Visualising martensite phase fraction in bulk ferrite steel by superimposed Bragg-edge profile analysis of wavelength-resolved neutron transmission imaging

Bragg-edge neutron transmission imaging, a wavelength-resolved neutron imaging method, is a unique method for materials characterization. This method can quantitatively visualise various crystalline microstructural information in bulk material over several-centimetres with sub-millimetre spatial resolution. In various forms of crystalline information, the martensite phase fraction in ferritic steel is significant for the characterisation of, e.g., contact surface of an induction-hardened gear, dual phase (DP) steel used for automobiles, and the cutting edge of Japanese swords. However, the martensite phase fraction in a ferrite-martensite steel has not been measured using conventional Bragg-edge analysis methods because the entire neutron transmission spectral pattern of the α'-martensite phase corresponds to that of the α-ferrite phase. However, the Bragg-edge profile of the martensite phase is slightly broader than that of the ferrite phase. For these reasons, we developed a new method for measuring the ferrite/martensite phase fraction from the superimposed Bragg-edge (sBE) profile composed of both sharp α{110} Bragg-edge and broad asymmetric α'{110}-α'{101} Bragg-edge. As a result, two-dimensional imaging and computed tomography of the martensite phase fraction in ferrite-martensite steel were reasonably achieved. In addition, we observed the sBE analysis method to have numerous advantages such as reasonable accuracy (∼5%), high precision and stability, and easy handling. Furthermore, we identified the suitability of an asymmetric crystal-lattice-plane-spacing distribution function for the determination of the α'{110}-α'{101} Bragg-edge profile, and found the blurred boundary by mixing unquenched and quenched regions in an induction-hardened steel rod. (author)