August 2020 Volume 2

FORGING RESEARCH

Figure 19: Schematic diagram of Baker-Nutting orientashion relationship [51]

the precipitation pinning and solid solution pinning forces must exceed or equal the grain growth driving forces. Optimally, in the RCR process the pinning force of the particles exceeds the driving force for grain growth, without exceeding the grain growth for recrystallization: < < (2-9) In analyzing precipitation in steels, one final aspect is of interest. When precipitates form in steel, they commonly form in a specific orientation, with respect to the phase they precipitated within. The first of these orientation relationships is labeled the Baker-Nutting orientation relationship. [49] This orientation relationship is defined by the following parameters: {100} − || {100} (2-10) 〈011〉 − || 〈010〉 (2-11) The presence of this orientation relationship implies that the precipitate in question precipitated in the ferrite in the steel. A second common orientation relationship is labeled the Kurdjumov Sachs orientation relationship. [50] This orientation relationship is defined by the following parameters: {110} − || {111} (2-12) 〈111〉 − || 〈110〉 (2-13) The presence of this orientation relationship implies that the precipitate in question precipitated in the austenite in the steel. Figure 19 shows a sample Baker-Nutting orientation relationship between a precipitate and a ferrite matrix. [51] Additionally, Figure 20 shows a schematic diagram of the rotation transformation which occurs during the austenite to ferrite transformation. [52] This rotation allows for the orientation relationship present between the austenite and the precipitate to remain detectable through the austenite to

Figure 20: Schematic diagram of rotation of austenite to ferrite transformation, allowing for retention of austenite-precipitate orientation relations through the transformation [52] 2.2.7 Chromium, Molybdenum andManganese Chromium and molybdenum are the two most pronounced hardenability alloying elements in steels. Additions of these elements to the steels cause shifts to longer times in the transformation temperatures, which is equivalent to a rightward shift on the CCT diagrams of the steels. This effect can be seen below in Figure 21, which displays the CCT diagrams for the M1 and M2 steels, which differ only in that steel M2 has half the quantity of Cr and Mo that steel M1 has. The rightward shift in the curves on steel M1, which has the higher Cr and Mo compositions, encourage the formation of non-ferritic/pearlitic microstructures, such as bainite at lower cooling rates, and martensite at elevated cooling rates. Additionally, Mo was seen to decrease the transformation start temperature of the steel in several of the works in the literature. [53, 54] Additionally, Radovic et al [55] shows that the addition of Cr and Mo to the V steel used in the experiment promotes the formation of a bainite sheave microstructure, through the suppression of ferritic/pearlitic and acicular ferritic microstructures. Furthermore, the hardenability multiplying factors of Cr and Mo for the calculation of the ideal diameter parameter (The diameter of a bar which can be quenched to produce a 50%martensitic microstructure at the center diameter) can be seen in Figure 22. [56] This figure shows Cr andMo as the most effective hardenability elements, along with Mn. 33 Figure 20: Schematic diagram of rotation of austenite to ferrite transformation, allowing for retention of austenite-precipitate orienta ion lations thr ugh the transformation [52]

Figure 21: CCT diagrams of steel M1 with high Cr, Mo (pictured left) and steel M2 with low Cr, Mo (picture right) from JMATPro Figure 21: CCT diagrams of steel M1 with high Cr, Mo (pictured left) and steel M2 with low Cr, Mo (picture right) from JMATPro

ferrite transformation. Figure 19: Schematic diagram of Baker-Nutting orientation relationship [51] Figure 19: Schematic diagram of Baker-Nutting orientashion relationship [51]

FIA MAGAZINE | AUGUST 2020 75