Nondestructive characterization of hardness variations in cold work tool steels using magnetic Barkhausen noise

Hardness is one of the most important features in mechanical properties of cold work tool steels which is altered by change in heat treating conditions. In tempering process of the steel, the hardness variations pattern is as follows: an initial moderate decrease in the hardness up to 400 C due to the recovery mechanism, an increase in the hardness at 500 °C related to the secondary hardening phenomena, and a continuous decrease at higher temperatures due to the carbide spheroidizing. In this paper, the potential of Barkhausen noise method is evaluated for detecting the hardness variations of AISI D2 steel parts during tempering treatment. To produce microstructural changes, two groups of samples austenitized at 1055 and 1080 °C, quenched in oil and finally each group tempered in the range 200-650 °C. The results show that the nondestructive method could be used to detect secondary carbides formation during tempering treatment.