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

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