August 2020 Volume 2

FORGING RESEARCH

2.3 Austenite Conditioning 2.3.1 SV Parameter

The goal of austenite conditioning is to increase the value of the parameter Sv, which represents the interfacial near-planar surface area per unit volume, since these are both nucleating sites for subsequent transformation and barriers to cleavage crack growth. This parameter is tied to the geometry of the austenite grains comprising the microstructure during high temperature deformation; Specifically, it increases when grains become smaller or when the grains become less spherical in shape, i.e., pancaked. With regard to the increase in Sv for a decrease in austenite grain size, Underwood [58] proposed that for equiaxed austenite grains, the Sv parameter and the grain size are related by the following relation: = 2 (2-14) This clearly shows the inverse relationship between the Sv parameter and the austenitic grain size. As the diameter Dγ decreases, the parameter Sv increases. Furthermore, Kozasu et al. [59] elaborate on the contribution of deforming the grains on the Sv parameter. As the grains are deformed, deformation bands are developed in the grains, and these furthermore contribute to the overall Sv value, as can be seen by the following relation: = ( )+ ( ) (2-15) Where Sv(GB) is the contribution from the austenite grain boundaries, and Sv(IPD) is the contribution from the austenite intragranular planar defects in the austenite. Where S v (GB) is the contribution from the austenite grain boundaries, and S v (IPD) is the contribution from the auste ite intragranular planar defects in the austenite.

Figure 27: Grain refinement of austenite during deformations in RCR process, compared to the CHR process [61,62] The core concept of the RCR schedule can be seen in Figure 27. Here, it is seen that the conventional hot rolling (CHR) does not induce significant grain refinement in the steel. In contrast, the RCR process technology is capable of refining the microstructure more significantly, due to the presence of grain coarsening inhibitors. 2.3.2 Recrystallization and Temperatures T 5 and T 95 Recrystallization in metals is a process by which a deformed microstructure is replaced by strain-free grains by a nucleation and growth process. [63] Under conditions of complete recrystallization, the entire microstructure will have been consumed by these deformation-free grains. Due to the nature of recrystallization being a growth and diffusion-controlled process, it is thus a function of temperature, as can be seen in Figure 28. Figure 27: Grain refinement of austenite during deformations in RCR process, compared to the CHR process [61, 62] The core concept of the RCR sc dule can be see in Figure 27. Here, it is seen that the conventional hot rolling (CHR) does not induce significant grain refinement in the steel. In contrast, the RCR process echnology is c pable of refining the microstructure more significantly, due to the presence of grain coarsening inhibitors. 2.3.2 Recrystallization and Temperatures T 5 and T 95 Recrystallization in metals is a process by which a deformed microstructure is replaced by strain-free grains by a nucleation and growth process. [63] Under conditions of complete recrystallization, the entire microstructure will have been consumed by these deformation-free grains. Due to the nature of recrystallization being a growth and diffusion-controlled proce thus a function of temperature, as can be seen in Figure 28.

Figure 26: Influence of SV parameter upon the ferrite grain diameter [60] The importance of the Sv parameter can be seen in Figure 26, from Speich et al. [60] In this figure, it is seen that as the Sv parameter is increased, the ferrite grain size decreases significantly. Figure 26: Influence of S V parameter upon the ferrite grain diameter [60] The importance of the S v parameter can be seen in Figure 26, from Speich et al. [60] In this figure, it is seen that as the S v parameter is increased, the ferrite grain size decreases significantly.

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Figure 28: Austenite recrystallization curves as a function of temperature and strain [47] Figure 28: Austenite recrystallization curves as a function of temperature and strain [47]

FIA MAGAZINE | AUGUST 2020 77 As can be seen from Figure 28, there are regions of temperatures and strains recrystallization does not proceed to completion, instead only partially recrystalliz microstructure. As such, the temperatures T 5 and T 95 are defined, with temperature T 95 bein