Detection of Decarburising Depth in Hadfield Steels Using a Multi-magnetic NDE Method

This paper reports the capability of a range of electromagnetic non-destructive evaluation (NDE) methods to characterise the decarburising phenomena in Hadfield steels. Four Hadfield steel samples were austenitised at 1050 °C for 1, 2, 3 and, 4 hours in an air furnace followed by water quenching to produce various decarburising depths. The decarburisation depth was characterised using hardness profile measurements, optical and scanning electron microscopy, and X-ray diffraction analysis. The results confirmed the occurrence of decarburising and presence of a surface martensite layer of 35 to 110 µm depending on austenitisation time. The microstructural change at the surface affects the magnetic properties of the sample and enables electromagnetic NDE methods to detect the transformed layers quantitatively. In the non-destructive characterisation step, the relationships between the austenitising time and the outputs from magnetic flux leakage (MFL), initial magnetisation curve (BH) and eddy current (EC) methods are reported. Comparing the results obtained from the three magnetic NDE methods, it can be concluded that MFL and EC (phase angle output) have the potential to quantify the surface microstructural changes occurring as a result of the decarburising process in Hadfield steels.