Texture development during austempering process of an AISI 4130 steel

Abstract

In this investigation, microstructural development and mechanical properties of an automotive grade AISI 4130 steel were explored through various heat treatment routes. In that regard, first of all the effect of austenitization temperature and time on the mechanical properties were assessed. Then, an optimum austenitization temperature was obtained for austempering at 400 °C and for the duration of 20, 40 and 60 min. Finally, under the optimum austenitization condition, austempering temperatures from 300 °C to 500 °C, with 50 °C interval, was evaluated in terms of mechanical properties changes and microstructural development. Optical microscope (OM) and Field Emission Scanning Electron Microscope (FE-SEM) equipped with Electron backscattered diffraction (EBSD) detector were employed for microstructural studies. Tensile tests, hardness measurements and Charpy impact tests were done to define the mechanical properties of the steels. Results show that austenitization at 900 °C for 15 min gives the highest hardness value. This was related to the development of bainite microstructure at the expense of martensite. Also, under different austenitization temperatures, austempering time of 20 min at 400 °C provides the highest strength level. As well, austempering temperature of 400 °C was determined as the best temperature to obtain optimum amount of bainite while strength and ductility levels are acceptable. {100} pole figures for some of the austempering steps were gauged and showed a random texture for the initial steel. However, after austempering process at different temperatures and times, textured structures with the main components of Cube ({100}<001>), Copper({112}<111>), S ({123}<634>), and Brass ({110}<112>)were obtained. Texture components were substantially changed with changing the austempering temperature and time. Out of these texture components, non-cube textures were developed for the best combination of austempering temperature and time. This latest finding could explain satisfactory levels of mechanical properties under the optimum condition of heat treatment.