Comparison of Microstructure and Mechanical Properties of a High‐Carbon Bainitic Steel Treated by Long‐Time Bainitic Austempering and Short‐Time Austempering Plus Tempering Processes

Abstract

Herein, the microstructure and mechanical properties of a high‐carbon bainitic steel treated by long‐time bainitic austempering and short‐time austempering plus tempering processes are compared. The multiphase microstructures are characterized by dilatometry, X‐ray diffraction, scanning electron microscopy, and transmission electron microscopy to correlate with mechanical properties. Results show that although long‐time austempering treatment can reduce the volume fraction of brittle martensite, no significant improvement is observed in fracture damage resistance. Besides, the cementite is prone to precipitation from the austenite at the later period of the long‐time austempering process. The cementite precipitation in austenite decreases the carbon content in retained austenite (RA) and consequently reduces the mechanical stability of RA. In contrast, the cementite has not been able to precipitate from austenite after short‐time austempering treatment, whereas the martensite is softened and the stability of RA is improved during subsequent tempering. Therefore, excellent mechanical properties are obtained in the samples treated by short‐time austempering plus tempering process: ultimate tensile strength, 1489 MPa, yield strength, 1014 MPa, total elongation, 33.2%, and the product of strength and elongation (PSE) of 48.4 GPa%, where PSE is increased by 27% compared with the sample after long‐time bainitic austempering.