Alternating current potential drop and eddy current methods for nondestructive evaluation of case depth
Huang, Yongqiang (2004) Alternating current potential drop and eddy current methods for nondestructive evaluation of case depth. PhD thesis, Iowa State University.
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Case hardening treatments offer a means of enhancing the strength and wear properties of parts made from steels. Generally applied to near-finished components, the processes impart a high-hardness wear-resistant surface which, with sufficient depth, can also improve fatigue strength. Applications range from simple mild steel pressings to heavy-duty alloy-steel transmission components. The characteristics of case hardening are the surface hardness, effective case depth, and depth profile of the residual stress. The specified case depth varies for different applications. It is useful to be able to measure the case depth nondestructively to ensure the specification is met. In the work outlined in this dissertation, the aim is to evaluate the properties of case hardened parts nondestructively. The case hardening process produces a change in the electromagnetic properties of the steel components in the near surface region. Consequently, the electrical conductivity and magnetic permeability have different values near the surface compared with those of the substrate. It is assumed that the conductivity and permeability variation with depth is indicative of the hardness profile allowing the case depth to be estimated from electromagnetic measurements. A two-layer model is adopted to approximate the case hardened steel parts as a homogeneous substrate layer surrounded by a homogeneous surface layer with uniform thickness. Alternating current potential drop (ACPD) theoretical calculations have been performed and compared with experimental measurements for both case hardened cylindrical rods and homogeneous metal plates. Driver and pick-up coils have been used for eddy current induction measurements on the cylindrical rod specimens. The multi-frequency measurement data are used to estimate the case depth by model-based inversion. The measured case depth is in reasonable agreement with the effective case depth from the measured hardness profile. Excellent agreement is observed between the measurement data and the theoretical calculation on homogeneous metal plates.
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