Freckle Formation in Ni-base Superalloys

Abstract

In order to reduce CO₂ emissions from fusel power plants, advanced ultra super critical (A-USC) steam turbine has been developing in Europe, the United States, and Japan. Candidate materials for A-USC steam turbine are Ni-base superalloys because 700°C exceeds the maximum service temperature of heat-resistant ferrite steels. Ni-base superalloys have superior high temperature properties, but they are well known as freckle prone material. One of the keys to the success of A-USC development is the availability of large size ingots for turbine shaft materials. There have been few studies on the productivity of Ni-base superalloys for large ingots. In this study, six A-USC candidate alloys were selected. Freckle tendencies of those A-USC candidate alloys were assessed using a horizontal directional solidification apparatus. The tested alloys tended to freckle in the following order: Alloy230>LTES700>Alloy625>Alloy706>Alloy617-Ti free>FENIX-700>USC141>Alloy617-Ti. We propose that the tendencies could be estimated from liquid density difference calculations.