Oxide defects in a vacuum investment-cast ni-based turbine blade
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12/29/04
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Oxide Defects in a Vacuum Investment–Cast Ni-Based Turbine Blade A.K.M.B. RASHID and J. CAMPBELL Samples from large turbine blades for power generation, investment-cast in vacuum from a nickelbased superalloy, were investigated. Samples were cut from regions near the top of the casting that contained freckle defects. The microstructures of these segregated regions were compared with those from nonsegregated adjacent regions using both optical microscopy and scanning electron microscopy (SEM). The segregated areas revealed a high density of carbides and a network of cracks. Samples were prepared by carefully fracturing along the cracks so as to observe the surfaces of the cracks. Observation by SEM revealed the presence of inclusions identified as films that appeared to have initiated the growth of carbides. Fractures of random areas of the nonsegregated alloy revealed that the films were numerous and widely distributed. In all cases, the films were principally oxides (and/or possibly nitrides) of aluminum and chromium. It was hypothesized that the films had originated by entrainment of the surface film on the liquid metal during the turbulent pouring of the casting. The films could, therefore, be assumed to be double, because the entrainment mechanism is a folding action. It follows that the doubled-over films constitute (1) the observed cracks and (2) the substrates for carbide precipitation. Evidence from other alloy systems is presented to support this conclusion.
I. INTRODUCTION
NICKEL-BASED superalloys are generally used for the production of high-performance turbines for power generation. The requirements for near-net shape, accuracy and surface finish dictate that the blades are cast in investment molds in a vacuum furnace. However, because of the necessity for the investment wax assembly to be robust, many investment castings use top-poured gating systems. This undesirable filling technique introduces the danger of the random entrainment of the surface of the liquid metal into the bulk of the casting (Figure 1). Despite the use of the vacuum for melting and casting, there is, of course, plenty of residual air in the vacuum environment to ensure that a surface film of oxide or nitride will form. This is especially true during the act of pouring, when the vigorous outgassing of the mold causes the “vacuum” (i.e., the dilute air environment) to be momentarily severely contaminated. The transient out-rushing of air as the mold is thermally shocked is usually clearly signaled on the vacuum gages of the furnace. During the turbulence of the pour, it is to be expected, therefore, that oxides and, possibly, nitride surface films (the aluminum and chromium in the alloy effectively gettering the residual gases in the vacuum chamber) will be formed on the falling liquid and, therefore, randomly folded into the melt, dry side to dry side. These unbonded interfaces, separating thin but stable oxide layers, will act as cracks in the liquid. Because of the mechanism of entrainment is a fol
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