with supply of the material or products. Part properties such as structure, hardness, or case depth, which are determined by the heat treatment process, are subject to variations. To ensure the correct heat treat- ment results, a fast and efficient nondestructive test method is needed. One solution that has significantly increased the effi- ciency of testing is using computer based multi-frequency eddy current testing stations with eight standard test fre- quencies and up to 16 test positions. These machines can test the microstructure, hardening and case depth, and ma- terial mingle. One of the advantages of modern eddy current testing systems using preventive multi-frequency testing (PMFT) is the ability to detect unexpected faults. WHY TEST AT 100%? Variation in product usually is in accord with statistical analysis and canbe predictedor estimated. For this reason, it is sufficient tomake a certain number of spot checks in order to conclude the general nature of the quality from the test data. Table 1 lists applications for material testing by eddy current. Problems can develop during manufacturing when processes are not subject to standard statistical distribu- tion. Each heat treatment batch is a separate production that is not related to the previous one, a spot check can only provide limited information about the entire production. First, one needs to know what may happen unexpectedly. Table 2 lists possible hardening faults. Keep inmind that ma- terials are not perfect and can have unexpected flaws. EDDY-CURRENT TESTING Eddy-current testing works with coils, generators, ac-current and ac-voltage, frequencies, field strength, and in- duction law. Eddy-current tests examinemetallurgicalmicro- structure, including mechanical features like hardness, case depth, or alloy. The eddy-current test does not provide ab- solute values (e.g., 56 HRC or 2.6 mm case depth) but it does detect fine differences inmicrostructurewithhigh sensitivity. In the production line, within a fraction of a second, a non- destructive 100% test for microstructure can measure case depth, hardness run out, hardness pattern, tensile strength, carbon content, soft spots, and surface decarburization. Having thesemeasurements, it is possible to take quick corrective activities to fix deviations from the specified struc- ture. With a suitable mechanical part handling arrangement, the transport from the hardening station to the test station takes only a few seconds. Faulty parts are immediately de- tected and separated, which provides enormous savings in time and costs. The energy used in eddy-current testing is very small, in the milliwatt range. Field strength is low and permeability is in the range of the initial permeability. Test frequencies rang- ing from Hz to hundreds of kHz levels provide information on undesired structures via the frequency dependent pen- etration depth of the eddy current and on the formation of permeability. Very small electrical signals require a very pre- cise evaluation in order to assure their differentiation from ambient interferences. To sort against an unexpected fault, the technique must have the ability to calibrate only with OK parts, a broad spectrum of tests and many test frequencies, it must be fast enough, and provide maximumwarranty to find unexpected errors. Eddy-current testing, especially with PMFT offers this NONDESTRUCTIVE HARDNESS TESTING OF HEAT-TREATED PARTS Preventive multi-frequency testing using eddy current ensures heat-treated parts meet specifications. A. Horsch, Arnold Horsch e.K., Remscheid, Germany TABLE 1 — EXAMPLES OF MATERIAL TESTING BY EDDY CURRENT (PMFT) Component Parameters tested Bearing parts Case depth, structure, and hardness pattern Axle components Case depth, structure, and hardness pattern Pinion pins and axles Case depth, structure, and hardness pattern Linear guide components Case depth, structure, and grinder burn Steering racks and the like Case depth, structure, and hardness pattern 8