Model for Strain-Induced Precipitation Kinetics in Microalloyed Steels

Abstract

Based on Dutta and Sellars's expression for the start of strain-induced precipitation in microalloyed steels, a new model has been constructed which takes into account the influence of variables such as microalloying element percentages, strain, temperature, strain rate and grain size. Although the equation given by these authors reproduces well the typical "C" shape of the precipitation start time (P s) curve, the expression is not reliable for all cases. Recrystallization-precipitation-time-temperature (RPTT) diagrams have been plotted thanks to a new experimental study carried out by means of hot torsion tests on approximately twenty microalloyed steels with different Nb, V and Ti contents. Mathematical analysis of the results recommends the modification of some parameters such as the supersaturation ratio (K s) and constant B, which is no longer a constant but a function of K s when the latter is calculated at the nose temperature (T N) of the P s curve. The value of parameter B is deduced from the minimum point or nose of the P s curve, where t 0.05 /T is equal to zero, and it can be demonstrated that B cannot be a constant. The new expressions for these parameters derive from the latest studies undertaken by the authors and this work represents an attempt to improve the model. The expressions are now more consistent and predict the precipitation-time-temperature (PTT) curves with remarkable accuracy. The model for strain induced precipitation kinetics is completed by means of Avrami's equation.

Theoretical and Experimental Nucleation and Growth of Precipitates in a Medium Carbon–Vanadium Steel

“…On the other hand, the carbon extraction replica technique does not easily detect precipitates with sizes of less than 1 nm. The results confirm that the time of 275 s deduced for the end of precipitation, to which the 17 s of the nucleation time must be added, coincides approximately with the experimentally determined precipitation end time [35].…”

From heterogeneous to homogeneous nucleation for precipitation in austenite of microalloyed steels

“…A new model for strain induced precipitation based on expression (2) has recently been published for any microalloyed steel and the following equation has been proposed [32]:…”

“…The exponent of the strain in equation (3) was determined experimentally, having obtained the following expression [32]:…”

“…In contrast, if the G  value is small, but large in absolute terms, nucleation will become preponderantly homogeneous. Given that this occurs whatever the austenite grain size [32], the loss of influence of heterogeneous nucleation on the dislocation nodes is due to a greater occurrence of homogeneous nucleation. According to equation (3), the grain boundaries continue to act as heterogeneous nucleation sites, and it has not yet been demonstrated whether their potential to promote heterogeneous nucleation will be reduced by the increase in driving forces for precipitation.…”

Precipitation model in microalloyed steels both isothermal and continuous cooling conditions

“…New adjustments have been made and in particular the influence of the temperature has been reassessed thanks to the performance of new calculations based on experimental results and new thermodynamic considerations. Although the model will be presented in its entirety, the quantitative influence of the aforementioned variables will simply be summarized, since the model has been recently published in its full version (Medina et al, 2014a;Medina et al, 2014b).…”