An investigation of the magnetic properties of high tensile steels

Abstract

This thesis describes an investigation of the magnetic properties of high tensile steels typical of those produced for the high pressure gas pipe-line industry. Results are presented for both bulk and small section samples, and the observed variations in magnetic behaviour as a function of orientation and position within the steel are described qualitatively by changes in steel metallography. The development of an automated double-crank Vibrating Sample Magnetometer, required for determining the magnetic characteristics of small samples, is also described which, without signal amplification, has a moment detectability limit of 10(^-4) e.m.u. The representation of the full magnetization loop by a Fourier series is investigated and the variations in harmonic amplitudes found for the range of steels considered here are compared to those predicted by theoretical models. The successful parameterization of the initial magnetization curve is also reported using a two parameter model (LnB = (k – H(^-1))LnA)), and linear relationships between the coercive field (H(_c)) and these parameters (k,LnA) are presented which permit the prediction of the initial magnetization curve of any similar steel from a knowledge of H(_c). Although the latter may be determined accurately by direct measurements of small samples, further linear relationships are indicated which allow the determination of the coercive field from knowledge of either the steel chemistry or metallography. The compatibility of the observed ferrite grain size dependent contribution to the coercive field with grain boundary domain wall pinning models is also investigated.