Microstructural and Residual Stress Evaluation of Bulk Martensitic Steels with Micrometer-sized Grains through Magneto-optical Kerr Effect

Abstract

The modernization of steel industry has brought up an interest in development of novel and fast characterization techniques that analyze the material from different aspects with the same instrument setup. In recent years, development of magnetic characterization techniques arose for investigating the microstructure and mechanical properties of certain steels [1]. However, these techniques display limited flexibility in terms of surface specific measurements and defect analysis that can become an important part in quality assurance of a produced material.Within this work, a novel utilization of magneto-optical Kerr effect (MOKE) microscopy for microstructural and stress evaluation of martensitic steels is discussed. MOKE has been proven to be a very powerful and versatile characterization technique [2], that allows investigation of magnetic domains and magnetic properties that emerge from the investigated material’s surface. With domain analysis and evaluation of magnetization change with external magnetic field, the surface magnetic properties can be correlated to the bulk magnetic properties of the material [3]. Furthermore, the technique allows high-resolution imaging and investigation of microstructural characteristics of the material without the usage of etchants that can potentially modify the microstructure and ending results. By correlating the microstructural attributes to the magnetic behavior of the material’s surface, the different phases of the material can be distinguished and morphologically analyzed. The method also allows clear identification of austenite in the matrix of martensitic steels, since austenite is non-magnetic opposite to other matrix phases. As a result, the method is highly versatile and allows investigation of steels on different aspects of its properties on a microstructural level.We present the applicability of MOKE on an example of martensitic high-speed steels with micrometer sized microstructural features. The presented data will provide insight into the correlation of the magnetic information to the microstructure. The novelty of this research lies in the application of MOKE microscopy on steels that are relatively magnetically hard and have a fine grain size (under 10 µm). The utilization of MOKE microscopy for phase and microstructure investigation on such steels has until now not been performed. To further display the practicality of MOKE, the steel is investigated with two different processing states, conventionally heat treated and deep cryogenic treated (DCT) [4]. DCT exposes the steel to sub-zero temperatures down to the temperature of liquid nitrogen (-196 °C) inducing further transformation of retained austenite into martensite, which is beneficial for the mechanical properties [5]. The presented examples display the capability of MOKE to identify small quantities of retained austenite of volumetric fraction under 1 %, which is challenging to evaluate with other commonly used techniques such as electron back scattering diffraction (EBSD) and X-ray diffraction (XRD) [6].In addition to microstructural analysis, we also explore the possibility to use MOKE for residual stress analysis. The technique involves the application of a Vickers indenter that imposes localized stress changes through an indentation imprint on the material’s surface. The resulting stress extends radially from the indentation and decreases in magnitude with distance from the edge of the indentation. With local magnetometry and domain analysis of the material’s surface before and after the stress state modification, a correlation to the residual stress of the material is established. The applicability of the method is presented on the example of the conventionally treated and DCT steel, for which the DCT is reported to reduce tensile residual stresses. MOKE analysis confirms the reduction of residual stresses as well as confirms the change of tensile residual stresses to compressive residual stresses as reported by previous researchers [7]. With the discussed results we present that MOKE microscopy shows high potential as a fast and in-depth microstructural and stress characterization technique applicable on an industrial scale. **