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PCC Structurals Inc. has experienced wide variations in hot tear susceptibility and microporosity formation in industrial gas turbine (IGT) castings poured from different master heats of FSX-414. Each master heat conformed to all specification requirements and foundry control limits. No direct statistical correlation could be identified between alloy chemistry and the occurrence of these casting defects. Empirical data from production castings suggested there to be an inverse relationship between hot tear propensity and microporosity development. Using data generated from a variety of FSX-414 master heats poured into test vehicle castings, a statistically-significant relationship was established between alloy chemistry and hot tearing by employing regression fit model analyses and by developing an intermediate statistical link with Differential Thermal Analysis (DTA) data generated from each trial casting. Statistically-based connections between alloy chemistry and microstructure were also explored. Introduction FSX-414 is an austenitic Co-Cr-Ni-W superalloy which is solid solution strengthened by M7C3, M23C6, M6C and MC primary carbides. The alloy was developed by the General Electric Company for its hot corrosion resistance and mechanical strength at service temperature as an incremental improvement over alloys X40 & X45. The nominal composition of cast FSX-414 is provided in Table I. Although FSX-414 was created more than 40 years ago, it is still widely used in power generation applications today; Precision Castparts Corporation will cast more than 120,000 kg of FSX-414 in 2008. The production castings used to develop Figure 1 are large (>40 kg) complex geometry FSX-414 IGT nozzle segments. The same fixed process was used to make each nozzle, from wax molding through casting and post-cast cooling parameters. Based on the fluorescent penetrant inspection (FPI) data presented in Figure 1, it is evident that there is significant variation in hot tear development between the different alloy master heats. Radiographic inspection of the same castings showed a similar degree of variation in microporosity development. This apparently unpredictable behaviour of the investment casting foundry’s two most critical response variables presents an engineering challenge that if not understood and properly managed, could rapidly lead to significant scrap and rework costs. Microporosity issues in cast FSX-414 must be addressed at the casting level as this alloy does not respond well to hot isostatic pressing (HIP), even at temperatures above 1200°C. Furthermore, the effects of HIP on the FSX-414 primary carbide microstructure and mechanical properties have not been thoroughly explored. As a result, the use of HIP for the repair of porosity defects in FSX-414 is not accepted practice. Previous studies conducted at PCC Structurals Inc. (PCC) using production casting data were not able identify a statistically significant link between variations in single element concentration and the propensity of FSX-414 to develop either hot tears or microporosity defects. This suggested that these key response variables were being driven by interactions between alloying elements and/or one or more lurking variables. Reviews of published literature regarding alloying additions made to cast cobalt-base alloys proved to be relevant only for relatively large changes in composition. [1-3,6,7] Although useful, this information is not specific enough to explain the variation in master heat performance that PCC experienced within its band of controlled alloy composition. As a result of this early work, PCC launched a comprehensive study to discover the root of the differences in casting quality which apparently are driven by different FSX-414 master heat chemistries. 0 50 100 150 200 250 0 100 200 300 400 500 600 700 800 900 Number of Parts Cast C um ul at iv e D ef ec tiv e C as tin gs ( C ou nt ) 0% 21% 0% 0% 16% 43% 19% 7% 32% 3% 55% 36% 78% 68% 75% 15% 0% 0% 5% ML Instantaneous Defect Rate Figure 1. Crow-AMSAA plot of Instantaneous FPI Defect Rate for hot tears in a single location for production FSX-414 IGT nozzle segments. Each series represents a change in metal lot (alloy chemistry). An increase in line slope corresponds to an increase in the instantaneous defect rate. Table I. Cast FSX-414 Nominal Chemical Composition (wt. %) C Mn Si P S Cr Ni Fe W B Co 0.25 0.70 0.75 0.03 max 0.02 max 29.5 10.5 2.0 max 7.0 0.008 Balance

A Statistical Analysis of Variations in Hot Tear Performance and Microporosity Formation Versus Composition in Investment Cast FSX-414