August 2020 Volume 2

FORGING RESEARCH

N also enhances the benefits of the V additions, which has its precipitation strengthening increments augmented at higher levels of N. This fact is clearly observed in Figure 13, from Fix et al. [37] Similar strengthening effects of N are observed by several other authors in the literature. [1, 14, 17] Medina et al. [20] also suggested that increasing quantities of N, when coupled with V, would also help to increase the extent of intragranular nucleation of ferrite due to the increased presence of VN precipitates on MnS particles. This additional nucleation of ferrite grains would result in a further refined final ferritic microstructure. 2.2.6 Vanadium and Titanium Precipitates: Solubility and Influences Within the V-Ti-N steel system utilized in the experiment proposed herein, V and Ti form several precipitate phases which heavily influence the performance of the steel. Principal among these precipitates regarding the properties of the steel are TiN, VN, and VC. Titaniumnitride in the system is utilized as the primarymethod of suppressing grain coarsening during the high temperature processing of the steel in the austenite temperature regime. [3, 14, 17, 18, 24, 37] Vanadium nitride serves the primary roles of increasing the nucleation rate during the transformation from austenite to ferrite, through the provision of an increased quantity of heterogeneous nucleation sites, as well as an increase in the precipitation hardening of the steel. [1, 19, 20, 38] Finally, vanadium carbide provides significant precipitation strengthening for the system when present in the form of a wide distribution of fine particles; However, this strengthening, as all precipitation strengthening methods, comes at the cost of a reduction in the toughness of the steel. [38, 39]

2.2.5 Nitrogen Nitrogen’s effect on the processing of the steel lies primarily in its interactions with, and its capability to enhance the effects produced by the microalloying additions in the steel, primarily Ti and V. As can be seen in Figure 11 and Figure 12 from Zheng et al. [17] , an increase in the quantity of N in the steel enhances the austenitic refinement benefits of the Ti. N enhances the grain coarsening reduction of Ti through manifestation of a finer distribution of TiN precipitates in the steel. [14, 17, 24-28, 37]

Figure 12: Influence of N and Ti presence upon the austenite grain size and coarsening behavior of the tested steels [17] : Influence of N and Ti presence upon the austenite grain size and coarsening behavior of the tested steels [17] Figure 12: Influence of N and Ti presence upon the austenite grain size and coarsening behavior of the tested steels [17]

Figure 14: Impact of precipitate and size on precipitation strengthening increment for 3 common precipitating elements [40] The relative precipitation strengthening effects of niobium (columbium), vanadium and titanium are shown in Figure 14. Here, all three microalloying additions are shown to be capable of significant strengthening increases. Additionally, Figure 14 shows Figure 14: Impact of precipitate and size on precipitation strengthening increment for 3 common precipitating elements [40] The relative precipitation strengthening effects of niobium (columbium), vanadium and titanium are shown in Figure 14. Here, all three microalloying additions are shown to be capable of significant strengthening increases. Additionally, Figure 14 shows the benefits of fine precipitation over large precipitation, reiterating the need for proper control of Ti and N in the

Figure 13: Influence of N content on precipitation hardening [37] Figure 13: Influence of N content on precipitation hardening [37]

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FIA MAGAZINE | AUGUST 2020 72