Evaluation of Fracture Surface of 11/4Cr-1/2Mo Steel by Residual Magnetization Induced from Inverse-Magnetostrictive Effect

Abstract

In this study, we propose a new technique to evaluate some properties during fracture propagation, such as stress at the crack surface and the propagating-route by measuring distributions of leaked magnetic flux vector from the residual magnetization in the vicinity of the fracture surface. The technique involves the application of an inverse-magnetostrictive effect in ferromagnetic materials, such as tempered 11/4Cr-1/2Mo steels below the ductile-brittle transition temperature. The maximum magnetic flux density was increased with impact absorption energy measured by Charpy impact test. The highest magnetic flux density located at the crack starting point, where a fish-eye type surface morphology was observed in fractographic analysis. It indicates the highest stress for fracture initiation at this point. According to the analysis, the change in the magnetic flux vectors corresponds with the direction of crack propagation, which was well explained from the magnetostrictive properties of iron. The measurement of magnetic flux density distribution will be useful for the fractographic analysis to discuss the in-situ phenomena that are difficult to obtain in previous methods.