Patent Application
20150217412
The present invention is directed to a weld filler metal for nickel base superalloys, and specifically to a nickel base weld filler metal that develops a high volume fraction of gamma prime phase in the weld repaired region.
Components located in the high temperature section of gas turbine engines are typically formed of superalloys, which includes nickel-base superalloys. High temperature sections of the gas turbine engine include the turbine section. In some types of turbine engines, the high temperature section may include the exhaust section. The different hot sections of the engine may experience different conditions requiring the materials comprising the components in the different sections to have different properties.
Turbine buckets or airfoils in the turbine section of the engine are attached to turbine wheels and rotate at very high speeds in the hot exhaust gases of combustion expelled by the turbine section of the engine. The turbine wheels with their buckets comprise a turbine stage and extract energy from the hot exhaust gases of combustion. A turbine engine has at least one turbine stage, but more typically includes a plurality of turbine stages, each extracting energy from the hot exhaust gases of combustion. The hot exhaust gases, after passing through a turbine stage and before entering a downstream turbine stage, are routed through stationary nozzles, sometimes referred to as vanes, which redirect the flow of the exhaust gases for proper impingement on the next turbine stage. These nozzles, since they experience the same environment as buckets or airfoils, desirably should have many of the same materials properties as the buckets or airfoils. For example, these components must simultaneously be oxidation-resistant and corrosion-resistant, while maintaining mechanical properties such as creep resistance and fatigue resistance at elevated temperatures experienced in a gas turbine exhaust. The nozzles do not experience all of the loadings that turbine stages are subject to, as they are stationary. Much of the stress experienced by nozzles is a result of high thermal stresses and, to a lesser degree, mechanical stresses such as aerodynamic loading. As a result, the nozzles must have excellent resistance to thermal fatigue and creep resistance, especially for large, multi-airfoil latter stage nozzles whose size and weight may make them subject to creep. At operating temperature, the nozzles support at least their own weight.
Nickel-base superalloys have typically been used to produce components for use in the hot sections of the engine since they can provide the desired properties that satisfy the demanding conditions of the turbine section environment. These nickel-base superalloys have high temperature capabilities, while achieving strength from precipitation strengthening mechanisms which include the development of coherent gamma prime precipitates. Alloys such as GTD® 222, GTD®-111, MAR-M®-247, WASPALLOY® and UDIMET® 500 are used to make latter stage nozzles working at temperatures of 1500° F. and above. Long period exposure to these temperatures and mechanical forces make the hot locations, such as nozzle leading edges prone to creep, while thermal mechanical fatigue may be experienced near cooling holes.
Such a nozzle always contains significant amount of strategic elements such as nickel, chromium, cobalt and tantalum. In addition, the cost of fabricating such a nozzle is high. So extending the life of a nozzle after indications develop is a cost-effective option and repair procedures have been developed to remove the indications to extend the life of a nozzle, while avoiding replacement costs. Removal of indications in a nozzle through blending and subsequent weld buildup with a filler material is currently employed to restore the structural integrity of a nozzle. However, weld repairs with the widely used filler such as Nimonic 263 have been repeatedly proven to experience premature indications after being returned to service.
What is needed is a weld repair that includes a weld repair alloy that is not as susceptible to cracking in the weld repair area in a high temperature environment, while being resistant to corrosion and high temperature oxidation.
A nickel base weld filler material that develops at least 27% (by volume) of the gamma prime (γ′) phase in the weld zone after post weld heat treat (PWHT) is set forth. A weld repair, as used herein, comprises a weld zone, in which weld filler material is deposited and some of the base material adjacent to the deposited weld filler is melted. The weld repair includes a heat affected zone (HAZ) adjacent to the deposited weld zone, as well as an unaffected base metal adjacent to the HAZ. The weld zone is characterized by columnar grains extending from the base material to the middle of the weld, which in turn precipitates up to 30% vol. γ′ phase after a predetermined PWHT. The weld zone microstructure is further characterized by an absence of the eta (η) phase. The HAZ, as is typical, displays some grain growth.
The nickel base weld filler material, hereinafter referred to as NiFillerX™ comprises, in weight percent, 0.03-0.13% C, 22.0-23.0% Cr, 18.5-19.5% Co, 1.8-2.2% W, 0.7-1.4% Nb, 2.2-2.4% Ti, 1.3-2.0% Al, 0.005-0.040% Zr, 0.002-0.008% B, up to 0.15% Mo, up to 0.35% Fe, up to 0.10% Mn, up to 0.10% Cu, up to 0.10% V, up to 0.15% Hf, up to 0.25% Si, and the balance Ni and incidental impurities. The nickel base weld filler material is characterized by an absence of Ta.
Advantageously, the nickel base weld filler material provides a structural weld repair that is crack resistant even in highly stressed areas.
The use of the nickel base weld filler material allows for repair and continued use of complex and expensive parts that develop indications after extended service. The use of this nickel base weld filler material also allows for repair of new nickel base articles that develop indications during casting but prior to service, such as in the thin trailing edge of nozzles or removal of indications in downstream fabrication operations that require weld buildup.
Because the nickel base weld filler material is free of Ta, the material is cheaper than the filler material that it replaces, Ta being a strategic material that haws limited availability in North America.
Other features and advantages of the present invention will be apparent from the following more detailed description of the preferred embodiment, taken in conjunction with the accompanying drawings which illustrate, by way of example, the principles of the invention.
A tantalum-free nickel base weld filler material is set forth. Weld joints made with the tantalum-free (Ta-free) weld filler material are particularly useful for weld repairs for certain high temperature nickel base materials that derive their strength from precipitation strengthening mechanisms which include the formation of γ′.
γ′ is the major precipitation strengthening phase in nickel base superalloys and is a stable face centered cubic (FCC) intermetallic precipitate comprising primarily Ni3Al. It exhibits long-range order to near its melting point of about 2525° F. Niobium (Nb), also referred to as columbium (Cb), Tantalum (Ta) and Titanium (Ti) may atomically substitute aluminum (Al) for up to 60%. Although beyond the scope of this disclosure, the γ′ precipitates in the matrix contribute to strengthening and creep and fatigue resistance by restricting dislocation movement through the grains. Furthermore, uniformly distributed γ′ particles in the range of 0.31.0 micrometers are more effective in resisting creep and fatigue than finer precipitates, as dislocations are less able to penetrate into or bypass these large γ′ particles.
Nickel base superalloys are usually used in the fabrication of parts used in the hot section of turbine engines, such as turbine buckets and turbine nozzles and combustion components. Latter stage turbine nozzles, stage 2 and later stages further to the rear of the turbine engine are of particular interest because of their large size and weight while being subject to high temperature and rapidly flowing exhaust gases. These parts thus must maintain their strength at high temperatures, while being subject to high temperatures of exhaust gases for long periods of time. Because of the high stress, these parts are subject to creep as a result of their exposure to high temperatures for long periods of time. For example, turbine nozzles, such as a turbine nozzle segment 10 shown in
Repairable indications such as these cracks are not limited to components operating in a turbine. New make nozzles sometimes have indications such as repairable cracks in the fillet between thin trailing edges 14 and the side walls during casting manufacture. Post-cast mishandling and mis-machining in downstream fabrications also can result in indications that require weld buildup.
Repairing the regions that develop indications in such articles is highly desirable because of their great expenses, but repairs must both restore the geometries of the structure as well as the mechanical and metallurgical properties of the materials comprising the structure. Otherwise premature indications will redevelop.
Repairs in nickel base superalloy parts or articles for hot gas path applications in gas turbines have been accomplished by welding, such as by arc welding processes including but not limited to tungsten inert gas (TIG) processes also referred to as Gas Tungsten Arc Welding (GTAW). A weld repair includes the addition of weld filler and the melting of adjacent base metal. Remelting of the thin layer of base metal obviously replaces the originally coarse grains with much finer ones, and therefore the solidified metal in the weld zone does not have the same microstructure as the microstructure of the article before welding. In addition, the heat affected zone (HAZ) between the fully remelt and unaffected base metal will invariably experience grain growth which is greatest closest to the weld zone.
Widely used fillers for repair of defects in such nickel base materials include Nimonic 263. The composition of NIMONIC® 263 is provided in Table 1. Nimonic 263 develops ˜8% γ′ phase, while the base material of latter stage nozzles contains at least 27% vol. γ′ phase particles. Thus, the weld repair zone using Nimonic 263 filler has much lower creep and fatigue capabilities than the base material. Moreover, other undesirable phases, such as η phases (Ni3Ti) and/or other TCP phases, easily form in the weld to further reduce the creep and fatigue resistance in the weld.
Welds made using NIMONIC® 263 filler and heat treated to develop a microstructure experience a design life shorter than desired because of the lower mechanical properties of the weld joint as compared to adjacent base material such as GTD®-222. Thus, NIMONIC® 263 is not a preferred weld repair material.
Indications may develop in weld joints with low volume fractions of γ′ at elevated temperatures of operation due to the linkage of creep voids. Creep voids generally form along the grain boundaries, resulting in the formation of an indication. In addition, eta (η) phases having a needle-like morphology may develop at and near the grain boundaries in the weld joint. η phases are Ni3Ti hexagonal close packed (HCP) structures that result from the transformation of stable Ni3Al(Ti) into metastable Ni3Ti. The η phases are to be avoided as they deteriorates the strength, ductility, creep and fatigue capabilities of γ′-containing nickel base superalloys.
Besides NIMONIC® 263, GTD®-222 filler has been developed, but its use in nickel base superalloy repair is very rare. This may be more related to the fact that the GTD-®222 filler is a more expensive alternative because it includes Ta as an expensive additive. Moreover, welds made with GTD-222 filler develop 11 phase needles after PWHT, which downgrade the creep and fatigue capabilities of the weld.
A new filler alloy, NiFillerX™, is thus provided that improve the properties of the weld zone to the same as, if not better than, the base metal. The chemical composition of NiFillerX™ is set forth in Table 2. The filler alloy nominally comprises 0.08% C, 22.5% Cr, 19.0% Co, 2.0% W, 1.1% Nb, 2.3% Ti, 1.8% Al, 0.02% Zr, 0.005% B, 0.15% Mo max., 0.35% Fe max., 0.10% Mn max., 0.10% Cu max., 0.10% V max., 0.15% Hf max., 0.25% Si max, and the balance essentially Ni and incidental impurities. Importantly, the nickel base weld filler alloy is further characterized by the absence of tantalum (Ta). The term “balance essentially nickel” or “balance of the alloy essentially nickel” is used to include, in addition to nickel, small amounts of impurities and other incidental elements, some of which have been described above, that are inherent in nickel base superalloys, which in character and/or amount do not affect the advantageous aspects of the superalloy.
NiFillerX™ weld rod can be prepared by ingot casting then wire drawing or by sintering of atomized powders. The alloy has sufficient ductility for swaging into thin wires for welding applications.
A NiFillerX™ weld in nickel base superalloy materials such as GTD®-222 can develop a microstructure, after heat treatment, having large precipitates of at least 27%, and preferably 30% γ′ by volume, while avoiding the formation of the 11 phase after high temperature exposure. As used herein, the term “large precipitates of γ′” means γ′ particles >0.3 micrometers and is understood to be used in relation to the precipitates of γ′ formed by other nickel base superalloy filler materials, and in particular, the precipitates of γ′ formed by NIMONIC® 263 filler.
One clear difference between NiFillerX™ and both Nimonic 263 filler and GTD®-222 filler is that NiFillerX™ includes more aluminum Al. While the weight percentage amount seems small (nominally, 0.6% by weight more than GTD®-222 and 1.35% more than Nimonic® 263), NiFillerX™ includes 33% more Al than GTD®-222 and 300% more Al than Nimonic® 263. The increase in the ratio of Al over Ti in NiFillerX™ moves it away from region that forms the η phase; the sum of Al+Ti also increases volume fraction of γ′, thereby creep and fatigue strength of the weld joint formed with NiFillerX™, when properly heat treated, is expected to be significantly improved. Another key difference between GTD®-222 filler and NiFillerX™ is that the latter does not include any of the expensive element tantalum (Ta), which lowers its cost.
A heat treatment is required to fully develop the microstructure of the NiFillerX™ weld joint using any precipitation hardenable nickel-base superalloy filler material. A preferred post weld heat treatment will precipitate and fully develop the γ′ in the weld joint. One PWHT comprises heating the article to a temperature of about 2000-2100° F. for about 2 hours followed by aging at a temperature of 1400-1600° F. for a period in the range of 2-8 hours to develop precipitates of γ′ of no less than 0.3 micrometers to provide the desired mechanical properties. It will be understood that lower aging temperatures should be accompanied by longer aging times. A preferred post weld heat treatment is a standard vacuum heat treatment that includes heating the article to a temperature of 2050° F. for two hours, followed by an aging treatment of 1475° F. for 4 hours.
Referring now to
A weld made with NiFillerX™ filler was tested using standard nondestructive testing techniques such as fluorescent penetrant testing, radiographic testing and ultrasonic testing. The nondestructive testing techniques disclosed no observable defects in the weld joint or in the HAZ. Subsequently such a weld was sectioned to examine the metallurgical quality under different magnifications.
The weld joint of the present invention was made by standard GTAW techniques, which is the preferred welding technique. However, the use of the filler material in a weld joint is not limited to applying the weld material in a weldment by GTAW techniques, as any other weld technique for repairing a defect in a nickel base superalloy article may be used. Thus, for example, depending upon the article and the required repair, shielded metal arc welding (SMAW), laser welding, gas metal arc welding (GMAW) and other techniques may be used.
The present invention provides a filler metal that results in a weldment having superior mechanical properties at a lower cost than current available filler metals. The filler metal is not dependent on the availability of limited strategic elements. The weld repair in the article allows the article when placed in service, to have a longer service life than weldments made of currently available filler material.
Both the size distribution and volume fraction of γ′ particles are crucial to impart a weldment using NiFillerX™ filler with superior mechanical and metallurgical properties under severe operation conditions during turbine operation. The particle size γ′ particles shall be no smaller than 0.3 micrometers and is preferred to be in the range of 0.5-1.0 micrometers. Though the elements in NiFillerX™ may vary in the specified range as shown in Table 2, the resultant γ′ phase shall be no lower than 27.0% by volume and is preferred to be in the range of 28.030.0% by volume.
While the invention has been described with reference to a preferred embodiment, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the appended claims.
