On the mechanism of nondestructive evaluation of cementite content in steels using a combination of magnetic Barkhausen noise and magnetic force microscopy techniques

The influence of carbon content in the form of globular cementite precipitates in unalloyed steels was macroscopically characterized by means of magnetic hysteresis loop and Barkhausen noise techniques. The choice of the frequency of the applied held has a strong influence on the Barkhausen noise profiles. At sufficiently high frequency (0.5 Hz) there are two peaks, one at lower held, the amplitude of which corresponds to the amount of ferrite and one at higher held, the amplitude of which corresponds to the amount of the cementite phase, respectively. Magnetic force microscopy and electron backscattered diffraction techniques were used to determine the magnetic and crystallographic microstructures of the steels. Cementite has its own domain structure and stray fields which influence the magnetization process of the steel by its own magnetic contribution. When an external magnetic field is applied, the magnetization process in ferrite occurs mainly at lower fields through the 180 degrees and 90 degrees domain walls. A higher field is required for the observation of 180 degrees domain wall movements in cementite. (C) 2013 Elsevier B.V. All rights reserved.