August 2020 Volume 2

FORGING RESEARCH

As Figure 16 shows, with decreasing temperature, the first element in the steel to experience precipitation with falling temperature is Ti, which begins to form well before the other microalloying elements. [39] Titaniumnitride, which has amuch lower solubility product than titanium carbide in the austenite region, has a complete dissolution temperature which exceeds the dissolution temperature of all other microalloying carbonitrides, and the melting temperature of the steel. [38] For example, evaluation of the empirical solubility products in Table 2 determines the solubility product of TiN in austenite at 1000°C to be [Ti%][N%] = 1.05*10 -8 (wt%)2 and the solubility product of TiC in austenite at 1000°C to be [Ti%][C%] = 1.26*10 -3 (wt%)2, thus demonstrating a much lower precipitation potential for TiC in austenite. [42] In Figure 16, it can also be seen that V does not begin to precipitate until the temperature has entered the austenite to ferrite transformation regime.[39] Additionally, the solubility products of VN and VC can be calculated from Table 2 to be 1382.8(wt%)2 and 0.181(wt%)2 respectively, demonstrating that precipitation in austenite is unfavorable for the compositions proposed herein.[42] It should be noted that experiments in the literature have shown the presence of chromium to decrease the chemical activity of N, and thus decrease the solubility product of the VN. [44] Stoichiometric ratio of TiN gives a Ti:Nmass ratio equal to 3.42. [45] As titaniumnitride begins to precipitate well before the other nitrides and carbides, there is a consequential depletion of Ti within solid solution, which reduces the formation of TiC to a small fraction. [39] Following the depletion of the Ti, the excess N then combines with the V in solid solution to formVN, which has a lower solubility in austenite than that of VC, which has a considerably higher solubility than any other microalloy carbide or nitride. [39] If the V concentration is greater than the stoichiometric ratio compared to the excess N remaining in solid solution ([V%] / [excess N%] > 3.64), then VN will precipitate until the depletion of the N in solid solution. In this super-stoichiometric condition, the excess V remains in solution for interphase precipitation or precipitation after transformation as vanadium carbides. [39] The importance of ensuring a fine precipitation of TiN is best illustrated through a consideration of the Zener pinning model. Included in Figure 17 is an illustration of the interaction of a spherical particle with a grain boundary. The maximum force a particle of this size can produce, designated the Zener pinning force, is calculated by: [46] Fz =π r γ (2-8) Where r is the radius of the individual particle and γ is the energy per unit area of a grain boundary. Analysis of this equation shows a direct relation between particle size and Zener pinning force of a particle in a 1 to 1 ratio. However, although individual particles will have a larger Zener pinning force at larger radii, the total Zener pinning forces present in the steel is a summation of the Zener pinning force of all individual particles, whose summation of volumes must necessarily equal the total volume fraction of the particles in the steel. Since the Zener pinning force of the particle increases linearly, and the volume of a particle increases in the third

degree with respect to the radius of the particle, at equivalent volume fractions, the total Zener pinning force present in the steel increases as the average particle size decreases.Thus, as stated, a fine dispersion of TiN precipitation is more effective in pinning austenite grain boundaries, further supporting the necessity of a substoichiometric Ti/N ratio.

Figure 17: Schematic diagram of the interaction of a spherical particle with a grain boundary [46] Figure 17: Schematic diagram of the in eraction of a spherical particle with a grain boundary [46] Figure 17: Schematic diagram of the interaction of a spherical particle with a grain boundary [46]

Figure 18: Forces in steel driving and opposing recrystallization and grain coarsening processes [1,47,48] Figure 18 shows the relevant driving forces for recrystallization and grain coarsening. In the top of this figure is shown on the left the driving force for recrystallization, and on the right the pinning force generated by a distribution of precipitate particles. In recrystallization suppression, these driving forces are equivalent, or the pinning force exceeds the driving force for recrystallization. In the bottom of this figure is shown on the left the driving force for grain growth, and on the right the summation of forces opposing grain growth. In order to avoid grain growth, the sum of 30 Figure 18: Forces in steel driving and opposing recrystallization and grain coarsening processes [1, 47, 48] 30 Figure 18: Forces in steel driving and opposing recrystallization and grain coarsening processes [1, 47, 48

FIA MAGAZINE | AUGUST 2020 74