Non-Destructive Characterization of Subsurface Plastic Deformation in Case Carburized Steel Using Magnetic Barkhausen Noise Technique

The effect of extreme loading in bending was investigated by magnetic Barkhausen Noise (MBN) and X-ray diffraction (XRD) in two types of inhomogeneous steels that widely used in gear industry. EN 36 and H8620 steels in the carburized, tempered and ground condition were investigated after unloading from high stress levels. The inhomogeneity arising from the variation in carbon content showed up clearly by double peaks MBN profiles before loading. The first MBN peak at low field revealed the soft subsurface region and the second peak at higher field revealed the hard surface layer. Residual stresses profiles were produced by XRD before and after loading to probe plasticity in the cross sections of both specimens. Barkhausen noise measurements showed a considerable change in the first peak height in both steels as a response to plastic deformation in tension and compression in the subsurface material. The height of the second peak remained unchanged in EN 36 specimen but increased slightly in the H8620 specimen. The residual stress measurements after unloading indicated that the subsurface materials after a depth of 0.4 mm in both specimens were yielded. The surface layer of the H8620 steel was also affected slightly by bending as revealed by an increase in the second MBN peak height and confirmed by XRD as indication of yielding. The experiment confirmed that the magnetic Barkhausen noise can be used to characterize yielding in inhomogeneous steels non-destructively.