Effects of chemical composition and austenite deformation on the onset of ferrite formation for arbitrary cooling paths

Abstract We present a computational method for calculating the phase transformation start for arbitrary cooling paths and for different steel compositions after thermomechanical treatments. We apply the method to quantitatively estimate how much austenite deformation and how many different alloying elements affect the transformation start at different temperatures. The calculations are done for recrystallized steel as well as strain hardened steel, and the results are compared. The method is parameterized using continuous cooling transformation (CCT) data as an input, and it can be easily adapted for different thermomechanical treatments when corresponding CCT data is available. The analysis can also be used to obtain estimates for the range of values for parameters in more detailed microstructure models

Modelling of austenite transformation along arbitrary cooling paths

Abstract A computational model based on the Johnson-Mehl-Avrami-Kolmogorov equation for simulating the onset and kinetics of austenite to bainite and martensite transformation has been fitted to experimental continuous cooling data for two different steels. We investigated how deformation below recrystallization temperature affected the transformation onset and kinetics in comparison to the same steel in the undeformed state. The fitted model can be used to simulate phase transformations occurring when the steel is cooled along any cooling path. The model can be fully coupled to heat transfer and conduction simulations in order to optimize cooling practice, for example in industrial thermomechanical processing of steel. The fitted model can also be used to predict the hardness of the steel after cooling

Structure-property correlations of a medium C steel following quenching and isothermal holding above and below the Mₛ temperature

Abstract The processing of advanced multiphase high strength steels often includes isothermal treatments around the martensite start temperature (Ms) for achieving a refined microstructure comprising bainite-austenite and/or bainite-martensite-austenite phase constituents. The objective of this research work was to investigate the structure-property relationship for a medium carbon, high-silicon DIN 1.5025 steel (Fe-0.529C-1.67 Si-0.72Mn-0.12Cr (in wt.%)) following isothermal holding close to the Ms temperature (~275°C) to enable low temperature austenite decomposition. For realizing multiphase microstructures, DIN 1.5025 steel samples were austenitized at 900°C for 5 min and then quenched to the isothermal holding temperatures 350 and 250°C for various times ranging from 5 to 3600 s. Microstructural investigation corroborated the formation of multiphase microstructure comprising tempered martensite, bainite, retained austenite, and fresh martensite in both the samples isothermally held above (350°C) and below the Ms (250°C) temperature. The sample isothermally held at 250°C showed a much more refined microstructure in comparison to that held at 350°C due to the presence of a fraction of initial martensite laths which acted as potential sites for bainite nucleation. Also, the evaluation of mechanical behaviour showed that the best tensile properties in terms of high tensile strength and good ductility were achieved in samples with high volume fractions of both interlath and blocky retained austenite, particularly those isothermally treated at 350°C for 200 s and at 250°C for 600 s, respectively

Flow Stress Behaviour and Static Recrystallization Characteristics of Hot Deformed Austenite in Microalloyed Medium-Carbon Bainitic Steels

In the past decade, efforts have been focused on developing very fine, medium-carbon bainitic steels via the low-temperature (typically 300–400 °C) ausforming process, which not only enables shorter isothermal holding times for bainitic transformation at low temperatures, but also offers significantly improved strength. This paper describes static recrystallization (SRX) characteristics of austenite in four medium-carbon 2%Mn-1.3%Si-0.7%Cr steels with and without microalloying intended for the development of these steels. The stress-relaxation method on a Gleeble simulator resulted in recrystallization times over a wide range of temperatures, strains and strain rates. Also, the occurrence of precipitation was revealed. Powers of strain (−1.7 to −2.7) and strain rate (−0.21 to −0.28) as well as the apparent activation energies (225–269 kJ/mol) were in the ranges reported in the literature for C-Mn and microalloyed steels with lower Mn and Si contents. The new regression equations established for estimating times for 50% SRX revealed the retardation effects of microalloying and Mo addition showing reasonable fits with the experimental data, whereas the previous model suggested for ordinary microalloyed steels tended to predict clearly shorter times on average than the experimental values for the present coarse-grained steels. The Boratto equation to estimate the non-recrystallization temperature was successfully modified to include the effect of Mo alloying and high silicon concentrations

Microstructural evolution in a high-silicon medium carbon steel following quenching and isothermal holding above and below the Ms temperature

Abstract In this study, the microstructural features evolved in a high-Si, medium-carbon steel (Fe-0.53C-1.67Si-0.72Mn-0.12Cr) subjected to quenching and isothermal holding at temperatures above and below the martensite start temperature (Ms) for one hour have been examined. Both laser scanning confocal and transmission electron microscopy were employed for detailed microstructural characterization, supported by dilatometry, X-ray diffraction, and hardness measurements. In the case of isothermal treatment above Ms at 300 °C, besides bainite transformation marked by typical S-shaped dilatation curve, high-carbon martensite is formed during the final cooling to room temperature. In the case of isothermal treatment below Ms at 250 °C, the initial martensite formation and subsequent carbon partitioning to austenite is followed by the formation of bainite containing carbides and some high-carbon martensite that forms during the final cooling to room temperature. Also, selected area diffraction patterns (SAED) for both of Q&B and Q&P heat treated samples showed there are extra weak diffraction spots, presumably due to the presence of omega phase (ω) as an intermediate phase during fcc (face- centered cubic) austenite to bcc (body- centered cubic) martensite transformation and is considered as a common substructure in bcc metals and alloys with a coherent interface with the matrix: aω = √2 × abcc and caω = √3/2 × abcc that appeared in twinning martensite or martensite regions with dislocations as a substructure

Dynamic softening kinetics of Al0.3CoCrFeNi high-entropy alloy during high temperature compression and its correlation with the evolving microstructure and micro-texture

Abstract To establish the characteristics and kinetics of dynamic softening in a Al0.3CoCrFeNi high–entropy alloy (HEA), isothermal compression tests were carried out in a suitable temperature range of 1273–1423 K at 10-2 and 10-1 s-1 in accord with our previous study. It was found that the discontinuous dynamic recrystallization (DRX) was the dominant microstructural reconstitution mechanism. The conditions of critical stress/strain for the onset of dynamic recrystallization were determined using the Poliak–Jonas analytical criterion. Further, a kinetic model was established based on the Avrami-type function in order to be able to predict the volume fraction of DRX. The DRX volume fraction expectedly increased with strain. The microstructural investigation of the isothermally compressed specimens revealed a good agreement with the proposed DRX kinetics model and validated its accuracy. Additionally, the evolution of DRX with strain was characterized by interrupting the test carried out at 1323 K/10-1 s-1 at different strains. The progress of DRX evolving as increased formation of new recrystallized grains further corroborated the predictions of the kinetic model. The micro-texture analysis revealed random texture in the recrystallized grains, whereas the unrecrystallized grains had shown their preferred orientation towards the <101> fiber texture

Characterization of hot deformation behavior of Al0.3CoCrFeNi high-entropy alloy and development of processing map

Abstract This study presents the characteristics of hot deformation behavior of a Al0.3CoCrFeNi high–entropy alloy in the temperature and strain rate ranges of 1023–1423 K and 10–3–10 s–1, respectively. The constitutive flow behavior was modeled based on the hyperbolic–sinusoidal Arrhenius–type equations and a mathematical relation was used to observe the influence of true strain on material constants. To define the hot workability of the alloy, a processing map was developed based on the principles of the dynamic materials model. Accordingly, a dynamic recrystallization (DRX) domain was identified as prudent for processing in the temperature and strain rate ranges of 1273–1423 K and 10–2–2 × 10–1 s–1 respectively, with a peak efficiency of ~45% at 1423 K/6 × 10–2 s–1. At lower temperatures (1048–1148 K) and strain rates (10–3–3 × 10–3 s–1), a dynamic recovery (DRV) domain was identified with a peak efficiency of 38% at 1123 K/10–3 s–1. A large instability regime occurred above 3 × 10–1 s–1 with an increased tendency of adiabatic shear bands. It extended to lower strain rates 10–2–10–1 s–1 at temperatures < 1123 K, manifested by localized shear bands and grain boundary cracking. At low strain rates (5 × 10–3–10–3 s–1) and temperatures (1148–1298 K), particle stimulated nucleation of new DRX grains occurred at B2 precipitates, though the efficiency of power dissipation dropped sharply to ∼9%

Constitutive modelling of hot deformation behaviour of a CoCrFeMnNi high-entropy alloy

Abstract Models describing the constitutive flow behaviour of a metallic material are desired for appropriate process design and realization of defect-free components. In this study, constitutive equations based on the hyperbolic-sinusoidal Arrhenius-type model have been developed to define the hot deformation characteristics of a CoCrFeMnNi high-entropy alloy. The experimental true stress-true strain data were generated over a wide temperature (1023–1423 K) and strain rates (10−3–10 s−1) ranges. The impact of strain rate and temperature on deformation behaviour was further characterized through a temperature compensated strain rate parameter, i.e. Zener-Hollomon parameter. Additionally, a mathematical relation was employed to express the influence of various material constants on true-strain ranging from 0.2 to 0.75. Typical third order polynomial relations were found to be appropriate to fit the true-strain dependency of these material constants. The accuracy of the developed constitutive equations was evaluated by using the average absolute relative error (AARE) and correlation coefficient (R); the obtained values were 7.63% and 0.9858, respectively, suggesting reasonable predictions. These results demonstrate that the developed constitutive equations can predict the flow stress behaviour of the alloy with a good accuracy over a wide range of temperature and strain rate conditions and for large strains

Characteristics of dynamic softening during high temperature deformation of CoCrFeMnNi high-entropy alloy and its correlation with the evolving microstructure and micro-texture

Abstract The characteristics of dynamic recrystallization (DRX) of a CoCrFeMnNi high–entropy alloy (HEA) was investigated via hot compression testing in the temperature range 950–1100 °C and at true strain rates of 10–2 and 10–1 s-1. The discontinuous DRX was found to be the dominant mechanism corroborating the microstructural evolution. The progress of the initiation of DRX was investigated in terms of critical strain/stress required using the Poliak–Jonas analytical criterion. Consequently, a new kinetic model based on Avrami–type function was established for the HEA to predict the DRX fractional recrystallization. It was revealed that the volume fraction of DRX grains increased with increasing strain. In the case of 10–2 s-1, steady–state flow was achieved after the completion of one DRX process cycle resulting in further straining, leading to the occurrence of dynamic restoration processes involving formation of substructures and generation and annihilation of dislocations inside the DRX grains which effectively increased the fraction of partially deformed DRX (substructured) grains. A good agreement between the proposed DRX kinetics model and microstructure observation results validated the accuracy of DRX kinetics model for CoCrFeMnNi HEA. The preferred orientation of the non–recrystallized grains was towards the formation of <101> fiber texture, whereas a random micro–texture is revealed in the recrystallized grains

Processing map for controlling microstructure and unraveling various deformation mechanisms during hot working of CoCrFeMnNi high entropy alloy

Abstract In the current study, the hot deformation characteristics and workability of a CoCrFeMnNi high entropy alloy was characterized using processing maps developed on the basis of dynamic materials model in the temperature range 1023–1423 K and strain rate range 10⁻³–10s⁻¹. The processing map delineated various deterministic domains including those of cracking processes and unstable flow, thus enabling identification of a ‘safe’ processing window for the hot working of the alloy. Accordingly, a deterministic domain in the temperature and strain rate ranges of 1223–1373 K and 10⁻²–5 × 10⁻¹s⁻¹, respectively, was identified to be the domain of dynamic recrystallization (DRX) with a peak efficiency of the order of ~34% at 1293 K and 3 × 10⁻²s⁻¹ and these were considered to be the optimum parameters for hot deformation. The DRX grain size was dependent on the deformation temperature and strain rate, increasing with the increase in temperature and decrease in strain rate, whereas DRX volume increased with the strain rate. At still higher temperatures (1403–1423 K) and lower strain rates (10⁻³–3 × 10⁻³s⁻¹), there was a sharp decrease in efficiency values from 27% to 5% thus forming a trough and the microstructure was characterized with coarse grains. In the instability regime, grain boundary cracking/sliding and localized shear bands manifested at temperatures <1223 K and strain rates <10⁻²s⁻¹. The increase in strain rate resulted in an intense adiabatic shear banding along with formation of voids. At 10s⁻¹ and temperatures >1398 K, microstructural reconstitution occurred in the shear bands leading to the formation of fine grains, presumably as a consequence of continuous recrystallization