Assessment of Test Methods for Freckle Formation in Ni‐base Superalloy Ingot

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 Nibase 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 700-760oC steam conditions were tested. The experiment revealed the freckling tendency of AUSC candidate alloys. Introduction The world is facing climate change caused by emissions of carbon dioxide (CO2) and other greenhouse gases from human activities. Reducing greenhouse gas emissions requires new technologies. Coal is the primary fuel for the generation of electricity in the U.S., Japan, and many other countries 1-3) . Coal fired power plants are one of the main sources of CO2 emissions. In order to reduce CO2 emissions from coal-fired power plants, the development of integrated gasification combined cycle (IGCC) technology and advanced ultra-supercritical (A-USC) steam cycle technology have been underway since the 1990’s. In the A-USC system, the operating temperature would be 700-760oC. It is said that a material usable at such high temperatures could be Ni-base superalloys. Some alloys are selected for the A-USC system for their mechanical properties at high temperatures. However Ni-base superalloys are known to be freckle prone. The size of steam turbine components would be much larger than gas turbine components. The freckling tendencies of the materials must be discussed in order to manufacture large components. Several methods for segregation evaluation were reported such as vertical directional solidification experiments and horizontal directional experiments. Vertical directional solidification (V-DS) experiments are widely used for Ni-base superalloy segregation tests. This experimental method originally comes from the macrosegregation evaluation method 4) for surface freckles on directional solidified turbine blades. Density inversion theory derived from V-DS experiments using NH4Cl aqueous solution 5) is known as one mechanism of freckle formation. V-DS experiments are relatively easy to perform because of their simple apparatus construction. If the freckle formation condition in large ingots can be estimated using V-DS experiments, the V-DS experiments would be expected to be a useful segregation test method for industries. In order to verify V-DS experiments as an evaluation method for channel segregation in large ingots, V-DS experiments were carried out using several Ni-base superalloys as candidate materials for an A-USC turbine.