Austenite Evolution and Solute Partitioning during Thermal Cycling in the Intercritical Range of a Medium-Mn Steel

Abstract

The evolution of austenite fraction and the associated solute partitioning during the intercritical annealing of medium-Mn steels are of great importance for austenite stabilization and the mechanical performance of this class of steels. In the present work, a 4.5Mn steel is subjected to a cyclic treatment and the evolution of the austenite fraction is measured with dilatometry. The evolution of austenite fraction and solute partitioning are simulated for a case where the starting time of the cyclic treatment is well before the equilibrium fractions have been established in the respective isothermal intercritical treatment. The evolution of austenite during thermal cycling in the intercritical range comprises of forward, inverse, and stagnant stages. The fraction of austenite formed decreases in each successive cycle while the kinetics of the evolution of austenite is controlled by the Mn diffusion in ferrite. Partitioning of Mn and C takes place from ferrite to austenite during the cyclic transformation. Due to the low diffusivity in austenite, wells form in the composition profiles in austenite of both Mn and C. These wells are the locus of the interfacial compositions of austenite, corresponding to the variation of the local equilibrium conditions during the thermal cycle.

Microstructural and Thermomechanical Simulation of the Additive Manufacturing Process in 316L Austenitic Stainless Steel

“…It is observed that upon heating in the 2 nd thermal cycle, the volume fraction of δ ferrite in part A-B remains constant, and then gradually increases in part B-C. Therefore, the parts A-B and B-C on the volume fraction curve are referred to as "stagnant" and "forward" stages respectively [10]. During cooling in the 3 rd thermal cycle, the volume fraction of δ ferrite in part C-D continues to increase.…”

Mechanism and Application of Reverse Austenitic Transformation in Medium Mn Steels: A Systematic Review

“…A local equilibrium (LE) model was often used to simulate the growth kinetics with the solute partitioning. [19,33,[56][57][58] The following Equation ( 3) is assumed in this model, i.e., the chemical potential of each component is equal at the interface in both austenite (γ) and ferrite (α) phases.…”

“…A local equilibrium (LE) model was often used to simulate the growth kinetics with the solute partitioning. [ 19,33,56–58 ] The following Equation (3) is assumed in this model, i.e., the chemical potential of each component is equal at the interface in both austenite ( γ ) and ferrite ( α ) phases.where and are the chemical potential of element i ( i = Fe, C, or Mn) at the interface in γ and α, respectively. To study the growth kinetics under this assumption, it is needed to solve Fick's law for all alloying elementswhere ∇ is Laplace operator, t is time, , , and are the diffusion flux, the diffusion coefficient, and the concentration of solute i ( i = C or Mn) in phase j ( j = γ or α ), respectively.…”

Investigation on Yield Point Elongation and Yield Strength of Fe–9Mn–1.5Si–1Al–0.05C Water‐Quenched and Air‐Quenched Steel Prior to Intercritical Annealing

“…Medium Mn steels with 3-12 (wt%) alloying element of Mn [1][2][3] have recently been defined as one of the third generations of advanced high-strength steels (AHSSs). Medium Mn steels generally consist of a dual-phase microstructure of ferrite (α) and austenite (γ) at room temperature after intercritical annealing (IA); in that, the addition of Mn to steels can promote the thermal stability of γ.…”