Evaluation of microstructures of variously heat treated carbon steel by magnetic coercivity measurement

Abstract

The microstructures of variously heat treated 0.85% carbon steel were evaluated by magnetic coercivity measurement. Austenitizing, isothermal transformation, continuous cooling or spheroidization heat treatment was carried out to produce various microstructures. Microstructural parameters such as prior austenite grain size, phase and pearlite interlamellar spacing were measured along with coercivity to investigate the relationships between them. Prior austenite grain size had little effect on the measured coercivity. Coercivity was observed to be high in order of martensite, pearlite and ferrite phase. The linear decrease of coercivity with the increasing pearlite interlamellar spacing was found. Therefore, coercivity was suggested as a potential nondestructive evaluation parameter for assessing the microstructures -pearlite interlamellar spacing and kind of phase -of 0.85% carbon steel. 1 Introduction Usual microstructure evaluations of steel products have been performed in a destructive way by cutting, polishing and using microscopical observation, which is selective, time-consuming, and laborious. There are increasing needs to introduce nondestructive evaluation techniques for better time saving and economic quality control of steel products.Magnetic method [1][2][3][4][5][6][7][8][9][10], by which correlations between material properties and magnetic parameters can be obtained, have been applied as one of nondestructive evaluation techniques. Although previous researchers have reported the effect of grain size [3][4][5][6][7], carbon content [4], morphology of carbide particles [8], applied stress [9] and plastic deformation [10] on magnetic coercivity in ferromagnetic steels, the correlations between magnetic coercivity and various microstructures of steels depending on specific heat treatment conditions are still lacking. In this research, it was attempted to investigate the variously heat treated microstructures (prior austenite grain size (PAGS), phase, pearlite interlamellar spacing (PIS)) of 0.85% carbon steel by magnetic coercivity measurement.

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“…8b is a mixture of ferrite and cementite essentially. However, due to large number of grain boundaries and dispersion of carbides, the movement of magnetic domain wall is still restricted, higher magnetic elds are required to overcome strong pinning sites and move domain walls [22][23], the coercivity level of 2# cylinder is still higher compared to that of the original cylinder. Moreover, the coercivity values determined using digital measurements were compared with the results obtained by the traditional HL method [24,34], a very good correlation was found between the two techniques (Fig.…”

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“…The grain boundaries act as pinning centers for dw pinning [ 30–32 ] and therefore result in larger coercivities. [ 33–37 ] Dislocations and residual stresses in the material due to deformation also lead to an increase in coercivity. [ 38–40 ] Residual stress results in preferred directions of magnetization and therefore in magnetoelastic anisotropy.…”