August 2020 Volume 2

FORGING RESEARCH

raises the standard deviation of the grain size distribution function. Curves such as those pictured in Figure 30 from Zheng et al. [17] can be used to display the grain coarsening temperature TGC, which occurs at the beginning of the upper curve, which represents the average size of the secondary coarsening grains. A vertical red line has been introduced into this figure, such as to aid in the estimation of the grain coarsening temperature for the V-Ti-N system. in Figure 30 from Zheng et al. [17] can be used to display the grain coarsening temperature T GC , which occurs at the beginning of the upper curve, which represents the average size of the secondary coarsening grains. A vertical red line has been introduced into this figure, such as to aid in the estimation of the grain coarsening temperature for the V-Ti-N system.

2.3.4 Deformation and Forging Passes In the recrystallization controlled forging process, the forging passes are implemented not just to shape the piece, but also to refine the austenitic microstructure, in preparation for the cooling and transformation. The forging passes implemented are conducted at large deformations, to maximize the contribution from the grain refinement, through higher recrystallization nucleation rates. These deformations thus increase the total SV of the microstructure through increasing the SVGB values, as can be seen in Figure 32. [60] Figure 33 also helps to illustrate the influence of the austenitic grain size upon the total SV value for the steel. [59] As can be seen from this figure, an increase in the ASTM grain size number (i.e. a decrease in the grain size) corresponds to an increase in the SV for the steel. It is important to note that the deformation temperature range for the RCF process occurs entirely between T RX (the temperature for 95% recrystallization) and T GC (the grain coarsening temperature). This temperature range is overlayed upon Figure 11 from Zheng et al. [17] in Figure 34 pictured below. Zheng et al. [17] also showed that this temperature range is 175°C larger for the V-Ti steels than for the V steels, allowing for higher temperature deformations and longer holding times, ultimately permitting lower forging loads and high flexibility in the manufacturing process. increase in the S V for the steel. It is important to note that the deformation temperature range f the RCF process occurs entirely between T RX (the temperature for 95% recrystallization) and T (the grain coarsening temperature). This temperature range is overlayed upon Figure 11 fro Zheng et al. [17] in Figure 34 pictured below. Zheng et al. [17] also showed that this temperature ran is 175 ° C larger for th V-Ti steels than for the V steels, allowing for higher temperatu deformations and onger holding times, ultimately permi ting lower f rging loads and hi flexibility in the manufacturing process.

Figure 30: Grain coarsening curves for the V-Ti-N steel systems [17] As can be seen in Figure 30, the additions of Ti and N to the V steel result in a remarkable increase in the grain coarsening temperature of the steel. This result is also shown elsewhere in the literature. [1, 3, 14, 24, 64] However, the influence of the Ti and N composition decreases significantly without the proper processing. In the literature, it is consistently stated that fine precipitates retard grain boundary motion to a higher degree, and thus further retard the coarsening of the microstructure. This can be seen in Figure 31, [64] which displays models developed by Zener [65] , Gladman [33] , and Hellman Hillert [66] . Each of these models shows increased grain refinement for precipitates of smaller sizes and/or larger volume fractions, both resulting in a higher density particle distribution. man [33] , and Hellman-Hillert [66] . Each of these models shows increased grain refinement for pitates of smaller sizes and/or larger volume fractions, both resulting in a higher density le distribution. 45 Figure 3 in coarseni g curves for the V-Ti-N steel ystems [17] As can be seen in Figure 30, the additions of Ti and N to the V steel result in a remarkable increase in the grain coarsening temperature of the steel. This result is also shown elsewhere in the literature. [1, 3, 14, 24, 64] However, the influence of the Ti and N composition decreases significantly without the proper processing. In the literature, it is consistently stated that fine precipitates retard grain boundary motion to a higher degree, and thus further retard the coarsening of the mic ostructure. This can be seen in Figure 31, [64] which displays models developed by Zener [65] ,

Figure 32: Deformation influence upon SV parameter contributors [60] Figure 32: Deformation influence upon S V par meter contributors [60]

Figure 31:Threemodels relating grain size to precipitate particle distributions [64] Figure 31: Three models relating grain size to ci itate particle distributions [64]

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FIA MAGAZINE | AUGUST 2020 79

Deformation and Forging Passes