August 2020 Volume 2

FORGING RESEARCH

As can be seen from Figure 28, there are regions of temperatures and strains where recrystallization does not proceed to completion, instead only partially recrystallizing themicrostructure. As such, the temperatures T 5 and T 95 are defined, with temperature T 95 being of far more importance in this research. T 5 represents the temperature for which the microstructure will contain only 5% recrystallized fraction at a given strain, while the T 95 for the same strain represents the temperature at which 95% of the microstructure will be recrystallized. For the RCF process, the refinement occurs via the repetitive recrystallization of the microstructure, and as such, all deformations in this process should occur above the T 95 temperature, will be elaborated n xt. Both ffec s can be seen in Figure 11 by analysis of the Ti and non-Ti curve locations, while the effect on the recrystallization of the sy te an b seen in Table 3 below [17] , where the T 95 /T RX temperature is estimated using the sectioning red line. where the maximum recrystallization and refinement is possible. As stated earlier, the primary benefit of the Ti additions to the V steel is depressing of the recrystallization temperatures, as well as the increase in the grain coarsening temperature, which will be elaborated next. Both effects can be seen in Figure 11 by analysis of the Ti and non-Ti curve locations, while the effect on the recrystallization of the system can be seen in Table 3 below [17] , where the T 95 /T RX temperature is estimated using the sectioning red line.

Table 3: Recrystallization at various temperatures for V-Ti-N steels [17] Table 3: Recrystallization at various temperatures for V-Ti-N steels [17]

microstructures in the 3-dimensional space displaying temperature, holding time, and strain level effects. Herein only the temperature effects are considered, as the time and deformation parameters are strictly defined by the production requirements of the industry partners. It should be noted that the author explains that the surface opens around the deformation axis at more severe values, allowing for more forgivable conditions (higher temperatures and holding times).

FIA MAGAZINE | AUGUST 2020 78 At higher temperatures, microstructures undergo a process known as grain coarsening, where larger grains in the microstructure grow at the expense of smaller grains. This process is driven by the will of the system to minimize the grain boundary energy per unit volume, i.e. the grain boundary area per unit volume. [63] Grain boundaries thus move towards their center of curvature, and sharp, or highly curved boundaries tend to straighten. Thus, the process is controlled by the motion of grain boundaries, a diffusional process. As a diffusional process, the motion of the grain boundaries is thermally activated, and depends upon the temperature. Included below in Figure 29 is a diagram from Siwecki et al. [14] which separates regions of fine and coarse 2.3.3 Grain Coarsening and Temperature T GC At higher temperatures, microstructures undergo a process known as grain coarsening, where larger grains in the microstructure grow at the expense of smaller grains. This process is driven by the will of the system to minimize the grain boundary energy per unit volume, i.e. the grain boundary area per unit volume. [63] Grain boundaries thus move towards their center of curvature, and sharp, or highly curved boundaries tend to straighten. Thus, the process is controlled by the motion of grain boundaries, a diffusional process. As a diffusional process, the motion of the grain boundaries is thermally activated, and depends upon the temperature. Included below in Figure 29 is a diagram from Siwecki et al. [14] which separates regions of fine and coarse microstructures in the 3-di ensional space displaying temperature, holding time, and strain level effects. Herein only the temperature effects are considered, as the time and deformation parameters are strictly defined by the production requirements of the industry partners. It should be noted that the author explains that the surface opens around the deformation axis at more severe values, allowing for more forgivable conditions (higher temperatures and holding times). Figure 29: Grain coarsening in T-t-ε space [14] In the presence of high stability, fine precipitates, which are insoluble up to very high temperatures, the temperature requirements for grain coarsening are increased, and coarsening occurs via abnormal grain coa sening. In t is proc ss, the microstructure remains relatively unchanged, save for a small number of grains which grow at excessively high rates. [33] This form of abnormal grain coarsening results in a bimodal distribution of grain sizes, and significantly Figure 29: Grain coarsening in T-t- ε space [14] In the presence of hi h stability, fine precipitate , which are in oluble up to very high temperatures, the temperature requirements for grain coarsening are increased, and coarsening occurs via abnormal grain coarsening. In this p ocess, the microstructure remai s relatively unchanged, save for a small number of grains which grow at excessively high rates. [33] This form of abnormal grain coarsening results in a bimodal distribution of grain sizes, and significantly raises the standard deviation of the grain size distribution function. Curves such as those pictured 2.3.3 Grain Coarsening and Temperature T GC