Assessment of Retained Austenite in AISI D2 Tool SteelUsing Magnetic Hysteresis and Barkhausen Noise.pdf

Assessment of Retained Austenite in AISI D2 Tool Steel Using Magnetic Hysteresis and Barkhausen Noise Parameters Saeed Kahrobaee and Mehrdad Kashefi (Submitted June 3, 2014; in revised form October 9, 2014; published online January 21, 2015) Inaccurate heat treatment process could result in excessive amount of retained austenite, which degrades the mechanical properties, like strength, wear resistance, and hardness of cold work tool steel parts. Thus, to control the mechanical properties, quantitative measurement of the retained austenite is a critical step in optimizing the heat-treating parameters. X-ray diffraction method is the most frequently used technique for this purpose. This technique is, however, destructive and time consuming. Furthermore, it is not applicable to 100% quality inspection of industrial parts. In the present paper, the influence of austenitizing tem- perature on the retained austenite content and hardness of AISI D2 tool steel has been studied. Additionally, nondestructive magnetic hysteresis parameters of the samples including coercivity, magnetic saturation, and maximum differential permeability as well as their magnetic Barkhausen noise features (RMS peak voltage and peak position) have been investigated. The results revealed direct relations between magnetic saturation, differential permeability, and MBN peak amplitude with increasing austenitizing temperature due to the retained austenite formation. Besides, both parameters of coercivity and peak position had an inverse correlation with the retained austenite fraction. Keywords Barkhausen noise, cold work tool steel, hysteresis loop, retained austenite 1. Introduction High level of hardness and wear resistance are among the main objectives of the heat treatment of AISI D2 cold work tool steel (Ref 1, 2), which is normally achieved by an optimized hardening and tempering process. In this regard, the presence of high amounts of retained austenite is the major problem which could negative