Cast Alloy 7 18 was evaluated in compression over a range of temperatures (900 to 1 120°C), strain rates (0.01 to 1.0 s-l), and true strains (0.15 to 0.7). Under most conditions evaluated, flow curves exhibited no peaks, indicating that dynamic recrystallization was not playing an important role. The percentage of static recrystallization was found to vary with temperature, strain rate, true strain, and time held at temperature after deformation. Recrystallized grain size values were also determined. A material model describing percent recrystallization and recrystallized grain size was established based on the experimental data. In addition, double hit compression tests were conducted to determine the effect of delay time between passes during ingot breakdown. For most cases the percent recrystallization after double hit was simply the sum of two individual hits under corresponding conditions.
The hot deformation behavior of a high nitrogen CrMn austenitic stainless steel in the temperature range 1173 to 1473 K (900 to 1200°C) and strain rate range 0.01 to 10 s À1 was investigated using optical microscopy, stress-strain curve analysis, processing maps, etc. The results showed that the work hardening rate and flow stress decreased with increasing deformation temperature and decreasing strain rate in 18Mn18Cr0.5N steel. The dynamic recrystallization (DRX) grain size decreased with increasing Z value; however, deformation heating has an effect on the DRX grain size under high strain rate conditions. In the processing maps, flow instability was observed at higher strain rate regions (1 to 10 s À1 ) and manifested as flow localization near the grain boundary. Early in the deformation, the flow instability region was at higher temperatures, and then the extent of this unstable region decreased with increasing strain and was restricted to lower temperatures. The hot deformation equation as well as the quantitative dependence of the critical stress for DRX and DRX grain size on Z value was obtained.
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