Coercivity and power spectrum of Barkhausen noise in structural steels

Abstract

Relative coercivities and power spectra of Barkhausen noise are measured for ferritic pearlitic C-Mn, tempered microalloyed and 9% Ni steels under unstressed and tensile stressed conditions. Coercivity depends on the type of material, stress state, amounts of pearlite and grain boundary precipitates, grain size and dislocation density. The stress dependence of coercivity is extremely strong in the Ni steel, relatively strong in the tempered steel at low stress levels and small in the ferritic-pearlitic steels. The power spectra exhibit clustering in elementary magnetic transitions, which can be expressed in terms of the mean pulse duration ϱ τ 0. This is inversely proportional to coercivity, incrasing as a function of grain size D approximately as D 1 2 . Tensile stress directed parallel to the magnetization prolongs ϱ τ 0 in the case of the ferritic- pearlitic and tempered steels but shortens it in the case of the Ni steel. The stress dependence of ϱ τ 0 and noise level are controlled by possible grain boundary phases and the amount of pearlite (lamellar carbides). Grain boundaries and lamellar precipitates tend to suppress displacements of the 90° domain walls, which increases the stress response of the noise at high stress level.