Determination of Sub-Surface Microstructure States by Micromagnetic NDT

Abstract

The functional behaviour of many components is determined by the intrinsic properties — residual stresses and microstructure — of sub-surface near zones. Therefore, in order to improve the static and dynamic mechanical properties of these surface near states, mechanical and/or thermal treatments such as laser hardening, inductive hardening, case hardening, nitriding, grinding and shot peening are applied. Depending upon the work-piece/component and the employed technique and machining parameters, the residual stress- and microstructure states will be changed between a few micrometers and several millimeters. To control the quality of these subsurface near states, metallographic inspections, x-ray methods and hardness measurements have been applied. Because all these conventional testing methods are expensive, time consuming and not suitable for real time control,^1–4 there is a considerable need of nondestructive testing (ndt) methods. So far ndt procedures are mainly restricted to eddy current methods,^5,6 ultrasonic techniques^7,8 and — in the case of ferromagnetic materials — micromagnetic techniques^1,9–11.