Technical Article
grain structure in the gage area is fine compared to the structure in the carrot shaped bar. Minimal machining will be required to be done to the hourglass shaped bars so the test area will consist of a fine grained structure. The room temperature tensile results
of the test bars are reported in Table II. All bars met the minimum yield, tensile and elongation requirements. Compar- ing the two test bar shapes shows that the hourglass type bar exhibited a lower yield strength, higher ultimate strength, and higher elongation than the carrot shaped test bars. The results of the stress rupture
testing are reported in Table III. The hourglass shaped bars passed all requirements under all test conditions. In addition, there was good agreement between the test bars. The carrot bars however, experienced multiple failures to pass requirements. Of the five samples tested, only 2 samples passed the minimum elongation requirement and no samples passed the minimum rupture life requirement. The differences between the carrot
and hourglass shaped test bars are likely due to the differences in the grain structures seen in Figures 3 and 4b. The fine equiaxed grain structure of the hourglass shaped bars should give the most consistent and reliable mechanical test results that will be representative of the alloy. This is because the equiaxed structure will not show a grain orientation dependence and smaller grain sizes typically result in increases in mechanical properties. The mechanical properties of the carrot shaped test bars will be highly dependent on the limited number of coarse grains found in the gage section and on the orientation of the coarse grains. The dependence on the grain orientation can lead to increased variability in mechanical testing results.
Summary Two different types of test bars, carrot shaped and hourglass shaped, were cast from the same heat of IN 713 LC. The grain structures in the gage sections of the bars were significantly
22 ❘ May 2019 ®
Figure 4: Hourglass shaped test bar of 713 LC. Note the fine-grained structure through the gage section of the test bar in photo (b).
Test Bar Shape
Carrot Carrot
Hourglass Hourglass
AMS 5377 REQUIREMENTS 690 MPa min. 760 MPa min. 5% min. Table 2: Room temperature tensile results for Carrot and Hourglass shaped test bars.
Yield Strength (MPa)
820 798 783 732
Ultimate Tensile Strength (MPa)
995 959
1014 974
4D Elongation (%) 10
11.2 13.6 14.1
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