State of the Art on Magnetic Properties Stress Correlation in Steels

摘要

Up to this moment, residual stresses are determined by strain gauges and drilled holes, both being surface techniques and able to monitor stresses only at certain points-areas of the steel surface. The motivation of our group during the last decades has been the determination of the stress tensor distribution in steels, using magnetic techniques. We offer a concrete method to the community of steel industry offering the possibility of stress gradient and therefore crack generation prediction based on magnetic non-destructive methods. A considerable amount of residual stresses is introduced during common manufacturing processes and operation in steels. Depending on their magnitude and distribution, residual stresses are rather useful in elucidating possible causes of failure of a material. During welding, the temperature range varies from the material's melting point to the room temperature. Additionally, the mechanical properties of the joint are temperature dependent and therefore, these are often degraded due to the presence of thermal gradients. Cooling to room temperature invokes stresses, which are inevitably incorporated to the material's residual stress. Therefore, the quantitative determination of the residual stresses is important for quality, integrity and performance of the welding joints. Any change in grains configuration, due to the existence of stresses results in rearrangement of the magnetic domains configuration. Thus, the magnetic signals contain information which is closely related to the microstructure of the examined ferromagnetic material. The magnetic methods are very sensitive to changes of both microstructure and applied stress (tensile stresses increase the magnetic noise, while compressive ones decrease it). Our method is based on a self-embedded nano-sensing element existing in magnetic steels: the magnetic domain wall. The domain wall displacement is dependent on the amount of stress field in its vicinity. The response of this sensing element is depicted in several sensing modes. Therefore, the magnetic response can be converted to residual stress values by using an appropriate calibration curve. However, all grade of steels with substantial microstructure differences should be separately considered for calibration.