Effect of Microstructural Variations on the Tensile and Fracture Toughness Properties of Alloy 718 Welds

Abstract

The effect of microstructural variations on the tensile and fracture toughness properties of Alloy 718 welds is reviewed. The microstructural parameters surveyed include heat treatment, longterm thermal aging and neutron irradiation. The two heat treatments that were characterized include the conventional heat treatment, per ASTM B637, and a modified heat treatment designed to improve the toughness of Alloy 718. Both heat treatments resulted in room temperature yield strengths in excess of 1000 MPa and total elongation values between 10 and 20%. The fracture toughness of modified heat treated (MHT) welds was far superior to that for conventional heat treated (CHT) welds, which supports the use of the modified treatment when fracture resistance is a primary design concern. In the as-welded condition, welds exhibited moderate strength levels and good fracture properties that preclude fracture concerns for most intermediate-strength applications. Fast-neutron irradiation significantly reduced ductility and fracture toughness for both heat treatments, but fracture resistance for the MHT weld was consistently higher than its CHT counterpart. Thermal aging at 566°C for times up to 20,000 hours resulted in a 10% increase in strength and two-fold reduction in ductility for both heat treated conditions. Longterm aging reduced the toughness of MHT welds by 25%. The aged CHT welds showed tremendous variability in fracture resistance, with some specimens exhibiting a very brittle response. The mechanical properties were correlated with microstructural variations associated with thermal aging, irradiation and heat treatment. Introduction Alloy 718 is a high-strength nickel-base superalloy that exhibits good weldability and strain-age cracking resistance. Its superior welding characteristics are associated with the sluggish precipitation kinetics of the primary strengthening y” phase. (‘z The sluggish hardening response results in a relatively high ductility fusion zone during cooling and aging. This permits relaxation of thermal and residual stresses and thereby improves strain-age cracking resistance. As a result of its good weldability, this alloy is used in the welded construction of components in the nuclear industry. Many of these components are highly loaded during service, so fracture control is an important design consideration. To assure the structural integrity of such components, a comprehensive understanding of the tensile and fracture responses for these welds is required. The effects of microstructural variations on the tensile and J,, fracture toughness properties for Alloy 718 gas-tungsten-arc welds were studied in a series of investigations conducted at Westinghouse Hanford Company,(3-6) and the results are reviewed in this paper. The parameters surveyed include the effects of heat treatment, long-term thermal embrittlement and neutron irradiation. The heat treatments, described in Table 1, included the conventional heat treatment, per ASTM B637, and a modified heat treatment that was developed to improve the notch toughness of welds.(7s8) Tensile and fracture properties for the as-welded material are also summarized. Superalloys 718,625,706 and Various Dcrivativcs Edited by E.A. Loria The Minerals, Metals &Materials Society, 1994