This paper discusses the basic principles of multi-frequency eddy current testing and explains how it can be used on high-volume production lines to detect faulty heat-treated parts based on case depth, hardness patterns, tensile strength, carbon content, soft spots, and surface decarburization. It also presents examples showing how the method is used in high-speed inspection of cam shafts, screws, balls of various sizes and materials, distance pins, and complex bolts.

As part of a project to identify nondestructive techniques to determine the surface hardness and case depth of carburized steel, Meandering Winding Magnetometers (MWM) measurements were evaluated. MWM technology is based on eddy current testing. Compared to traditional eddy current testing, MWM measurements integrate the generating coil and detection coil into a thin, flexible sensor that can be applied to complex geometries. Conductivity and permeability are measured with MWM equipment to evaluate the properties of carburized steel. For this study, samples of 8620 were carburized to selected hardness and case depths. MWM technology was determined to be an effective method to detect surface hardness. The results of this study are presented and discussed in this paper.

Eddy current is a non-destructive testing technique proven for use in heat treat and material structure verification. Modern multi-frequency eddy current instruments can test for conditions such as misplaced case, shallow case, short heat, short quench, and delayed quench. Eddy current testing offers many benefits over traditional heat treat validation methods. Unlike sample testing processes using cut, polish, etch, and visual inspection techniques, eddy current testing provides a clean, fast, and repeatable process that can perform in-line inspections of all parts produced. Eddy current inspections have traditionally focused on symmetrical parts such as wheel bearings and gears. However, advances in robotics have paved the way for cost-effective inspection of non-symmetrical, complex components that would have previously required multiple test stations. Robotics also provides a low-cost way to retest, null, and periodically proof the testing process using multiple conditions of masters. This has been difficult and expensive with other types of automation and operator involvement.