Investigating the Effects of Quenching and Tempering on Steel Microstructures by Magnetic Barkhausen Noise Method

As-quenched and quenched-tempered steels were characterized by Magnetic Barkhausen Noise (MBN) method in order to contribute to monitoring and optimization of the heat treatment processes by nondestructive methods. Identical austenitization and quenching procedures were applied to SAE 1040 and SAE 4140 specimens. Then, some of the specimens were tempered at 200^oC and 600^oC for 2 h. Microstructures of the samples were characterized by metallographic examinations and hardness measurements. Next, the amplitude, position and frequency spectrum of the MBN signals were evaluated by using a commercial Barkhausen noise system. The results showed that as tempering temperature increases, in contrast to the reduction in hardness, the MBN response of the microstructure increases due to the enhancement of domain wall displacement with softening of the martensite. MBN method is a promising tool for evaluating microstructure of quenched and tempered steels after establishing the quantitative relationships between MBN parameters and the microstructural parameters.