Modeling the Precipitation Kinetics of Cementite in Bainite in 0.17% Carbon Steel

Abstract

Nanoscale cementite can be obtained in 0.17% carbon steel during isothermal treatment at 500oC after ultra fast cooling (UFC) and thermo-mechanical treatment. The precipitation strengthening contribution to yield strength was more than 250 MPa, when the heat treatment time was less than 20 min. The carbon diffusion is impacted by Mn and Si, which are redistributed during the precipitation process. All the effects induced by substitutional elements can be manifested through the restricted carbon diffusion, which is equal to the carbon diffusion multiplied by adjustable parameters. Based on this, a kinetic model has been adapted to simulate the precipitation behaviors of cementite involving the evolution of the number density per unit volume, radius of cementite over time, and the evolution of carbon concentration in matrix. An excellent agreement in mean radius of particles between the predictions of the model and experimental observations was obtained. It was found that the nucleation period of cementite was very short and did not exceed 0.2 s, and there was an overlap between the nucleation period and the growth period, and the coarsening period began at about 1s. In the growth stage, the carbon concentration in the matrix dropped rapidly and the mean radius of particles increased quickly. In the coarsening stage, the carbon concentration remained unchanged and the number of particles per unit volume fell sharply.