Assessment of test methods for freckle formation in Ni-base superalloy ingots

Abstract

Ni-base superalloys are well-known as freckle prone materials. Various researchers have reported on several types of test methods for investigating the freckle formation conditions in Ni-base superalloy ingots. Although varieties of test methods have been studied, there seem to be few articles comparing the test methods. In order to reveal the characteristics of the test methods, some of the test methods were compared. This study shows which test method could be more suitable for the freckling condition in a large ingot. Also, the incompleteness of some of the freckle formation theories is discussed. Using a selected test method, the candidate Ni-base superalloys for an advanced ultra-supercritical (A-USC) steam turbine that would be operated in 700-760°C steam conditions were tested. The experiment revealed the freckling tendency of A-USC candidate alloys.