Artificial Neural Network Modelling of the Effect of Vanadium Addition on the Tensile Properties and Microstructure of High-Strength Tempcore Rebars

In high-strength rebar, the various microstructures obtained by the Tempcore process and the addition of V have a complex effect on the strength improvement of rebar. This study investigated the mechanism of strengthening of high-strength Tempcore rebars upon the addition of vanadium through artificial neural network (ANN) modelling. Various V contents (0.005, 0.072 and 0.14 wt.%) were investigated, and a large amount of bainite and V(C, N) were precipitated in the core of the Tempcore rebar in the high-V specimens. In addition, as the V content increased, the number of these fine precipitates (10–30 nm) increased. The precipitation strengthening proposed by the Ashby–Orowan model is a major contributing factor to the yield-strength increase (35 MPa) of the Tempcore rebar containing 0.140 wt.% V. The ANN model was developed to predict the yield and tensile strengths of Tempcore rebar after the addition of various amounts of V and self-tempering at various temperatures, and it showed high reproducibility compared to the experimental values (R-square was 93% and the average relative error was 2.6%). ANN modelling revealed that the yield strength of the Tempcore rebar increased more significantly with increasing V content (0.01–0.2 wt.%.) at relatively high self-tempering temperatures (≥530 °C). These results provide guidelines for selecting the optimal V content and process conditions for manufacturing high-strength Tempcore rebars.

Kinetics of Austenite Phase Transformations in Newly-Developed 0.17C-2Mn-1Si-0.2Mo Forging Steel with Ti and V Microadditions

This work presents the results of phase transformation kinetics during continuous cooling in newly developed high strength low-alloy steel (HSLA). Initial theoretical calculations for the determination of heat treatment parameters were conducted. To determine the structural constituents formed due to the austenite decomposition the dilatometry approach was used. The material was cooled down from the austenitization temperature of 1000 °C with cooling rates between 0.1 °C/s to 60 °C/s. Then, light and scanning electron microscopy investigations were carried out. The microstructure after cooling at rates between 0.1 °C/s up to 1 °C/s is mainly ferritic with some fraction of granular bainite. Increasing the cooling rate led to formation of a higher fraction of bainitic ferrite. At 60 °C/s the microstructure was mainly bainite with some fraction of ferrite. To determine the presence of retained austenite, color etching using Klemm solution was used. The results show that the increase of cooling rate decreases the amount of retained austenite in the microstructure of the steel. Hardness measurements were made to determine the changes in the mechanical properties as a function of the cooling rate.

Effect of Selected Cooling and Deformation Parameters on the Structure and Properties of AISI 4340 Steel

The paper is focused on investigation of the high-strength AISI 4340 steel at various temperature and deformation conditions. The article is divided into two specific analyses. The first is to examine the dilatation behavior of the steel at eight different cooling rates, namely, 100, 10, 5, 1, 0.5, 0.1, 0.05 and 0.01 °C·s^-1. The mapping of the phase transformations due to varying cooling rates from the austenitizing temperature of 850 °C allows the construction of the CCT diagram for a given high-strength steel. These dilatation curves were also compared with the metallography of the selected samples for the proper construction of the CCT diagram. A further analysis of the high temperature deformation of high strength steel AISI 4340 was performed in the range of temperature 900-1200 °C, and the strain rate was in the range from 0.001 to 10 s^-1 with maximum value of the true strain 0.9. Changes in the microstructure were observed using light optical microscopy (LOM). The effect of hot deformation temperature on true stress, peak stress and true strain was investigated. The hardness of all deformed samples, depending on the temperature, the deformation rate and the peak stress σ_p overall together related with hardness, has also been evaluated.

Dilatometric Analysis of the Austenite Decomposition in Undeformed and Deformed Low-Carbon Structural Steel

This paper aims to analyze the effect of deformation on the phase transformation kinetics of low-carbon structural steel. The steel used for the investigation was subjected to two different dilatometric analyses using a DIL 805A/D device. The first analysis was to determine the phase transformation kinetics without deformation of austenite before cooling. Then, the analysis under deformation conditions was conducted to investigate the deformation effect on the transformation kinetics. Microscopic studies by light microscopy were performed. The essential part of the research was hardness analysis for different cooling rates and the creation of continuous-cooling-transformation (CCT) and deformation continuous-cooling-transformation (DCCT) diagrams. It was found that the deformation of the samples before cooling increases a diffusion rate in the austenite resulting in the corresponding increase of ferritic, pearlitic, and bainitic start temperatures, as well as shifting the austenite transformation product regions to a longer time. The increase of the transformation area and a decrease in grain size are observed for the deformed samples.

Effects of Austenitization Temperature and Pre-Deformation on CCT Diagrams of 23MnNiCrMo5-3 Steel

The combined effect of deformation temperature and strain value on the continuous cooling transformation (CCT) diagram of low-alloy steel with 0.23% C, 1.17% Mn, 0.79% Ni, 0.44% Cr, and 0.22% Mo was studied. The deformation temperature (identical to the austenitization temperature) was in the range suitable for the wire rolling mill. The applied compressive deformation corresponded to the true strain values in an unusually wide range. Based on the dilatometric tests and metallographic analyses, a total of five different CCT diagrams were constructed. Pre-deformation corresponding to the true strain of 0.35 or even 1.0 had no clear effect on the austenite decomposition kinetics at the austenitization temperature of 880 °C. During the long-lasting cooling, recrystallization and probably coarsening of the new austenitic grains occurred, which almost eliminated the influence of pre-deformation on the temperatures of the diffusion-controlled phase transformations. Decreasing the deformation temperature to 830 °C led to the significant acceleration of the austenite → ferrite and austenite → pearlite transformations due to the applied strain of 1.0 only in the region of the cooling rate between 3 and 35 °C·s^-1. The kinetics of the bainitic or martensitic transformation remained practically unaffected by the pre-deformation. The acceleration of the diffusion-controlled phase transformations resulted from the formation of an austenitic microstructure with a mean grain size of about 4 µm. As the analysis of the stress-strain curves showed, the grain refinement was carried out by dynamic and metadynamic recrystallization. At low cooling rates, the effect of plastic deformation on the kinetics of phase transformations was indistinct.

Glass Forming Ability of Amorphous Drugs Investigated by Continuous Cooling and Isothermal Transformation

The aim of this study was to investigate the glass forming ability of 12 different drugs by the determination of continuous cooling and isothermal transformation diagrams in order to elucidate if an inherent differentiation between the drugs with respect to their the glass forming ability can be made. Continuous-cooling-transformation (CCT) and time-temperature-transformation (TTT) diagrams of the drugs were developed in order to predict the critical cooling rate necessary to convert the drug from the melt into an amorphous form. While TTT diagrams overestimated the actual critical cooling rate, they allowed an inherent differentiation of glass forming ability for the investigated drugs into drugs that are extremely difficult to amorphize (>750 °C/min), drugs that require modest cooling rates (>10 °C/min), and drugs that can be made amorphous even at very slow cooling rates (>2 °C/min). Thus, the glass forming ability can be predicted by the use of TTT diagrams. In contrast to TTT diagrams, CCT diagrams may not be suitable for small organic molecules due to poor separation of exothermic events, which makes it difficult to determine the zone of recrystallization. In conclusion, this study shows that glass forming ability of drugs can be predicted by TTT diagrams.