Hideaki Ikehata

Calculation for grain growth rate of carbon steels by solute drag model considering segregation effect of each substitutional element

Abstract Based on the solute drag model, a practical model incorporating the segregation effect is proposed to calculate grain growth rates in carbon steels. The segregation effect is modelled using two factors: the difference in atomic diameter between a solvent and a substitutional element, and the solubility of a substitutional element. By including the segregation energy, the proposed model enables the simulated retardation of grain growth by the addition of microalloying elements. The calculated grain growth rate by the proposed model shows reasonable correspondence between grain growth rates for experimental and calculated results. The temperature dependence of the grain growth rate is also well simulated.

Modeling Growth and Dissolution Kinetics of Grain-Boundary Cementite in Cyclic Carburizing

In vacuum carburizing of steels, short-time carburizing is usually followed by a diffusion period to eliminate the filmlike cementite (θGB) grown on the austenite (γ) grain boundary surface. In order to obtain the θGB amount during the process, the conventional model estimates the amount of cementite (θ) with the equilibrium fractions for local C contents within a framework of the finite difference method (FDM), which overestimates the amount of θGB observed after several minutes of carburizing. In our newly developed model, a parabolic law is assumed for the growth of θGB and the rate controlling process is considered to be Si diffusion rejected from θ under the isoactivity condition. In contrast, the rate constant for the dissolution of θGB is considered to be controlled by Cr diffusion of θ. Both rate coefficients (α) were validated using multicomponent diffusion simulation for the moving velocity of the γ/θ interface. A one-dimensional (1-D) FDM program calculates an increment of θGB for all grid points by the updated diffusivities and local equilibrium using coupled CALPHAD software. Predictions of the carbon (C) profile and volume fraction of cementite represent the experimental analysis much better than the existing models, especially for both short-time carburization and the cyclic procedure of carburization and diffusion processes.

The Solute Drag Model to Calculate Grain Growth Rate at High Temperatures in Carbon Steels

Based on the solute drag model, a practical model incorporating the segregation effect is proposed to calculate grain growth rates in carbon steels. The segregation effect is modeled using two factors: the difference in atomic diameter between a solvent and a substitutional element, and the solubility of a substitutional element. By including the segregation energy, the proposed model enables the simulated retardation of grain growth by the addition of microalloying elements. The calculated grain growth rate by the proposed model shows reasonable correspondence between grain growth rates for experimental and calculated results. The temperature dependence of the grain growth rate is also well simulated.

Modeling of Growth and Dissolution of Grain Boundary Cementite in Vacuum Carburizing Process

In order to predict microstructures during vacuum carburizing, the model which simulates not only the carbon(C) diffusion but also growth/dissolution of cementite(θ) is required. For development of a new model we applied vacuum carburizing to low alloy steels and analyzed the distribution of C and θ by GD-OES and image analysis of microstructures. The C in retained austenite(γ) phase after carburizing was also measured by lattice constants obtained from XRD. We also simulated multi-component diffusion with γ matrix and θ layer to analyze a velocity of the moving interface. The new carburizing model was proposed based on the findings, which suggest that C in γ phase at the carburizing surface is supersaturated and corresponds to C concentration for metastable equilibrium condition to graphite. The growth and dissolution of the θ follow a square root of time with the coefficients controlled by diffusion of Si in γ and Cr in θ respectively. The required parameters such as diffusivity coefficients are obtained by the CALPHAD method. The calculated C distributions and volume fractions of θ represent the experimental results.