Hot Deformation Flow Curves Research Articles

Prediction of Hot Deformation Flow Curves of 1.4542 Stainless Steel

The ability of four constitutive equations, Johnson–Cook (JC), Zerilli–Armstrong (ZA), Arrhenius-type constitutive equations and a newly developed phenomenological model for describing the hot flow behavior of 1.4542 stainless steel was evaluated. The hot flow curves were obtained from hot compression tests in the temperature range of 900–1050 °C and at strain rates of 0.001–1 s−1. The JC model was not able to predict the softening part of the flow curves owing to the separated effects of strain, strain rate and temperature on the flow stress. The original ZA model was found to be useful at large strains but the overall consistency between the experimental and the calculated flow curves was not good. The subsequent modifications of the ZA model for low strain levels resulted in acceptable predictions. It is observed that the Arrhenius-type predicted flow curves well agree with the experimental results especially at low strain rates. However, at high strain rates where the steady state condition is shifted to large strains, some deviations were observed at low strain levels. Finally, a phenomenological model was constructed based on the nonlinear estimation of work hardening. The flow curves developed by the model could predict the experimental ones with satisfactory precision.

Evaluation of the Kinetics of Dynamic Recovery in AISI 321 Austenitic Stainless Steel Using Hot Flow Curves

The trend in the variations of the flow stress, obtained in the hot flow curves of materials, reflects the type of microstructural changes that occur during hot deformation. It is also possible to evaluate the kinetics of the relevant microstructural events directly from flow stress data. In the present study, a method for obtaining the kinetics of dynamic recovery from hot deformation flow curves has been proposed and carried out to evaluate the fraction of dynamic recovery in AISI 321 austenitic stainless steel during hot compression deformation in the temperature range of 800–950 °C. Results show that the rate of dynamic recovery is considerably increased by increasing strain rate. It has also been concluded, that the effect of deformation temperature on the kinetics of dynamic recovery is insignificant compared to the effect of strain rate. The flow behavior in a high temperature deformation reflects the type of microstructural changes that occur during deformation and is also possible to evaluate the kinetics of the relevant microstructural events directly from flow curve data. In the present study, a method to evaluate the fraction of dynamic recovery in AISI 321 austenitic stainless steel during hot compression in the temperature range of 800–950 °C has been proposed and carried out. Results indicate that the dynamic recovery process is considerably increased by increasing the strain rate and temperature.

A simple constitutive model for predicting flow stress of medium carbon microalloyed steel during hot deformation

The constitutive behavior of a medium carbon microalloyed steel during hot working over a wide range of temperatures and strain rates was studied using the Johnson–Cook (JC) model, the Hollomon equation, and their modifications. The original JC model was not able to predict the softening part of the flow curves and the subsequent modifications of the JC model to account for the softening stage and the strain dependency of constants were not satisfactory owing to the uncoupled nature of the JC approach regarding strain rate and temperature. The coupled effect of these variables was considered in the form of Zener–Hollomon parameter (Z) and the constants of the Hollomon equation were related to Z. This modification was found to be useful for the hardening stage but the overall consistency between the experimental flow curves and the calculated ones was not good. Therefore, a simple constitutive model was proposed in the current work, in which by utilization of the peak stress and strain into the Hollomon equation, good prediction abilities were attained. Conclusively, the proposed model can be considered as an efficient one for modeling and prediction of hot deformation flow curves.

Modeling the hot deformation flow curves of API X65 pipeline steel

In this paper, a new simple model was developed based on a power function of Zener–Hollomon parameter and a third-order polynomial function of strainm (m is a constant) to model the hot flow stress of API X65 pipeline steel. The results of the developed equation for modeling the hot flow stress of API X65 pipeline steel was compared with two other constitutive equations, including the Arrhenius equation with strain-dependent constants and an equation with the peak stress, peak strain, and four constants. The performance of all three equations was compared with each other using a root-mean-square error (RMSE) criterion. According to the results obtained, the developed model showed better performance than that of the Arrhenius equation, but worse performance than that of the equation with the peak stress and peak strain. The simplicity of the proposed model is its advantage over the other considered constitutive equations. However, its performance is in an acceptable range.

Development of dynamic recrystallization maps based on the initial grain size

A series of hot compression tests at the temperature range of 900–1100°C and strain rate of 0.01–1s−1 was carried out using a 304L stainless steel with different initial grain sizes. The hot deformation flow curves, the corresponding strain hardening curves, and the resulting microstructures were analyzed in order to reveal the occurrence of dynamic recrystallization (DRX). Results showed that despite the shape of flow curves, DRX occurs in the range of deformation conditions examined in the current work. Some DRX maps were developed by considering the effect of initial grain size and deformation conditions. These maps showed that the effect of austenite grain size on the progress of DRX in the temperature–strain rate coordinate is considerable and should be taken into account in thermomechanical processing. Since the occurrence and the extent of static and metadynamic recrystallization highly depend on the completion of DRX during deformation, the full DRX maps based on grain size can be used to find the appropriate working conditions based on the desired microstructures in the hot deformation schedules, especially in the dynamic recrystallization controlled rolling.

Modeling and Prediction of Hot Deformation Flow Curves

The modeling of hot flow stress and prediction of flow curves for unseen deformation conditions are important in metal-forming processes because any feasible mathematical simulation needs accurate flow description. In the current work, in an attempt to summarize, generalize, and introduce efficient methods, the dynamic recrystallization (DRX) flow curves of a 17-4 PH martensitic precipitation hardening stainless steel, a medium carbon microalloyed steel, and a 304 H austenitic stainless steel were modeled and predicted using (1) a hyperbolic sine equation with strain dependent constants, (2) a developed constitutive equation in a simple normalized stress-normalized strain form and its modified version, and (3) a feed-forward artificial neural network (ANN). These methods were critically discussed, and the ANN technique was found to be the best for the modeling available flow curves; however, the developed constitutive equation showed slightly better performance than that of ANN and significantly better predicted values than those of the hyperbolic sine equation in prediction of flow curves for unseen deformation conditions.

Prediction of hot compression flow curves of Ti–6Al–4V alloy in α + β phase region

Prediction of material flow behavior is essential for designing the forming process of any material. In this research, experimental flow curves of Ti–6Al–4 V alloy were obtained using the isothermal hot compression test done at 750–950 °C with 50 °C intervals and constant strain rates of 0.001, 0.005 and 0.01 s −1. For prediction of hot deformation flow curves two methods of modeling were applied. In the first method, an entire flow curve was modeled using Sellars equation. In the second one, modeling of a flow curve up to the peak point was carried out with Cingara model, and modeling beyond that was performed with a model developed based on the Johnson–Mehl–Avrami–Kolmogorov (JMAK) theory. The accuracy of each model was examined through a statistical method. Results showed that flow curve modeling using Cingara model and JMAK theory leads to results that are more consistent with the experimental data.

Modelling of flow curves for hot deformation

A new mathematical model for hot deformation flow curves is presented. The developed expression can be used for austenite or ferrite deformation, for monotonically increasing flow curves as well as for those with a maximum or with oscillating shape. By experimental determination of 4 flow curves, the parameters for the flow curve field can be calculated. Experiments have been carried out using aluminium‐ killed mild steels in hot compression tests with a range of deformation temperature between 700 and 1250°C and of strain rate between 0.1 and 20 s‐1. The accuracy of the new approach and the ability to meet the different flow curve shapes proved to be very good.