Effect of Ti/V ratio on thermodynamics and kinetics of MC in γ/α matrices of Ti-V microalloyed steels

Through the solubility product theory of the ternary secondary phase, classical nucleation theory, and Ostwald ripening theory, a model was established to describe the thermodynamics and kinetics of (Ti, V)C precipitates in austenite/ferrite (gamma/alpha) matrices. The model was used to calculate the volume fraction, precipitation-temperature-time (PTT) curve, and nucleation rate-temperature (NrT) curve of MC (M = Ti, V) precipitates in gamma/alpha matrices in Ti-V microalloyed steels with various Ti/V ratios, which is verified by hardness tester, transmission electron microscopy and energy-dispersive X-ray spectroscopy. The calculations indicate that, by decreasing Ti/V ratio from Ti4V0 steel to Ti0V4 steel, the complete-dissolution temperature decreases monotonically from 1226 to 830 degrees C, and the equilibrium volume fraction of MC precipitated from austenite decreases from 0.333% to 0.091% at 900 degrees C. Moreover, the maximum nucleation temperature of MC precipitated from alpha matrix decreases from 748 to 605 degrees C and the fastest precipitation temperature decreases from 844 to 675 degrees C as Ti/V ratio decreases. PTT and NrT diagrams of MC precipitated from alpha matrices in different Ti-V microalloyed steels all exhibit C-shaped and inverse C-shaped curves. In addition, both theoretical calculation and experimental results show that when tempered at 600 degrees C for 100 h, Ti2V2 steel shows the largest hardness value of 312 HV among the three steels tested because it has a larger volume fraction (0.364%), a larger precipitate density (1689 mu m(-2)), and the smallest average size (8.4 nm) of (Ti, V)C precipitates. The theoretical calculations are consistent with experimental results, which indicates that the thermodynamics and kinetics model for (Ti, V)C precipitates is reliable and accurate.

Automated summary

A model was established to describe the thermodynamics and kinetics of (Ti, V)C precipitates in Ti-V microalloyed steels, which was verified by experimental results. By decreasing the Ti/V ratio, the complete-dissolution temperature, equilibrium volume fraction, and nucleation temperature of MC precipitates all decrease monotonically. Both theoretical calculations and experimental results confirm the reliability and accuracy of the model for (Ti, V)C precipitates.