Variations in Fracture Toughness for Alloy 718 Given a Modified Heat Treatment

Heat-to-heat and product-form variations in the J_IC fracture toughness for Alloy 718 were characterized at 24, 427, and 538°C using the multiple-specimen J_R-curve method. Six different material heats along with three product forms from one of the heats were tested in the modified heat treated condition. This heat treatment was developed at Idaho National Engineering Laboratory to improve the impact toughness for Alloy 718 weldments, but it has also been found to enhance the fracture resistance for the base metal. Statistical analysis of test results revealed four distinguishable J_IC levels with mean toughness levels ranging from 87 to 190 kJ/m² at 24°C. At 538°C, J_IC values were 15 to 20 percent lower than room temperature toughness levels. Minimum expected values of J_IC (ranging from 72 kJ/m² at 24°C to 48 kJ/m² at 538°C) and dJ_R/da (27 MPa at 24 to 538°C) were established based on tolerance intervals bracketing 90 percent of the lowest J_IC and dJ_R/da populations at a 95 percent confidence level. Metallographic and fractographic examinations were performed to relate key microstructural features and operative fracture mechanisms to macroscopic properties.