Isothermal precipitation kinetics of carbides in undercooled austenite and ferrite of a titanium microalloyed steel

A new method for isothermal precipitation kinetics of carbides in undercooled austenite and ferrite has been established based on measuring the strength increments. Precipitation-time-temperature (PIT) diagrams of carbides were determined with this method performed on a Gleeble simulator, for a low-carbon titanium microalloyed steel in a temperature range from 530 degrees C to 670 degrees C. Fine precipitates were studied by using high resolution transmission electron microscope (HRTEM), and their volume fractions together with their attributions to strength increments were calculated and analyzed. Results show that the strength increments are mainly attributed to the Wee. interphase precipitation whose nano-scale TiC precipitates obey the Baker-Nutting orientation relationship with respect to the ferrite matrix: (100)(TiC) parallel to (100)(alpha-Fe) and [011](TiC) parallel to [001](alpha-Fe). The PIT curves obtained have a classic C-shape with their noses at about 600 degrees C, where the strength increment reaches 114 MPa after isothermally holding for 1 h. Additionally, the corresponding precipitation kinetics curves are presented in an S-shape, and a modified form of Avrami formula is introduced to describe their growth periods. The presented mathematical expression fits the experimental data quite well. Consequently, the proposed new method is intuitive, effective and convenient, particularly suitable for detecting the precipitation kinetics in low temperature range. (C) 2016 Elsevier Ltd. All rights reserved.