In-Situ Synchrotron X-ray Diffraction Studies on Effects of Plastic and Elastic Loading on bcc Phase Transformations of a 3rd Generation 1 GPa Advanced High Strength Steel

Abstract

In this paper, we describe the effects of mechanical loading on bcc-to-bcc phase transformations of an Advanced High Strength Steel during cooling. In-situ synchrotron diffraction was employed to measure time-temperature-load diffraction patterns. Calculations were made of the volume fractions of the phases, the transformation kinetics, and the austenite lattice parameter during cooling and simultaneous loading. In addition, volume fractions and lattice parameters of retained austenite at room temperature under different loading conditions were obtained. The results show that applying a load during cooling of the fcc phase significantly increases the volume fraction of a bcc phase before the start of the martensitic transformation. The kinetics of phase transformations were affected by the applied loads. The volume fraction and lattice parameter of retained austenite at room temperature vary in different samples and the highest retained austenite and the largest lattice parameter were obtained in the sample subjected to the highest load.

Effect of Carbon Partitioning and Residual Compressive Stresses on the Lattice Strains of Retained Austenite During Quenching and Isothermal Bainitic Holding in a High‐Silicon Medium‐Carbon Steel

“…The volume fractions of different microstructural constituents, such as ferrite, bainite, martensite, and RA, determine the final mechanical properties in these groups of advanced high strength steels (AHSS). [18][19][20] Depending on the alloy type, chemical composition, and parameters of these novel heat treatment processes, such as the reaustenitization temperature, quenching rate, final quench stop temperature to enable isothermal bainitic transformation and/or carbon partitioning, the heat-treated steel samples may contain varied amounts of RA, which can impart a wide range of properties in the end products. [3,[21][22][23][24][25][26][27][28] In these novel steel concepts, the role of RA is very complicated depending on the high complexity of the multiphase structures and often has beneficial effects, but at times can have some negative effects…”