Manufacturing and properties of a large alloy 706 disc made by the open die forging process

Abstract

Nickel base alloy 706 has extensive application history for rotating parts in land based gas turbines combining high strength and toughness over the whole range of the service temperatures. This material is typically finish forged in a closed die press. The size of parts needed for the land-based gas turbines is very large requiring the use of the largest forging presses in the world. Finish forging of alloy 706 and other nickel-based alloys for rotating parts in gas turbines require homogeneous structure and properties throughout the forging. Many experts considered that open die forging process would not be able to achieve such uniformity. An experiment was done to determine if a well-controlled open die forging process could achieve the necessary uniformity in microstructure and properties for a large gas turbine part made of alloy 706. This paper will describe the details of the forging and heat treatment process. Results of mechanical tests and metallographic examination will be also discussed. Issues with process control and repeatability for open die forging are also discussed.