Ultrasonic Determination of Case Depth and Surface Hardness in Axles

Abstract

Generally, NDI methods for determining case depth are based on inferring case depth indirectly through measuring electromagnetic or elastic 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 applications⁴. However, they lack sensitivity if the case depth is deep (e.g. greater than 5 mm in steel parts) because of the compromise required between a sufficiently high frequency to obtain adequate coupling and a sufficiently low frequency to obtain adequate penetration of the part. Further, custom probes are required for inspection of components with different geometries. When the steel is hardened, the eddy current techniques are dependent on changes in the magnetic permeability and the electrical conductivity of the steel, whereas the ultrasonic techniques are dependent on changes in the elastic modulus of the steel. It is not generally known whether the changes in the electromagnetic properties or the elastic properties correlate better with the hardness. Further, since the same hardness can be achieved through different processing paths, it is not clear if the correlation for either set of properties is unique or is dependent on the specific processing path.