Insight into the microstructural characterization of ferritic steels using micromagnetic parameters
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I. INTRODUCTION
MANY investigations over the past five decades prove that micromagnetic parameters such as coercive force (Hc), residual induction (Br), maximum permeability (mmax), maximum induction (Bmax), magnetic Barkhausen emission (MBE), etc., can be used to predict the microstructural state as well as the mechanical properties such as hardness, yield strength, tensile strength, etc., in ferromagnetic materials.[1–4] It has been found that the microstructural features such as dislocations, grain boundaries, second-phase precipitates, etc., strongly affect the domain wall movement and, hence, the magnetic parameters.[1–12] Quantitative relations have been obtained between grain size and magnetic parameters (Hc and MBE).[l,3,4,7,8] Similarly, Hc has been related with average size and volume fraction of inclusions and second-phase precipitates. [l,9–12] However, the theory of ferromagnetism is not able to explain the observed quantitative relationship between the magnetic and microstructural parameters, particularly in tempered structural steels.[13] For example, a difference of more than five times has been observed between the actual and theoretically predicted values of coercive force in 0.2 pct carbon steel.[14] Normally, when a steel is subjected to heat treatment or exposed to high temperature, microstructural features evolve in a complex manner. For example, the dislocations dominate the microstructure in the quenched and early stage of tempering, whereas the variations in grain size and carbide precipitation dominate the later stage of tempering/aging. The magnetization process and, hence, the micromagnetic properties for a particular microstructural state are influenced by the synergistic interactions of domain walls with these microstructural features. However, in most of the studies V. MOORTHY, Scientific Officer “E,” S. VAIDYANATHAN, Scientific Officer “F,” BALDEV RAJ, Director, and T. JAYAKUMAR, Scientific Officer “G,” are with the Metallurgy and Materials Group, Indira Gandhi Centre for Atomic Research, Kalpakkam-603 102, India. B.P. KASHYAP, Professor, is with the Department of Metallurgical Engineering and Materials Science, Indian Institute of Technology, Bombay-400 076, India. Manuscript submitted April 27, 1999. METALLURGICAL AND MATERIALS TRANSACTIONS A
carried out to date, the micromagnetic parameters are correlated with only one microstructural feature at a time, ignoring the effects of other features. For example, the magnetic properties are correlated with grain size without considering the effect of precipitates. Similarly, they are related to the size and/or volume fraction of precipitates without taking into account the effect of grain boundaries. Even though the micromagnetic parameters are found to be influenced by the variations in the size and distribution of both grains and precipitates in tempered steels, the individual variation during various stages of tempering/aging has not yet been resolved. Recently, the present authors have established the influence of different tempered
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