Hot Cracking Susceptibility of Nickel Base Alloy 718

Nickel base alloy 718, a precipitation-hardenable alloy, is used in the sodium-water heat exchanger of fast reactor steam generators. Although Alloy 718 is considered to be weldable, it is susceptible to fusion zone and heat affected zone (HAZ) cracking during welding. Two heats of 718 of thicknesses 2 mm and 4 mm were tested for hot cracking susceptibility using the moving torch Varestraint hot cracking test. A single bead technique was used to determine fusion zone and base metal HAZ cracking susceptibility. Alloy 600, a solid solution alloy, was also tested for comparison. The hot cracking tests showed that the Alloy 718 was much more susceptible to fusion zone and HAZ cracking than 600. The fusion zone cracking susceptibilities of the two heats of Alloy 718 are shown not to depend on linear heat input values used in the testing. This is presumably due to the differing cooling rates as slower cooling rates lead to a higher cracking tendency despite lower heat input.