Imaging a weld cross-section using a novel frequency feature in multi-frequency eddy current testing

In this paper, a novel frequency feature (the frequency at which the real part of the inductance reaches a specified value) is proposed for imaging an X70 welding cross-section. It is derived that this frequency feature is related to the permeability of the sample and therefore the imaging results obtained from this feature are a reflection of the electromagnetic properties (in particular permeability) map of the sample. The resulting images are then compared with the weld macrograph and the hardness map. The shape and contour of three weld zones (fusion zone, heat-affected zone and base metal) can be identified. This novel imaging method has a high sensitivity to changes in microstructure and, by adjusting the inductance value it crosses, the frequency range can be adjusted to suit the capability of a particular instrument. © 2019 British Institute of Non-Destructive Testing. All rights reserved.