Case Depth Measurement Using Surface Acoustic Wave Velocity Dispersion

Abstract

Case hardening is a metallurgical process typically to increase the fatigue and wear resistance of steel components. The case refers to the hardened layer that is formed in this process, and the depth of the case is critical to the components’ performance. Currently, to inspect the quality of a batch of material that has gone through the hardening process, one or more of the parts are sectioned with an abrasive wheel, polished flat and the hardness profile measured using a microhardness indenter. This method of inspection can take several hours, often with the production line stopped, until the results are known. It does have the advantage of measuring directly the desired material properties but has the obvious disadvantages of costly manufacturing down-time, unnecessary scrappage of a production item and assumes that the material properties of other samples in the batch are similar to the one inspected. As a promising alternative approach, nondestructive inspection (NDI) permits a 100% inspection of the batch which is not economically viable using destructive inspection methods. Generally, NDI methods are based on inferring case depth indirectly through measuring electromagnetic or mechanical properties of the part using eddy current or ultrasonic probes [1–4]. Eddy current systems are commonly used for case depth measurements and are known to be reliable for many applications [4]. However, they lack sensitivity if the case depth is deep (e.g. greater than 5 mm in steel parts) and custom probes are required for inspection of components with different geometries.