Evaluation of tempering induced changes in the hardness profile of case-carburised EN36 steel using magnetic Barkhausen noise analysis

The effect of tempering on the magnetic Barkhausen noise (MBN) signal profile was studied in case-carburised EN36 steel using a range of magnetic excitation frequencies and a number of frequency ranges for analysis of the MBN signal. The MBN level generally increases with tempering due to coarsening of the microstructure. With higher values of excitation frequency, f(EX), the MBN profile exhibits a single peak, but with low values of f(EX), double peaks are observed. The MBN peak obtained with higher f(EX) was found to correlate well with hardness changes in a region, down to 100 mum below the surface. The analysis of the MBN signal produced with low f(EX), in narrow frequency ranges selected by software frequency filtering, showed variations in the extent of changes in the relative height of the two MBN peaks in the profile. After taking into account the skin depth-frequency relation for the MBN signal, variations in the values of the two MBN peaks in different analysing frequency ranges were found to correlate well with hardness variations at different depths down to 425 mum below the surface. An empirical relationship has been established between the hardness-depth profile and the MBN measurements. (C) 2002 Elsevier Science Ltd. All rights reserved.