Controlling the Thermal Stability of a Bainitic Structure by Alloy Design and Isothermal Heat Treatment

Abstract

The aim of this work was to develop a novel bainitic steel that will be specifically dedicated to achieving a high degree of refinement (nano- or submicron scale) along with increased thermal stability of the structure at elevated temperatures. The material was characterized by improved in-use properties, expressed as the thermal stability of the structure, compared to nanocrystalline bainitic steels with a limited fraction of carbide precipitations. Assumed criteria for the expected low martensite start temperature, bainitic hardenability level, and thermal stability are specified. The steel design process and complete characteristics of the novel steel including continuous cooling transformation and time–temperature–transformation diagrams based on dilatometry are presented. Moreover, the influence of bainite transformation temperature on the degree of structure refinement and dimensions of austenite blocks was also determined. It was assessed whether, in medium-carbon steels, it is possible to achieve a nanoscale bainitic structure. Finally, the effectiveness of the applied strategy for enhancing thermal stability at elevated temperatures was analyzed.

Consequences of thermite welding on the microstructure of the heat affected zone of a carbide-free bainitic steel rail

“…For higher temperature, but lower than Ac1 (500°C -700°C), a so-called direct decomposition is observed. Austenite transforms into discrete particles of cementite in a ferritic matrix [64,[66][67][68]. This is typically observed inside SC-Ac1 and SC-HT (520 ±20°C to 745 ±20°C calculated), where austenite films are partly transformed into fine interlath carbides (Figure 18).…”

“…Sourmail et al [64] indicated that austenite can be decomposed or destabilized following mainly two mechanisms. At temperature lower than 550°C, corresponding to SC-LT region, a so-called indirect decomposition is mainly observed where tiny amounts of cementite precipitate inside austenite [64,[66][67][68]. Resulting transformation depletes the remaining austenite from carbon, hereby destabilizing it [66,[69][70][71].…”