Patent Application
20120328903
This disclosure relates to superalloy components, such as components that are used in turbine engines.
Superalloy materials, such as nickel or cobalt-based superalloys, are known and used to fabricate components that are subject to severe operating environments. As an example, airfoils that are used in the high temperature section of gas turbine engines may be made of superalloy material. The superalloy material is typically cast into the desired shape and subjected to post-cast processing steps, such as grinding, polishing and grit blasting, to finish the component.
Disclosed is a method to limit surface zone recrystallization in a superalloy article. The method includes limiting recrystallization in a surface zone of a superalloy article by treating the superalloy article in an oxygen-containing environment to introduce oxygen into the surface zone in an amount sufficient to pin any new grain boundaries in the surface zone. In an embodiment, the recrystallization that occurs under a recrystallization condition of 1080° C./1976° F. for 4 hours is limited by first treating the superalloy article in an oxygen-containing environment at a treatment temperature of 800-900° C./1472-1652° F. to introduce the oxygen into the surface zone.
Also disclosed is a superalloy article that includes a superalloy body that has a surface zone. The surface zone includes oxygen in an amount sufficient to pin any new grain boundaries in the surface zone that occur under a recrystallization condition of 1080° C./1976° F. for 4 hours.
The various features and advantages of the disclosed examples will become apparent to those skilled in the art from the following detailed description. The drawings that accompany the detailed description can be briefly described as follows.
In the illustrated example, the superalloy article 20 generally includes a blade section 22 that extends between a tip 24 and a base 26. The base 26 is connected to a platform 28 and root portion 30 for securing the compressor blade within an engine.
The superalloy article 20 is formed from a superalloy material, such as by casting the superalloy material into the shape of the blade or other article. In some examples, the superalloy material is a nickel-based or cobalt-based superalloy material, such as, but not limited to HASTELLOY, INCONEL, NIMONIC, Waspaloy, Rene alloys, HAYNES alloys, INCOLOY, or single crystal alloys.
In one example, the superalloy article 20 is a single-crystal, nickel-based superalloy. In a further example, the single-crystal, nickel-based superalloy has a nominal composition of 6.5 wt. % chromium, 9 wt. % cobalt, 0.6 wt. % molybdenum, 3 wt. % rhenium, 6 wt. % tungsten, 5.6 wt. % aluminum, 1 wt. % titanium, 6.5 wt. % tantalum, 0.1 wt. % hafnium and a balance of nickel and any incidental impurities.
The superalloy article 20 is subjected to post-solidification processing steps, such as grinding, polishing and grit blasting, to finish the superalloy article 20. Such processing steps can produce residual stresses and/or increased defect density in the microstructure of the superalloy material of the superalloy article 20. Residual stress and/or increased defect density promotes recrystallization in the surface of a superalloy material upon exposure to elevated temperatures in subsequent processing steps, and particularly when the temperature exceeds the gamma prime phase solvus temperature.
For the superalloy article 20, recrystallization at the surface debits creep and fatigue performance and can increase oxidation. Creep rupture life can be reduced by up to a 50%. The reduction in creep performance is thought to be a result of easier slip propagation in the recrystallized areas from a higher amount of slip systems favorably oriented relative to applied stresses, and to the degradation of the gamma prime distribution. Additionally, the grain boundaries of the recrystallized areas are also initiation points for void formation during creep, especially in the alloys of the last generations, in which content of so-called grain boundary strengthening elements (Zr, B, C) is low. In some examples, creep failure can also initiate at the interface between the recrystallized area and the single crystal because of the different stiffness between the recrystallized area and the single crystal and precipitation compounds present along the interface. The oxidation rate increases because of oxygen diffusion along the recrystallized grain boundaries. As will be described in further detail, the superalloy article 20 has been treated according to the disclosed method in order to limit surface recrystallization that might otherwise occur under recrystallization conditions. That is, the described treatment effectively increases the surface zone recrystallization temperature by doping the surface zone with an oxygen dopant.
Through the disclosed method that will be described below, the surface zone 34 includes an oxygen dopant in an amount sufficient to pin any new grain boundaries in the surface zone 34 that occur under a recrystallization condition of 1080° C./1976° F. for 4 hours. The amount of oxygen dopant that is needed to pin grain boundaries is a function of the material composition, treatment temperature to introduce the oxygen and level of mechanical stress at the surface zone 34 (e.g., from machining grit blasting, etc.), which, with the teachings of this disclosure, can all be easily experimentally determined.
In comparison, the subsurface zone 36 includes less oxygen than the surface zone 34. The amount of oxygen within the surface zone 34, however, is not so high as to produce a continuous oxide scale on the surface of the superalloy article 20. That is, the oxygen is in solution (doped) within the microstructure of the superalloy material and/or forms fine oxide compounds that are discrete, discontinuous phases within the surface zone 34. Additionally, the amount of oxygen is not so high as to deplete the superalloy material of gamma prime phase 38. As an example, the amount of gamma prime phase 38 in the subsurface zone 36 in terms of volume percentage is equal before and after the introduction of oxygen into the surface zone 34.
Turning now to the disclosed method of treatment, the superalloy article 20 is treated in an oxygen-containing environment to introduce, or dope, the oxygen into the surface zone 34 in an amount sufficient to pin any new grain boundaries in the surface zone 34 to thereby limit recrystallization in the surface zone 34. In general, recrystallization occurs at a lower temperature in the surface zone 34 than in the subsurface zone 36, because of mechanical stress in the surface zone 34. That is, the surface zone 34 has a lower recrystallization temperature than the subsurface zone 36. The oxygen dopant effectively raises the recytallization temperature of the surface zone 34 to thereby limit recystallization. However, the recrystallization temperature of the surface zone may still be lower than the recrystallization temperature of the subsurface zone 36.
As an example, the superalloy article 20 is treated in air at a treatment temperature of 800-900° C./1472-1652° F. for two hours, although the time and temperature within the given range, and optionally pressure, can be varied depending on the composition of the superalloy material and processing history of the superalloy material with regard to mechanical processing. The treatment introduces oxygen into the surface zone 34, but not in such a high amount as to deplete the gamma prime phase 38 in the underlying subsurface zone 36. That is, the selected conditions for the disclosed method are insufficient for recrystallization and excessive oxidation. The treatment temperature is therefore lower than the recrystallization temperature of the surface zone 34, which as described above is lower than the recrystallization temperature of the subsurface zone 36.
The following examples show microstructures of a superalloy material according to the disclosed method of treatment in comparison to microstructures of the same nominal composition of superalloy material for comparative treatments to show the effectiveness of the disclosed method.
In a further comparative example shown in
The sample was then treated under the recrystallization condition of 1080° C./1976° F. for 4 hours. As shown in
In a further example, portions of the superalloy article 20 that are not to be treated may be masked to block oxygen from infiltrating into the superalloy material. As an example, the blade section 22 of the superalloy article 20 may be masked, as shown by the cross-hatched lines in
Although a combination of features is shown in the illustrated examples, not all of them need to be combined to realize the benefits of various embodiments of this disclosure. In other words, a system designed according to an embodiment of this disclosure will not necessarily include all of the features shown in any one of the Figures or all of the portions schematically shown in the Figures. Moreover, selected features of one example embodiment may be combined with selected features of other example embodiments.
The preceding description is exemplary rather than limiting in nature. Variations and modifications to the disclosed examples may become apparent to those skilled in the art that do not necessarily depart from the essence of this disclosure. The scope of legal protection given to this disclosure can only be determined by studying the following claims.
