Isothermal Bainitic Transformation in High-Alloyed C + N Steel: Influence of Carbon and Nitrogen on Microstructure and Mechanical Properties

Abstract

In order to gain insight into the bainitic transformation of carbon-and nitrogen (C + N)-alloyed steels, first investigations were conducted on the steel DIN EN X30CrMoN15-1. In particular, the influence of C and N on the transformation characteristics, resulting microstructure and associated mechanical properties was subjected to detailed analysis. Quenching dilatometry was employed to conduct isothermal heat treatments, concurrently acquiring data on transformation behavior and assessing suitable heat treatment parameters for mechanical properties. Scanning electron microscopy (SEM) and electron backscatter diffraction (EBSD) studies demonstrate the presence of bainitic microstructures with retained austenite following isothermal heat treatments of 24 hours. Annealing temperatures of 200 °C yielded nanoscale plate-like bainite structures with plate widths below 40 nanometers and hardness exceeding 600 HV10. Wavelength dispersive X-ray spectroscopy (WDS) measurements were able to demonstrate that during bainite formation the N diffusion behavior is comparable to C. TEM studies were employed to examine the nanoscale microstructure in detail and revealed Fe2C and Cr2N precipitates.