Freckling Tendencies of A-USC Candidate Ni-based Superalloys

Freckling Tendencies of A-USC Candidate Ni-based Superalloys By Koji Kajikawa, Masaru Tanaka, Shigeru Suzuki, and Hitohisa YamadaA bstract In order to reduce CO2 emissions from fossilpower plants, advanced ultra super critical (A-USC) steam turbine development was started in Europe in the 1990s, followed by the United States and Japan in the 2000s. Since 700-760°C is beyond the maximum service temperature of heat-resistant ferritic steels, Ni-based superalloys are candidate materials for A-USC. Ni-based superalloys have superior high temperature properties; however, the alloys are well-known as freckle prone materials. One of the keys to the success of A-USC development is the usability of large Ni-based superalloy ingots for turbine shaft materials. There have been few studies on the productivity of Ni-based superalloys for large ingots. In this study, the freckling tendencies of several A-USC candidate alloys were investigated using a horizontal directional solidification apparatus to evaluate the large ingot productivity. The experiment revealed the freckling tendency of A-USC candidate alloys.IntroductionThere has been a growing concern about the potential impact of CO2 emissions from coal-fired power plants. It is considered that CO2 emissions are a major cause of global warming. The efficiency of the power plant is a key value that affects both the fuel costs and the amount of CO2 emitted to the environment. The power generation efficiency of a fossil power plant depends on the steam temperature and pressure because a steam turbine is a thermodynamically based system. Higher steam temperatures and increased pressures will significantly improve the thermal cycle efficiency. Current ultra-supercritical (USC) systems operate with the steam at temperatures of 593-600°C. In the Advanced-ultra supercritical (A-USC) system, the operating temperature would be 700-760ºC1,2). In order to meet the requirements for advanced steam turbine materials including sufficient strength, steam oxidation resistance and temperature capability at 650-760oC, the use of Ni-based superalloys is being considered. One of the key issues of the A-USC power system is the usability of materials for the system. For example, A-USC steam turbine rotor shafts would require forgings in the several tens of tons class. Only a few studies on steam turbine sized Ni-based superalloy forgings have been reported3), even though large Ni-based superalloy ingots are susceptible to freckling during solidification4). There have been few freckling criteria data for wrought Ni-based superalloys. However, the inherent freckling tendencies of these alloys may be unclear to users. Information on the productivity of large ingots of these alloys is extremely important for manufactures. This paper presents the freckling tendencies of A-USC candidate alloys obtained from experimental study.Experimental ProcedureCandidate materials for the A-USC system were selected for this experiment. Most of the alloys are chosen from A-USC candidate alloys listed in European, American, and Japanese projects. The test alloys and those chemical compositions are summarized in Table 1. The horizontal directional solidification furnace used for this study is schematically illustrated in Figure 1. In this resistance heat furnace, a rectangular Al2O3 crucible with an air cool chiller was placed. The inner size of the crucible was about 55Wx170Lx230Hmm.Table 1. Chemical composition of test alloys (mass %)Six thermocouples were inserted in the crucible to measure the thermal history of the metal during solidification. The test alloy was melted in an induction melting furnace and poured into the crucible. The pouring temperature was approximately 50-100°C higher than the liquidus point of the alloy. Just after pouring, cooling air was blown into the chiller. The molten alloy was horizontally directional-solidified from the chiller side to the opposite side. The flow rate of cooling air was optimized to produce freckle plumes in the middle of the ingot. The temperature profile in the liquid alloy was measured until the alloy solidified completely. Chemical analysis of the alloy was by an X-ray fluorescence method.The solidified ingot was cut horizontally into cross sections on the plane of tips of the thermocouples. The cross section planes were polished and macro etched using a mixture of copper (II) diammonium chloride saturated aqueous solution and hydrochloric acidora mixture of hydrogen peroxide, hydrochloric acid and water. If some spots were observed on the plane, the ingot was cut vertically on the plane of freckle spots in order to observe the profiles of the freckle plumes. If spots were blurred and unclear, the suspicious portion was clipped out and an EPMA analysis was performed to judge whether the spots are freckles or uneven etching.Figure 1 Schematic of the horizontaldirectional solidification furnaceThe solidification condition at the location where the freckle spot closest to the chiller was found is considered as the freckling appearance critical condition. The solidification conditions such as the cooling rate (ε)) and solidification rate (R) at the freckle spot are obtained from the data of the six thermocouples.Figure 2 Macro structure of the Alloy617 ingotFigure 3 Macro structure of the Inconel 740 Ingot ResultsFigures 2 and 3 show the macrostructures of vertical cross sections of horizontally directional-solidified ingots of Alloy617 and Nimonic105 respectively as examples. Ni-based superalloys have a wide variety of chemical compositions and the amounts of alloying element contents are also spreading in some range. Therefore every alloy has its own unique freckle profile. Freckle plumes would grow close to the direction of the sum of the solidification direction vector and the flow vector owing to the density difference between the solute enriched liquid and the base liquid5). According to this mechanism, freckles (channel type segregations) could be divided into two types; one is a solute-enriched liquid growing upward and the other is a solute enriched liquid growing downward. Alloy617, 625, and LTES700 showed downward type freckles. On the other hand, Haynes230, 282, Inconel740, Udimet720Li, Nimonic105, Waspaloy, and USC141 alloys showed upward type freckles. Even within the group of upward type freckling alloys, the freckle plume shapes were different and there were distinctive freckle plume profiles. For example, a lot of thin plumes were observed in Alloy 617 as shown in Figure 2. On the other hand, Inconel740 in Figure 3 showed relatively few but thick freckle plumes. The shape of the freckle plumes could depend on the temperature interval between liquidus and solidus, and the solidification conditions at the freckled portion. If the temperature interval of the alloy is wide, that means the mushy zone of the alloy in solidification is wide and the local solidification time is longer than for narrow temperature interval alloys. In this case, dendrite arm spacing become wide and the freckle plumes will be thick. If the alloy is freckle prone, freckles form in high cooling rate regions and conditions. The dendrite arm spacing in the high cooling rate region is narrow. Therefore the freckle plumes would be thin.An example of the temperature profiles obtained by the thermocouple from the test ingot is shown in Figure 4. Although the thermocouple locations may vary a little owing to the working accuracy of the refractory attached protection sheaths for the thermocouples and the pressure of the liquid metal at pouring, thermocouple locations were measured for each ingot and do not affect the solidification condition analysis.DiscussionThere are several criteria for freckle formation. In this paper, the criterion proposed by Suzuki and Miyamoto6) was used, because the criterion derived from the channel segregation formation condition in their testingots is similar to that used in this study. Suzuki andMiyamoto reported that the critical freckle formation condition could be estimated from the cooling rate ε (°C /s) and solidification rate R (mm/s), and freckles form at the condition of ε•R 1.1α. (1)Figure 4 The temperature profile of the Inconel 740 ingotFigure 5 Freckling tendencies of Ni-based superalloysThe term α here is the alloy inherent value. In other words, α means the freckling critical condition of the alloy (α=ε•R 1.1αcrit ). The cooling rate at the growth front and the growth rate were derived from the cooling curves measured by the thermocouples and the distance between the chiller and each thermocouple. There are several theories about the solid fraction of freckle initiation. In this paper, the solid fraction, fs=0.35, for the growth front was assumed from reports7) that showed the value is ing the temperature profiles and the thermocouple locations in the ingots, the cooling rate ε (°C /s) and solidification rate R (mm/s) can be determined. Figure 5 shows the ε•R 1.1 crit values of the Ni-based superalloys derived from these horizontal solidification experiments. In this figure, smaller values of ε•R 1.1 crit means the materials is less freckle prone. The density of the liquid alloy can be calculated from the equation shown below because the summation of molar volumes of the elements in the liquid metal could give a good estimate 8). (2)where Xi is the molar fraction of element i, Mi is the molar weight of element i, and Vi is the molar volume of element i. Figure 18 shows the relationship between the calculated liquid densities using the latest data of liquid metal densities9-11). Since Li and Mills have reportedthat adding Al increases the Ni-based alloy density because of the strong bonding between Al and Ni11), the effect of Al addition on the density was corrected using their compensation formula: (3)where Y={X Al -0.3164X Cr -0.0835X Co }. Thermo-Calc12) with Ni-data13) was used for this calculation. Although negative values cannot be obtained from the ε•R 1.1 equation, in Figure 6, downward type freckles are plotted as negative values in order to distinguish them from upward type freckles. The plotted data of Alloy718, Hasteloy X, and Nimonic80A, which were not tested in this experiment, are recalculated values of previously reported data5). The scatter of the data points is considered as coming from errors of the partition coefficients and density values. It seems that the Thermo-Calc data base still needs correction for some particular systems14, 15). This diagram implies that channel segregation could be estimated from a liquid density difference calculation.Figure 6 Relation between liquid density differences andfreckling conditionsAs mentioned above, dendrite arm spacing influences the size of freckle plumes. Secondary dendrite arm spacing, λ2, depends directly on the cooling rate. Secondary dendrite arm spacing affects permeability in the mushy zone. It is said that freckle formation is owing to interdendritic flow. Figure 7 shows the relationship between freckle spot size and cooling rate around the solid fraction, fs=0.35. It seems that freckle spot size is related to the cooling rate. The data points are a little scattered. This may be due to the difference of chemical composition. Primary dendrite arm spacing, λ1, also has an influence on permeability. However it is considered that measuring primary dendrites is not practical as discussed later.There are several freckle formation criteria using theRayleigh number. One of the equations16) is shown below. (4)Figure 5 The relationship between freckle spot size and cooling rate around the solid fraction fs=0.35For a Rayleigh number calculation, primary dendrite arm spacing, λ1, is required to calculate the permeability in the mushy zone. This may make it difficult to use the Rayleigh number for wrought ingots because it is difficult to measure the primary dendrite arm spacing in wrought ingots. Wrought ingots are not directional-solidified and primary dendrite arm spacing varies with cross section. Another difficulty with the Rayleigh number criterion is the availability of accurate physical properties for the calculation. For example, the viscosity of liquid metals is still under study. Although the predicted formula for the viscosity of an alloyed liquid metal is reported, there is some question about the accuracy of the viscosity of pure elements used for the formula. It is said that the error range of dynamic viscosity η and thermal diffusivity DT is about 5-10%8). Thermal expansion of liquid metal is difficult to measure and is affected by small quantities of alloyed elements. Therefore the accuracy of the Rayleigh number criterion may not be sufficient and therefore not yet a practical method for industries. Even though the Rayleigh number could be calculated using reported physical properties, it would be necessary to perform several experiments in order to obtain parameters for precise estimation of freckle formation. This issue was also pointed by Yang et al.17) They indicated that the data generated by some superalloy databases show a large discrepancy from experimental data. However, several new measuring methods for liquid metal physical properties have been emerging18, 19). When we obtain accurate physical properties of liquid metals in near future, the Rayleigh number criterion may be more popular for freckling prediction in forging materials.On the other hand, the ε•R1.1 equation uses relatively simple parameters. It uses only two solidification parameters. Certainly, the calculation shows somerange of error as described above and it would needto be checked by a horizontal directional solidification experiment, but the equilibrium partition coefficient canbe estimated from Thermo-Calc and so on using thediagram shown in Figure 6. This diagram is also usefulfor alloy designs with low segregation materials. In fact,this idea has been leveraged for the development of aNi-based superalloy for the A-USC system20) and newwrought alloy development is expected.ConclusionsThe freckling tendencies of several A-USC candidatealloys were assessed using a horizontal directional solidification apparatus. The results are summarizedbelow.1) T he freckling tendencies of Ni-based superalloyscould be classified using freckling criteria ε•R1.1α2) T he order of the freckling tendencies ofthe tested alloys is as follows: Haynes230>Nimonic105>Udimet720Li> Alloy625>LTES700>Waspaloy>Inconel740 > Haynes282>FENIX700>USC141> Alloy617. Interms of freckle formation, Haynes282, FENIX700,USC141, and Alloy617 are better candidates forA-USC steam turbines.3) F reckling tendencies could be estimated from aliquid density calculation and the ε•R1.1 valuesof alloys, which were obtained from horizontaldirectional solidification experiments.4) M ore accurate liquid density data is needed toobtain better estimates of freckling tendencies.Also, the Thermo-Calc database for Ni-basedalloys should be refined in order to allow moreaccurate assumptions.5) T he freckle spot size could be related to thecooling rate.6) T he freckling prediction method described here isa useful method for designing less freckle pronealloys. ■References1) S. Kjaer, F. Klauke, R. Vanstone, A. Zeijseink, G.Weissinger, P. Kristensen, J. Meier, R. Blum, K.Weighardt: Powergen Europe 2001, Brussels,(2001)2) R. Viswanathan, R. Purgert, S. Goodstine, J.Tanzosh, G. Stanko, J. P. Shingledecker, B. Vitalis:5th Inter. Conf. on Adv. in Mater. 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