2007•Journal of Achievements of Materials and Manufacturing EngineeringRequires access
Determination of the stability of retained austenite in TRIP-aided bainitic steel
Adam Grajcar
Open publisher page 21 citations
Abstract
Purpose: The aim of the paper is to determine the influence of the isothermal holding temperature in a bainitic range of medium-carbon steel on the stability of retained austenite. Design/methodology/approach: The heat treatment of the medium-carbon steel in order to obtain a bainitic structure with the retained austenite was realized. A range of the holding temperature of the steel under isothermal bainite transformation conditions was from 250 to 550°C. To investigate the structure, optical and transmission electron microscopy were employed. An amount of the retained austenite was determined using X-ray diffraction. The mechanical stability of retained austenite was determined by means of the tensile test. Findings: It was found that after the optimum heat treatment the investigated medium-carbon steel has a structure of the ferritic bainite with a 19% fraction of retained austenite. Increasing the isothermal holding temperature results in obtaining the lower (350°C) and upper bainite (450°C). The retained austenite occurs as regular grains or thin foils between bainite laths. The steel is characterized by an excellent strength-ductility balance. A kinetics of the strain-induced martensitic transformation of the retained austenite has a two-stage character. Research limitations/implications: To determine with more detail the stability of retained austenite the knowledge of the austenite carbon content as a function of the isothermal holding is required. Lattice parameter measurements with X-ray diffraction is planned for this purpose. Practical implications: The established heat treatment conditions can be useful for manufacturing crash-related elements in the automobile industry. Originality/value: It should be stressed that the developed conditions of the heat treatment concern the medium-carbon steel, compared with usually investigated low-carbon steels. The kinetics of the strain-induced transformation for TRIP-aided bainitic steel was determined, too.