Microstructure refinement of steels using fluctuation in solid solution

Abstract

This latest project focuses on creating homogeneous refinement of steel by manipulating the incubation precursor stage before a process called precipitation. The project is interdisciplinary and draws on the extensive imaging expertise available at Ehime University. In addition to Kobayashi, who besides coordinating the project, is also a specialist in the use of transmission electron and scanning electron microscopy, Professors Masato Ueda at Kansai University and Koichi Hiraoka at Ehime University, bring expertise in electric resistivity and nuclear magnetic resonance imaging. The project is funded by the Japanese Society for the Promotion of Science (JSPS) through its Grants in Aid programme and builds on earlier work on the development and imaging of fine microstructures in titanium and other alloys. By utilising state-of-the-art imaging techniques, Kobayashi’s team found that there was significant compositional fluctuation in the incubation stage prior to precipitation. He says: ‘One of our biggest challenges has been in identifying the exact location of the solute atoms in the matrix at different stages. Currently, we are conducting structural and compositional fluctuation detection experiments at temperature in situ under the transmission electron microscope.’ To date, a variety of imaging techniques have been employed to study rapidly-quenched samples after running selected heat regimes during the incubation stage. Careful analysis of the structure which Kobayashi says: ‘is sensitive to structural and compositional differences in the solid solution,’ and allows those fluctuations to be deduced. These analyses have enabled the team to home in on ideal heat regimes that lead to finely dispersed, stable and homogeneous micro-structured materials. The main findings of the team are that by reducing the temperature following a period of normal incubation, to a level between that of incubation and precipitation and holding it ‘isothermally’ for a pre-determined time, a finer microstructure is induced during precipitation. These experiments have been conducted on steel containing minute quantities of several other elements including carbon, manganese, nickel, chromium, silicon and titanium, that was designed to induce martensitic transformation. Martensite is one of the more useful forms of iron that has a lath-like structure at the microscopic level. Imaging has shown that the isothermal holding period during incubation results in much smaller and more finely-distributed laths of martensite than are achieved during the normal manufacturing process. Kobayashi says: ‘We are now looking at using this technique to examine the effect of compositional fluctuation during incubation on the acceleration of bainitic transformation in steels.’ Heat regimes, including precipitation, have been developed to promote either martensite or bainite iron forms in steel that are stronger and have greater utility.