Figure 2: Porosity present in the transverse direction of a cast 347 steel test bar.


by the requirements to fill the mold completely and make a sound casting. The shape of the casting will


also impact the cast grain size. Large sections will typically have larger grain sizes than small sections due to the more efficient heat extraction in the small sections which will help to minimize grain growth. In the case of cast tensile bars, the size and shape of the section of the casting that will become the gage section can influence the grain size. The gating of the tensile bars may


also play a role in the final grain size of the gage section. A gate connected to the gage may promote a sound casting with minimum porosity, however it may also serve as a heat source. This could reduce the temperature gradient in that area of the tensile bar and cause grain growth to occur. This brief review of casting variables


is not meant to be an exhaustive list and the book Castings[2]


is recommended for a more in depth discussion of the metallurgy behind the casting process.


Experimental Procedure Two types of mechanical test bars were evaluated for this study. One type was a carrot-style test bar (Figure 3) and the other type was an hourglass style (Figure 4). Both bars were cast from a heat of IN 713 LC, the nominal chemistry of which is presented in Table I. Both types of mechanical test bars were sectioned, polished, and etched to allow the grain structure to be evaluated. The mechanical testing performed was room


® by John Campbell


Figure 3: (a) Carrot-style test bars of 713 LC. Note the presence of large equiaxed grains in the center of the tensile bars. (b) Schematic overlay of the part of the test bar that will become the gage section once the test bar is machined. Note the gage section will consist of a predominately coarse grain structure.


C 0.06% Cr 12% Ni Bal. Mo 4.3%


Nb/Cb 2%


Table 1: Nominal composition of IN 713 LC


temperature tensile testing and stress rupture testing and were compared to the requirements of AMS 5377.


Results and Discussion The grain structure of the carrot shaped bars can be observed in Figure 3. There is no apparent porosity in the bars, which indicates that the carrot shape is capable of producing a sound casting. The grains in the center of the bar, however, are


large relative to the overall size of the bar. When the test bars are machined to produce tensile bars that will meet ASTM E8, the gage section will consist predominately of the large grains. This is illustrated schematically in Figure 3b where the dashed red lines indicated the gage section of a tensile bar that would be machined from the carrot bar. Figure 4b presents the grain structure of the hourglass shaped bars. The


May 2019 ❘ 21 Ti 0.7% Al 5.8% B 0.007% Zr 0.06%


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