Analysis of dynamic recrystallization through austenite grain reconstruction of additively manufactured martensitic M789 steel

This work examines the hot compression behavior of additively manufactured M789 steel via laser powder bed fusion (LPBF) at temperatures ranging from 850 °C to 1050 °C and strain rates between 0.01 s-1 and 1 s-1. Combined corrections for friction and deformation heating were applied to the flow curves to acquire accurate stress-strain data, followed by phenomenological and physical-based constitutive modelling. In particular, the application of Sellars and Tegart hyperbolic sine model, the Kocks-Mecking model and the Johnson-Mehl-Avrami-Kolmogorov equation facilitated the development of physical material models that accurately depict the behavior of LPBF-fabricated M789 steel, highlighting the inadequacy of traditional models, such as the modified Johnson-Cook model, in capturing the complexities and metallurgical phenomena associated with high-temperature deformation. The activation energy for deformation of M789 steel was determined to be lower than that of conventional M250 steel, indicating an accelerated recrystallization process at elevated temperatures. The study also introduces a novel quantitative analysis of dynamically recrystallized grains via combined austenite parent grain reconstruction (based on the Kurdjumov-Sachs orientation relationship combined with the Markov clustering technique) and shape factor measurements, confirming the occurrence of dynamic recrystallization (DRX). The findings suggest that DRX more readily occurs at higher strain rates and higher deformation temperatures, whereas at lower temperatures, a lower strain rate is preferred to generate a higher fraction of DRX grains along with finer grains. Additionally, the work indicates that specific variants from the Kurdjumov-Sachs orientation relationship are favored during the solidification process in LPBF, which seem to disappear when the alloy is subjected to its austenitization temperature.