Thermal and microstructural softening behaviors during dynamic recrystallization in 3Cr20Ni10W2 alloy

In this work, microstructure evolution of dynamic recrystallization (DRX) behavior in Cr12MoV die steel was investigated via experiments and simulations systematically. Firstly, hot compression tests were performed to obtain the true stress–strain curves. Based on the experimental results, the flow stress model was established, and Avrami equation was developed to model the kinetics of DRX. Then, the cellular automaton (CA) model was established to describe DRX behavior. In order to obtain more accurate simulation results, a microstructure enhancement, extraction and conversion program based on fingerprint image enhancement algorithm was developed to generate real initial microstructure which could be directly used in CA simulation. With real initial microstructure generation, good agreement between simulated and experimental results was achieved, indicating the high accuracy of the established CA model. Finally, the CA model was employed to investigate the hot deformation behavior of Cr12MoV die steel under multiple thermomechanical conditions. It could be found that a lower strain rate was beneficial to the occurrence of DRX. When the strain rate was beyond 1.0 s−1, the DRX fraction was very small. This work would provide a significant guidance to optimize the hot working process of Cr12MoV die steel or some other similar steels.

Microstructure Evolution Modeling and Simulation for Dynamic Recrystallization of Cr12MoV Die Steel During Hot Compression Based on Real Metallographic Image

The high-temperature deformation characteristics of an ultrahigh strength steel is studied by hot compression tests under the deformation temperatures of 920–1160 °C and strain rates of 0.01–10 s−1. By employing the metalloscope and transmission electron microscope, the influences of deformation parameters (deformation temperature and strain rate) upon microstructure evolution are analyzed in detail. Results show that the hot deformation activation energy of the studied steel is estimated as 332.3 kJ/mol. The microstructure presents the typical dynamic recrystallization (DRX) features, and the DRX behaviors are mainly dominated by the discontinuous DRX mechanism under different deformation parameters. Under the low strain rate or the high deformation temperature, the continuous DRX behavior caused by the rotation of subgrains takes place. For forecasting the deformation stress of the studied steel, a physically-based constitutive model is constructed by combining dislocation density theory and DRX kinetics. The reproduced deformation stresses well close to the measured data indicates that the developed physically-based constitutive model enjoys a preferred prediction ability to forecast the high-temperature deformation behaviors for the studied steel.

High-Temperature Deformation Characteristics and Constitutive Model of an Ultrahigh Strength Steel

High N-containing austenitic stainless steels have long been used as materials in orthopedic implants. For almost three decades, research has shown that ASTM F-1586 steel is an alternative for orthopedic applications involving severe loads and long implant survival rates in the human body. However, several studies have detected impaired mechanical strength of prostheses during use as a result of manufacturing processes. In this research work, the dynamic softening of this material is characterized based on a constitutive analysis of the stress–strain curves under conditions resembling those of industrial manufacturing, obtained by continuous isothermal hot torsion tests at different temperatures (900ºC-1200 °C) and strain rates (0.01–10s−1). The results indicate that the hot deformation apparent activation energy (Qdef = 594 kJ/mol) is high compared to other types of 300 series stainless steel, as are the ratios between critical (σc), peak (σp), steady state (σss) and saturation (σsat) stresses: σo/σp = 0.69, σc/σp = 0.94, σss/σp = 0.68 and σsat/σp = 1.01. These high values suggest competition between the mechanisms of work hardening (WH), dynamic recovery (DRV) and dynamic recrystallization (DRX), with delay in the onset and progression of DRX kinetics, significantly affected by the moderate stacking fault energy (γsfe ∼ 68.7 mJ/m2), solute atoms (Nb,N) and by fine Z-phase precipitates (CrNbN) at the grain boundaries, which favor softening with intense DRV. Thus, as can be seen, the parameters of WH (h), DRV (r) and DRX (t0.5, n) determine the shape of the stress–strain curves. 

Constitutive analysis of stress-strain curves in dynamic softening of high Nb- and N-containing austenitic stainless-steel biomaterial

In the present study, restoration and texture evolution mechanisms were investigated during the semi-solid hot compression of AA7075 aluminum alloy using electron backscatter diffraction and x-ray diffraction analysis. The results indicated that the restoration mechanism of the alloy at the semi-solid state can be encountered as a combination of continuous dynamic recrystallization (CDRX) and formation of dilatant bands. CDRX occurred because of the homogeneous increase in misorientation and lattice rotation adjacent to grain boundaries. On the other hand, dilatant bands were found to be formed as a result of sliding and rotation of the globular solid grains. Deformation at low strain rates promoted the role of strain accommodation in the liquid phase as well as subsequent formation of dilatant bands. Following hot compression at semi-solid state, a weak fiber texture of $$ (200)\left\| {\text{CD}} \right. $$ formed at low strain rates, whereas $$ (220)\left\| {\text{CD}} \right. $$ developed at high strain rates.

Effect of Semi-solid Deformation on the Restoration Mechanisms and Texture Evolution in AA7075

The compression deformation of 30Cr2Ni4MoV steel at different temperatures and strain rates is carried out on Gleeble 1500 thermal mechanical simulation tester. Based on the experimental flow curves, the strain hardening rate curves (θ = dσ/de versus σ) are derived, from which the characteristic stresses and strains are identified. Meanwhile, the dependences of the characteristic stresses and strains on Zener-Hollomon parameter are determined and the results show that the value of the critical stress of dynamic recrystallization is close to the value of the steady stress. With the aid of the experimental flow curves, the Avrami equation is employed to describe the kinetics of dynamic recrystallization. The time exponent (n) is expressed as a power law function of Zener-Hollomon parameter and the Avrami constant (k) is determined as a function of half of the time for the complete dynamic recrystallization (t
 50). Furthermore, a constitutive model is presented based on the rule of mixtures when the dynamic recrystallization occurs. Validation of the constitutive model is implemented and the simulated results agree well with the experimental results.

Dynamic recrystallization behavior and constitutive modeling of as-cast 30Cr2Ni4MoV steel based on flow curves

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Dynamic recrystallization behavior and constitutive modeling of as-cast 30Cr2Ni4MoV steel based on flow curves