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Young's modulus for ferritic and austenitic steel at high temperatures is typically acquired through acoustic emission tests. In this study, we applied an inverse solution method to consider the effect of a temperature gradient on samples under experimental conditions. Wachman model, Varshni models, and power model were used to describe the influence of temperature on the sound velocity of steel. The constants for these equations were acquired using an optimization scheme that minimized the error between calculated and measured travel times of sound in the samples. Using equations developed for the change in velocity under various temperatures, we calculated Young's modulus at low and high temperatures with acceptable accuracy.This method is non-destructive, and can be applied to measure Young's modulus at different temperatures using a single sample. The acquired Young's modulus values were compared with the results of a modified uniaxial tensile test that accurately measures strain at elevated temperatures. The results showed good agreement between Young's modulus values obtained using the two methods, demonstrating that incorporating the non-linear dependency of the sound velocity on temperature in Young's modulus calculations leads to higher accuracy at low and high temperatures.

The static recrystallization (SRX) behavior of 42CrMo steel was investigated by a non-isothermal hot compression test. The conventional empirical equation for SRX was applied to a finite element (FE) simulator using a nonconventional manner of parameter characterization. Parameter characterization was conducted by applying the FE-coupled optimization method to cylinder compression, which is an axisymmetric upsetting process. An accurate and practical flow stress model was used to precisely determine the critical strain value and define the instant when SRX occurred. The results revealed that the microstructural evolution during SRX was substantially influenced by temperature, strain rate, degree of deformation, and initial austenite grain size. The predicted recrystallized SRX grain sizes exhibited good agreement with the measured values when the model's parameters for the calculation of grain size were obtained using a FE-coupled optimization scheme.

We present a direct method to calculate by which to calculate flow-related dynamic recrystallization (DRX) kinetic parameters, including the peak strain and the strain at 50% DRX which are essential when predicting microstructural evolution. The method is based on an accurate general description of flow curves using the general and improved C-m models in which C and m are defined, at various strains and temperatures, as functions of the strain rate. The method eschews mathematical modeling of these parameters when determining the volume fraction of dynamically recrystallized grains and greatly increases the practical utility of microstructural prediction. A finite element approach based on the direct method and the Avrami kinetic model is utilized to reveal the DRX behavior of the magnesium alloy AZ91D. A comparison among our present approach, other approaches, and experiments reveals that the new method predicts the DRX kinetics and the grain sizes during microstructural evolution with remarkable accuracy.

In this study, a promising model is presented to describe the flow stress curves with acceptable accuracy as well as generality. In this model, peak strain, peak stress, steady-state stress and hardening and softening constants are uncoupled and expressed as either bi-linearly interpolated or closed-form functions of temperature and strain rate. A practical method to obtain material constants is developed to render the model applicable in practice, which is based on optimization scheme. The flow stress curves of four materials are obtained using the new model and compared with experiments or the other models at some specific temperatures and strain rates. The comparison has revealed that the new model is feasible and general to describe the flow stress curves of various metals and alloys at different temperatures and strain rates with acceptable accuracy. Keywords: Flow stress model, Closed-form function, Bi-linear interpolation, Softening, Hardening

The temperature dependency of the dynamic recrystallization (DRX) activation energy and strain rate exponent, which are major material properties in the Johnson–Mehl–Avrami–Kolmogorov (JMAK) model affecting the DRX phenomenon, are quantitatively presented in experimental and numerical ways. A finite element analysis-based optimization method was used to acquire the material properties of the JMAK model. The first and second stages of a three-stage hot forging process for a bearing outer race were used to obtain and verify all of the material properties, including the DRX activation energy and the strain rate exponent of the JMAK model, which were assumed to be constants or functions of temperature. The predicted grain size after the third stage obtained with the optimized material properties was compared with the experimental values to validate the acquired material properties and reveal the dependence of the two major material properties on temperature. The comparison showed that the difference between the measured and predicted grain sizes was significantly smaller for temperature-dependent material properties, indicating that the DRX activation energy and strain rate exponent are highly temperature-dependent. Keywords: Activation energy, Dynamic recrystallization, Grain size, JMAK model, Strain rate exponent, Temperature effect