Abstract
Corrosion-resistant, nickel-base alloy 686 (UNS N06686) is a fastener material for marine applications. Yield strength in excess of 150 ksi (1034 MPa) can be achieved by cold working, while retaining significant ductility, toughness, impact strength and Resistance to hydrogen-induced cracking. A qualification program is being conducted by the U.S. Navy (USN) in collaboration with a Nickel Alloy manufacturing plant to qualify alloy 686 bolting for use in new ship designs. Intergranular fracture was encountered in a USN four point bend test in air and also in a step load intergranular test program with alloy 686 bar of size 2.0 inches (50.8 mm) and 2.5 inches (63.5 mm) diameters. However, slow strain rate testing conducted at the plant location confirmed the toughness of the material. Metallographic investigation revealed the presence of mu phase in alloy 686 bar of sizes over 1 inch diameter. A phase diagram of alloy 686 developed using a phase predicting software predicts the presence of mu phase below 1100°C. It was perceived that intergranular mu phase can be reduced to a large extent by using a faster quench technique. Various sizes of bar products were manufactured using an improved quench technique after annealing. The microstructure of the bar produced in this manner showed a significant reduction in mu phase.
