Influence of Microstructure on the Mechanical Properties and Work-Hardening Behavior of High-Carbon Nanostructured Bainitic Steel

The microstructure-properties relationship of the low-alloy, high-carbon nanostructured bainitic steel obtained by heat treatment, including austenitization and cooling followed by isothermal nanobainitic transformation at 280℃ for 72 h, was investigated. Detailed characterization of the obtained microstructure was performed using light optical, scanning, and transmission electron microscopy. These analyses reveals that the microstructure of tested nanobainitic steel consists of bainitic ferrite lath with an average size of 84 ± 21 nm and retained austenite with two different morphologies: (i) thin films with an average size of 64 ± 19 nm and (ii) blocks with a size of a few micrometers. The carbon concentrations in the film-type retained austenite and blocks of retained austenite were determined through X-ray synchrotron radiation diffraction analysis. The concentrations are 1.81 ± 0.09 wt.% and 1.39 ± 0.06 wt.%, respectively. The total amount of retained austenite in the microstructure is 48.0 ± 1.8 vol.%, and the dominant crystallographic orientation relationships between the microstructure constituents were determined to be Nishiyama-Wassermann. The minor K-S relationship was also recognized from the SEM/EBSD results. Tensile strength of the nanostructured steel was tested, and yield strength was found to be high. At an elongation of 7.2%, the tensile strength reached a significant level, while the average hardness was 490 ± 7 HV.