Hasan, Sk Md

Given the critical role that metastable retained austenite (RA) plays in advanced high-strength steel (AHSS), there is significant interest in obtaining a comprehensive understanding of its stability, to achieve excellent mechanical properties. Despite considerable attention and numerous studies, the significance of individual contributions of various microstructural factors (size, crystallographic orientation, surrounding phases, etc.) on the stability of RA remain unclear, partly due to the difficulty of isolating the direct effects of these factors. In this study, we examined the influence of microstructural factors while minimizing the effect of chemical composition on the mechanical stability of RA. We accomplished this by comparing the austenite (γ) stability in two distinct microstructures: a two-phase RA/martensite microstructure and a one-phase γ microstructure, both with nearly identical γ compositions. We employed in situ high-energy X-ray diffraction during uniaxial tensile testing conducted at both room temperature and 100 °C, facilitating the continuous monitoring of microstructural changes during the deformation process. By establishing a direct correlation between the macroscopic tensile load, phase load partitioning, and the γ/RA transformation, we aimed to understand the significance of the microstructural factors on the mechanical stability of the RA. The results indicate that very fine RA size and the surrounding hard martensitic matrix (aside from contributing to load partitioning) contribute less significantly to RA stability during deformation than expected. The findings of this study emphasize the critical and distinct influence of microstructure on γ/RA stability.

In this study, we simulated the solidification behavior, microsegregation, and heat treatment in Fe–xMn–7.5Al–5Cr–1.0C lightweight steels using the CALculation of PHAse Diagrams method. The solidification paths and microsegregation of these steels were calculated with the Scheil-Gulliver model and equilibrium calculator in the Thermo-Calc® software. At the same time, thermodynamic calculations predicted heat-treatment temperatures for different steels. The transformation path of the Fe–xMn–7.5Al–5Cr–1.0C (x = 18 and 20 wt.%) lightweight steels is as following: liquid → liquid + δ-ferrite → δ-ferrite + γ-austenite → γ-austenite → γ-austenite + M7C3, according to equilibrium and Scheil’s calculations. In case of 25 wt.% Mn steel, the two-phase region of δ-ferrite and γ-austenite is absent in the transition path. The segregation behaviour of solute elements in the liquid, ferrite, and austenite phases were predicted using the Scheil model. The heat treatment temperature for single-phase formation is expected to be between 900 °C and 1030 °C.

In the current study, 7 wt.%Ni alloy steel was prepared, hot rolled, and heat treated according to popular quenching, lamellarization, and tempering treatments. The inter-critical lamellarization temperature was varied and the microstructure-property correlation was evaluated in each stage of heat treatment to understand the metallurgical aspects. Optical and detailed electron microscopic techniques were used to characterize and quantify the microstructures. Mechanical responses under uniaxial and impact loading were also recorded for all the studied samples. Tempered martensite with blocky and lamellar morphology, along with retained austenite and ɛ-martensite, were observed in the microstructures after the above-mentioned heat treatment. The lamellarization at 700 ℃ leads to a more uniform distribution of alloying elements and, therefore, promotes the formation of finer retained austenite with uniform distribution, compared to 650 °C lamellarization temperature. The presence of lower matrix strain and uniformly distributed fine retained austenite provides the highest toughness with moderate strength in the 700 °C samples. ɛ-martensite is expected to provide the necessary strength to balance the softening arising due to tempered martensite and retained austenite. Moreover, the uniformly distributed fine and filmy-shaped retained austenite provides thermal stability, and arrests crack propagation, enhancing toughness. The XRD results after impact toughness show that the γ-ε-α transformation takes place during the -196 °C temperature, and during impact toughness testing, ε-α transformations also provide the toughening in the Ni-700+590 sample.