Characterisation of microstructure in steels by magnetic techniques - An overview

Abstract

Magnetic techniques are highly sensitive for microstructural characterisations in ferromagnetic materials like steels and can be used in the areas of quality control and remaining life assessments. Tile basis of these techniques are related to the domain nucleations and domain dynamics with respect to the various types of microstructural features such as grain boundaries. second phase precipitates, residual stresses, dislocations etc. The microstructural features act as sites for magnetic domain nucleation and pinning centres against magnetic domain wall motions -both the phenomena being responsible for the magnetic behaviour of a ferromagnetic material like steel. Different types of domain walls (180 degrees and 90 degrees walls) interact differently with the microstructural features. There are two broad classes of magnetic techniques, namely analysis of magnetic hysteresis loop parameters and the analysis of magnetic and acoustic Barkhausen noise signal. The hysteresis loop parameters are coercivity. retentivity, permeability, differential permeability etc. The reasons for a generally observed lack of straight forward relationships is due to complex domain patterns and their complex interactions with the microstructural features. The paper discusses the principles based on which various types of magnetic parameters call be used in specific application cases. Examples are given to show the relevance of the various techniques.