A continuum model accounting for the effect of the initial and evolving microstructure on the evolution of dynamic recrystallization

Laser assisted forming is a process which is increasingly being adopted by the industry. Application of heat by a laser to austenitic stainless steel (ASS) sheet provides local control over formability and strength of the material. The hot forming behavior of ASS is characterized by significant dynamic recovery and dynamic recrystallization. These two processes lead to a softening stress-strain response and have a significant impact on the microstructure of the material. Most of the research performed on hot forming of ASS focuses on dynamic recrystallization and then specifically on the behavior of the annealed state, consisting of relatively large equiaxed austenite grains. However, in industry it is common to use cold rolled ASS sheet which is a mixture of austenite and martensite. Application of a laser heat treatment to the cold rolled grades of ASS induces a socalled ‘reverse’ transformation of martensite to austenite which, depending on the exact time-temperature combinations, leads to an austenite grain size in the range of nanoto micrometer. It is known from experiments that the effect of initial grain size on dynamic recrystallization is significant, especially on the initial stages of recrystallization. Therefore any continuum model capable of describing hot forming of cold rolled ASS should include the effect of the initial grain size. In this work a physically based continuum model for dynamic recrystallization is presented which accounts for the effect of the initial and evolving grain size on the evolution of dynamic recrystallization. It is shown that the initial grain size can be accounted for by incorporating its effect on the availability of preferred nucleation sites, i.e. grain edges. The new model is compared to experimental results and it is shown that the model correctly predicts accelerated recrystallization with decrease in grain size and that there is a weak dependence of the dynamically recrystallized grain size on the initial grain size. Furthermore predicted recrystallized grain sizes are in good agreement with the experimentally measured values

Dynamic recrystallization mechanisms and twining evolution during hot deformation of Inconel 718

The hot deformation behavior of an IN718 superalloy was studied by isothermal compression tests under the deformation temperature range of 950–1100 °C and strain rate range of 0.001–1 s-1 up to true strains of 0.05, 0.2, 0.4 and 0.7. Electron backscattered diffraction (EBSD) technique was employed to investigate systematically the effects of strain, strain rate and deformation temperature on the subgrain structures, local and cumulative misorientations and twinning phenomena. The results showed that the occurrence of dynamic recrystallization (DRX) is promoted by increasing strain and deformation temperature and decreasing strain rate. The microstructural changes showed that discontinuous dynamic recrystallization (DDRX), characterized by grain boundary bulging, is the dominant nucleation mechanism in the early stages of deformation in which DRX nucleation occurs by twining behind the bulged areas. Twin boundaries of nuclei lost their ¿3 character with further deformation. However, many simple and multiple twins can be also regenerated during the growth of grains. The results showed that continuous dynamic recrystallization (CDRX) is promoted at higher strains and large strain rates, and lower temperatures, indicating that under certain conditions both DDRX and CDRX can occur simultaneously during the hot deformation of IN718.Peer ReviewedPostprint (author's final draft

Inferring dynamic recrystallization in ferrite using the kinetics of static recrystallization

A general relationship between the kinetics of dynamic and static recrystallization is developed. It is predicted that conventional dynamic recrystallization will occur whenever the deformation time exceeds the adjusted start time for static recrystallization. This approach is verified using data for austenite and lead. It is then applied to current and previous work on ferrite. The model provides support for the contention that conventional dynamic recrystallization occurs in low carbon ferrite if deformation is carried out at high temperatures and low strain rates. In the present work, which was carried out at 700 &deg;C, evidence for dynamic recrystallization was observed for strain rates less than around 0.01 s&minus;1. At higher strain rates, the model predicts a critical strain for the onset of dynamic recrystallization that exceeds the critical strain for the beginning of the recovery steady-state region. While the model allows dynamic recrystallization to begin in this region, the critical strain for its onset is expected to increase rapidly with increasing strain rate and decreasing temperature once steady state has been reached. <br /

Study of two bovine bone blocks (sintered and non-sintered) used for bone grafts: physico-chemical characterization and in vitro bioactivity and cellular analysis

High-temperature compression and electron backscatter diffraction (EBSD) techniques were used in a systematic investigation of the dynamic recrystallization (DRX) behavior and texture evolution of the Inconel625 alloy. The true stress–true strain curves and the constitutive equation of Inconel625 were obtained at temperatures ranging from 900 to 1200 °C and strain rates of 10, 1, 0.1, and 0.01 s−1. The adiabatic heating effect was observed during the hot compression process. At a high strain rate, as the temperature increased, the grains initially refined and then grew, and the proportion of high-angle grain boundaries increased. The volume fraction of the dynamic recrystallization increased. Most of the grains were randomly distributed and the proportion of recrystallized texture components first increased and then decreased. Complete dynamic recrystallization occurred at 1100 °C, where the recrystallized volume fraction and the random distribution ratios of grains reached a maximum. This study indicated that the dynamic recrystallization mechanism of the Inconel625 alloy at a high strain rate included continuous dynamic recrystallization with subgrain merging and rotation, and discontinuous dynamic recrystallization with bulging grain boundary induced by twinning. The latter mechanism was less dominan

Static grain growth in an austenitic stainless steel subjected to intense plastic straining

The post-dynamic recrystallization of an ultrafine grained 304-type austenitic stainless steel was studied during annealing at 800 and 1000°C for 7.5 to 480 minutes. The initial ultrafine grained microstructures have been developed by continuous dynamic recrystallization during isothermal multidirectional forging to a total strain of ~4 at temperatures ranging from 500 to 800°C. The post-dynamic recrystallization involves a rapid softening at early stage of annealing followed by a sluggish decrease of hardness upon further annealing. A transient recrystallization at early annealing stage results in somewhat heterogeneous microstructures in the samples subjected to previous deformation at relatively low temperatures of 500-600°C. This structural heterogeneity disappears with increasing the annealing tim

Investigation of flow stress behavior of AISI 4340 steel in thermomechanical process

In this study, flow stress behavior of AISI 4340 steel in thermomechanical process was investigated under temperature and strain rate ranges of 1173 to 1373 K and 0.01 to 1 s-1, respectively. In flow curves, mechanisms such as work hardening (WH), dynamic recovery (DRV) and dynamic recrystallization (DRX) occurred. It was also discovered that the flow stress decreases with the increase of deformation temperature and the decrease of strain rate. Flow stress curves declared that in low-strain rate and high temperature, dynamic recrystallization overcome work hardening. Also, decreasing temperature led to dynamic recovery and incomplete dynamic recrystallization. Work hardening rate-stress curves depicted that the presence of a turning point expresses dynamic recrystallization mechanism and sub-boundaries are formed at the beginning of where a turning point occurs. In partial dynamic recrystallization, the microstructure was consisted of long grains reshaped because of deformation and some recrystallized grains that nucleated around those reshaped long grains. The results also indicated that at temperature of 1373 K, stress value of σsf, for strain rate of 0.01 s-1 was increased from 27.8 MPa to 96.5 MPa and also for strain rate of 1 s-1 and stress of σc was increased from 32.3 MPa to 105 MPa. The significance of the approach used in this work was any increase in strain rate leads to accelerating dislocation movements. Therefore, dislocations will hit the barriers sooner and will be stopped and also, as a result of delayed dynamic recovery due to dislocations movements, dynamic recrystallization is also delayed

Modeling and simulation of viscoplasticity, recrystallization, and softening of alloyed steel during hot rolling process

Hot rolling is one of the most important and complex deformation processes in steel manufacturing and is essential to final product quality. The objective of this study is to investigate viscoplasticity, dynamic recrystallization, and static softening of alloyed metal during hot rolling process. Gleeble hot compression tests were performed to provide experimental stress-strain curves at different temperatures and strain rates. An inverse finite element analysis was performed to calibrate the experimental curves. Viscoplastic models including a Johnson-Cook (JC) model, a Zerilli-Armstrong (ZA) model, and a combined JC and ZA model were developed. Dynamic recrystallization behavior was investigated and modeled based on single hot compression test. Work hardening rate curve and dynamic recovery curve were modeled to calibrate the kinetics of dynamic recrystallization. Double hit tests were designed and performed and static softening model was developed at varying interpass time, pre-strain, temperature, and strain rate. Subroutines accounting for developed viscoplasticity, dynamic recrystallization, and static softening were developed and implemented into a three-dimensional finite element model of round bar hot rolling. The combined JC and ZA model demonstrated better agreement with experimental data than other traditional models. Dynamic recrystallization occurred throughout the round bar during hot rolling and is significantly influenced by the plastic strain and temperature. Static softening occurred rapidly in the beginning of interpass and then slowed down. Compared to rolling speed, rolling temperature demonstrated more significant influence on dynamic recrystallization and static softening during round bar hot rolling --Abstract, page iv

Simulation of dynamic recrystallization using irregular cellular automata

Computer simulation is a powerful tool to predict microstructure and its evolution during dynamic recrystallization. Cellular Automata (CA), as one of the most efficient methods proposed to simulate recrystallization and grain growth. In this work, recrystallization and grain growth phenomena were modelled by using a two dimensional irregular CA method. Initial grain size, nuclei density and orientation of each grain were variables which have been used as entering data to the CA model. Final grain size, orientation of each grain, dislocation density and stress-strain curve were the results which have been resulted to validate the current model. Considering the model assumptions, it is shown that the CA can successfully simulate dynamic recrystallization

Validation of a model for static and dynamic recrystallization in metals

In this paper, modifications are proposed to a phenomenological plasticity model to account for the evolution of recrystallization and the resultant softening behavior. The novel model includes internal state variables representing dislocation density and the spacing between geometrically necessary subgrain boundaries. In order to capture both single and multiple peak recrystallization, the model tracks the evolution of recrystallized volume fractions for multiple cycles of recrystallization, and has a set of state variables for each volume fraction. A rule of mixtures is used to determine the average stress. The model is capable of capturing static recrystallization as well as both single and multiple peak dynamic recrystallization. Material parameters are fit to data from monotonic compression tests on copper for a wide range of temperatures and strain rates. The model is then validated by using the same parameter set to predict multiple-stage response in which samples are compressed, held at temperature for various lengths of time, and then compressed further. The model predicts both the static recrystallization that occurs between loading stages as well as the dynamic recrystallization occurring during the second loading stage