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This paper reviews the effects of microstructural variations on the fracture toughness properties of Alloy 718 base metal and welds at 24 to 538°C. Seven different base metal lots, including five base metal heats and three different product forms from one of the heats, were tested in both the conventional (ASTM B637) precipitation treatment condition and a modified treatment condition that was developed to improve the fracture resistance for this superalloy. A gas-tungsten-arc weld was tested in both heat treatment conditions and the as-welded condition. Significant heat treatment and heatto-heat variations in fracture toughness were found and the results were analyzed statistically to establish minimum-expected toughness values for use in fracture control analyses. In the conventional heat treatment (CHT) condition, the presence of coarse second phase precipitates, 6 phase in the base metal and 6 plus Lavesphase in the weld, controlled the fracture behavior by causing premature microvoid nucleation and growth. The higher annealing temperature used during the modified heat treatment (MHT) dissolved these coarse particles and suppressed premature microvoid coalescence. This accounted for the improved fracture resistance exhibited by MHT materials. Heat-to-heat variations in fracture toughness behavior were attributed to differences in precipitate morphology and alternate secondary fracture mechanisms. INTRODUCTION Alloy 718 is a high strength nickel-base superalloy that possesses excellent corrosion and oxidation resistance, coupled with good tensile and creep properties. As a result, this alloy is used extensively in structural applications in the aerospace, nuclear, cryogenic and petrochemical industries. In addition, Alloy 718 has been selected for several welded applications because it exhibits superior weldability relative to most superalloys. The enhanced weldability characteristics are associated with the sluggish precipitation kinetics of the primary strengthening y" (bodycentered-tetragonal Ni,Nb) phase[l,2]. This sluggish age hardening behavior results in a relatively high ductility heat-affected-zone and fusion zone during cooling and aging. This permits relaxation of residual stresses and thereby improves the strain-age cracking resistance. Superalloy 718-Metallurgy and Applications Edited by E.A. Loria The Minerals, Metals 8 Materials Society, 1989 517 For most structural components, this superalloy is given a conventional heat treatment (CHT) per ASTM 8637: annealed 1 h at 954°C and air cooled to 24"C, aged 8 h at 718°C and furnace cooled to 621"C, aged at 621°C for a total aging time of 18 h and air cooled. This heat treatment, however, has contributed to a series of failures in welded structures by severely reducing the ductility and impact toughness of the weld fusion zone[3-51. The inferior toughness was attributed to the presence of Laves phase in the CHT weld metal. To increase ductility and fracture properties, the Idaho National Engineering Laboratory[5,6] developed a modified heat treatment (MHT): annealed 1 h at 1093°C and cooled to 718°C at 55"C/h, aged 4 h at 718°C and cooled to 621°C at 55"C/h, aged 16 h at 621°C and air cooled. The slower cooling rate reduced thermal stresses and the high annealing temperature dissolved the Laves phase, which restored adequate impact toughness to the fusion zone. Many Alloy 718 components are highly loaded during service, so fracture control is a primary design consideration. Structural integrity assessments of such high strength structures require a comprehensive understanding of the fracture toughness characteristics for this superalloy. Since this alloy is metallurgically complex, involving precipitation of several phases, its fracture resistance is expected to be strongly influenced by heat treatment, processing history, melt practice and alloy composition. A series of investigations[7-111 were conducted at Westinghouse Hanford Company to evaluate the influence of microstructural variations on the fracture properties for Alloy 718 base metal and welds and the results were reviewed in this paper. The parameters examined in these studies included the effects of heat treatment, product-form and heat-to-heat variations. Fracture toughness tests were performed using linear-elastic and elastic-plastic (J ) test techniques. (K c) Toughness values for the wrough I material were statistica f F y analyzed to establish minimum-expected toughness levels that account for material variability. Metallographic and fractographic examinations were also performed to relate key microstructural features and operative fracture mechanisms to macroscopic properties. EXPERIMENTAL MATERIALS AND PROCEDURES Five heats of Alloy 718 were examined in the CHT and MHT conditions. In addition, three product forms (plate, round bar and upset forging) from a single heat were studied. Details of the material supplier, product form and melt practice for the seven material lots are summarized in Table I. A material lot is defined as a two letter code that designates a particular heat/product form combination: the first letter denotes the heat: the second letter indicates the product form (i.e., forging). P for plate, B for bar and F for A gas-tungsten-arc (GTA) weld was also tested in the as-welded, CHT and MHT conditions. The welding procedures used to manufacture this weld were detailed in Reference 7. Chemical analyses and tensile properties for the test materials were reported in References 9-11. The CHT wrought metal exhibited lo-20% higher yield strength and O-10% higher ultimate strength levels relative to its MHT counterpart. Ductility values were generally found to be insensitive to heat treatment. In contrast to the base metal response, yield strength levels for the age-hardened welds were relatively insensitive to heat treatment, while the ultimate strength for the CHT weld was slightly lower than that for the MHT weld. The two heat treated welds exhibited comparable uniform and total elongation values, but the reduction in area for the MHT weld was much greater than that for its CHT counterpart. The strength in the as-welded condition was approximately half of that for the aged materials.

Fracture Toughness Variations for Alloy 718 Base Metal and Welds