August 2020 Volume 2

of these steels to room temperature, the phase volume fractions of the steels were analyzed. The data found herein served to design the cooling paths for the final trials which will occur on MFC production lines. Figure 43 below shows an example of the use of CCT diagrams provided by JMATPro in order to determine the approximate WET values for the cooling experiments. The cooling and transformation studies proposed herein comprise initially of the austenite conditioning processes determined in the previous experiments. Upon completion of these previous steps, the steel was cooled to a WET, where it was held for a time which varied upon the anticipated phase transformation. Upon further cooling of these steels to room temperature, the phase volume fractions of the steels were analyzed. The data found herein served to design the cooling paths for the final trials which will occur on MFC production lines. Figure 43 below shows an example of the use of CCT diagrams provided by JMATPro in order to determine the approximate WET values for the cooling experiments.

FORGING RESEARCH

A third strengthening mechanism is present in the steels’ designs in the form of solute strengthening. While C is the most prevalent of solute strengthening additions, many other elements are added for this purpose as well. Figure 23 [56] displays quite well the strengthening effect of additions of many of the elements in the current steels’ designs. Apart from the strengthening mechanisms previously mentioned, the most prevalent remaining mechanism is dislocation strengthening. This strengthening mechanism is generally accrued through the deformation passes of the steel. However, this mechanism is not present in notable quantities in the steels present herein, because the high temperatures at which the deformations occur. The recrystallization of the microstructure following the deformation nucleates strain-free austenite grains, [61] and thus eliminates the dislocations from which the dislocation strengthening would derive. However, the formation of bainite and/ or martensite, being displacive transformations involving shearing, will result in high dislocation densities leading to the possibility of very high strength being attained in the final forging. ■

Figure 43: Steel M1 CCT diagram with approximate WET selections 2.5.3 Strengthening Employed Multiple strengthening mechanisms were employed within the steels in the current experiment. Grain boundary strengthening is by far the most prevalent strengthening mechanism, as the foremost purpose of the project is the refinement of the microstructure of the steel, to amplify mechanical properties via the Hall-Petch equation. In addition, the benefit to toughness of fine austenite grains is also recognized. However, several other strengthening mechanisms are present in the steels. The V presence in the steel primarily serves to provide a source of precipitation strengthening in the steel. Because the steel is sub stoichiometric in the Ti:N ratio, complete precipitation of TiN particles leaves excess N for the precipitation of VN or VCN, which precipitates at a lower temperature than TiN, and has significant precipitation hardening effects.[1] Additionally, upon the depletion of N in the steel, the V further precipitates as VC, increasing the precipitation hardening increment furthermore. Precipitation hardening, however, also serves to decrease the toughness of the steel, [38, 39] and thus the current experiments primarily utilize toughness favoring grain boundary strengthening over, or at least in conjunction with, the detrimental precipitation hardening effects. 58 Figure 43: Steel M1 CCT diagram with approximate WET selections

FIA MAGAZINE | AUGUST 2020 83