Effect of cooling rate on the nucleation and growth of large TiC particles in Ti-Mo steel

Abstract

The mechanism and kinetics of large TiC precipitates in Ti-Mo steel were investigated in situ with a high-temperature confocal laser scanning microscope and analytically evaluated through thermodynamic calculations in Thermo-Calc. The size of the austenite grains and TiC gradually decreased with increasing undercooling. During the solidification process, strong solute diffusion fields were formed via undercooling, which inhibited the nucleation of TiC; hence, the second phase precipitation temperature was lower than the equilibrium precipitation temperature. Moreover, there was a critical cooling rate between 0.2 and 1.5 K/s that rendered the capacity of γ formation equal to that of TiC formation. The nucleation location of TiC can be changed from grain boundary to grain interior by increasing undercooling. The growth pattern of TiC conformed to the Avrami equation; at higher undercooling, the time exponent was small, and the relationship between the growth length and time was approximately linear. During actual cooling, the driving force exceeded a critical value under a specific degree of undercooling, and the γ/TiC interface became morphologically unstable, leading to a cellular/dendritic pattern.