BS in Materials Engineering
Materials Engineering Department
Schlosser Forge Co. forges low pressure turbine (LPT) cases from the nickel based superalloy 718 Plus using the ring rolling forging process. Schlosser has been experiencing low room temperature ductility problems in a number of their 718 Plus LPT cases. LPT cases are required to pass a 4D tensile test at room temperature with a minimum of 15 percent elongation. The ductility problem was reduced by increasing the forging temperature from 1825˚F to 1850˚F. Analysis was performed on 718 Plus samples taken from forged LPT cases that failed the room temperature ductility requirement and ones that passed the requirement. The analysis required conducting grain size measurements and delta phase volume fraction measurements to relate these microstructural characteristics to low room temperature ductility. Samples of 718 Plus were mounted and polished for metallography in accordance with ASTM E3-11. Samples were etched with modified Kalling’s reagent for 3 minutes and grain size measurements were taken per ASTM E112-10. Grain sizes varied only slightly between the low and acceptable ductility samples, with an average grain diameter of 44.9 μm. Grain size variation was not contributing to the low room temperature ductility problem. The samples were then re-polished to remove the previous etch and re-etched with modified Kalling’s for 30 seconds. The 30 second etch allowed the delta phase precipitates to be revealed while leaving the grain boundaries un-etched. Volume fraction measurements of the delta phase precipitates were taken per ASTM E562-11. Results showed the low ductility sample contained the highest amount of delta, about 7.5 percent by volume. The amount of delta was found to be inversely proportional to ductility as well as forging temperature.