METHODS FOR PROCESSING NICKEL-BASE ALLOYS INVENTORS

Kevin Bockenstedt

Ramesh S. inisandram

BACKGROUND OF THE TECHNOLOGY FIELD OF TECHNOLOGY

[0001] The present disclosure relates to methods for heat treating powder metallurgy nickel-base alloy articles. The present disclosure also is directed to powder metallurgy nickel-base alloys produced by the method of the present disclosure, and to articles including such alloys. DESCRIPTION OF THE BACKGROUND OF THE TECHNOLOGY

[0002] Powder metallurgy nickel-base alloys are produced using powder metallurgical techniques such as, for example, consolidating and sintering metallurgical powders. Powder metallurgy nickel-base alloys contain nickel as the predominant element, along with concentrations of various alloying elements and impurities, and may be

strengthened by the precipitation of gamma prime (γ') or a related phase during heat treatment. Components and other articles produced from powder metallurgy nickel- base alloys, e.g., discs for gas turbine engines, typically undergo thermo-mechanical processing to form the shape of the articles, and are heat treated afterwards. For example, the articles are forged and isotherma!iy solution heat treated at a temperature below the γ' soivus (subsolvus), followed by quenching in suitable medium, e.g. , air or oil. A solution heat treatment below the γ' soivus can result in a fine grain

microstructure. The solution heat treatment may be followed by a lower temperature aging heat treatment to relieve residual stresses that develop as a result of the quench and/or to produce a distribution of γ' precipitates in a gamma (γ) matrix. [0003] In conventional processes, forged powder metallurgy nickel-base alloy articles are placed in a furnace at a start temperature in the furnace that is within 30°C of the solution heat treatment temperature. The furnace set point is then recovered so that the articles reach the solution heat treatment temperature as fast as possible for completing the required heat treatment. However, the likelihood of critical grain growth in the articles may be increased by this conventional method of heat treating. Thus, there has developed a need for improved methods that overcome the limitations of conventional processes that increase the likelihood of critical grain growth in powder metallurgy nickel-base alloy articles.

SUMMARY

[0004] The present disclosure, in part, is directed to methods and alloy articles that address certain of the limitations of conventional approaches for heat treating powder metallurgy nickel-base alloy articles. Certain embodiments herein address limitations of conventional processes regarding the heat treat recovery time for solution heat treating, e.g., the time it takes for powder metallurgy nickel-base alloy articles to reach the solution heat treatment temperature. One non-limiting aspect of the present disclosure is directed to a method for heat treating a powder metallurgy nickel-base alloy article comprising: placing the article in a furnace at a start temperature in the furnace that is 80°C to 200°C below a gamma prime solvus temperature; increasing the temperature in the furnace to a solution temperature at a ramp rate in the range of 30°C per hour to 70°C per hour; solution treating the article for a predetermined time; and cooling the article to ambient temperature. In certain non-limiting embodiments of the method, the ramp rate is in the range of 50°C per hour to 55°C per hour.

[0005] Another non-limiting aspect of the present disclosure is directed to a powder metallurgy nickel-base alloy article prepared by a process comprising: placing the article in a furnace at a start temperature in the furnace that is 80°C to 200°C below a gamma prime solvus temperature; increasing the temperature in the furnace to a solution temperature at a ramp rate of 30°C per hour to 70°C per hour; solution treating the article for a predetermined time; and cooling the article to ambient temperature. BRIEF DESCRIPTION OF THE DRAWING

[0006] Features and advantages of the methods and alloy articles described herein may be better understood by reference to the accompanying drawings in which:

[0007] Figure 1 is a flow chart of a non-limiting embodiment of a method for heat treating a powder metallurgy nickel-base alloy article according to the present disclosure;

[0008] Figure 2 is a graph plotting the temperature in the furnace as a function of time for a non-limiting embodiment of a method for heat treating a powder metallurgy nickel- base alloy article according to the present disclosure; and [0009] Figure 3 is a graph plotting the temperature in the furnace relative to solution temperature as a function of time for another non-limiting embodiment of a method for heat treating a powder metallurgy nickel-base alloy article according to the present disclosure.

[0010] it should be understood that the invention is not limited in its application to the arrangements illustrated in the above-described drawings. The reader will appreciate the foregoing details, as well as others, upon considering the following detailed description of certain non-limiting embodiments of methods and alloy articles according to the present disclosure. The reader also may comprehend certain of such additional details upon using the methods and alloy articles described herein. DETAILED DESCRIPTION OF CERTAIN NON-LIMITING EMBODIMENTS

[0011] in the present description of non-limiting embodiments and in the claims, other than in the operating examples or where otherwise indicated, all numbers expressing quantities or characteristics of ingredients and products, processing conditions, and the like are to be understood as being modified in ail instances by the term "about." Accordingly, unless indicated to the contrary, any numerical parameters set forth in the following description and the attached claims are approximations that may vary depending upon the desired properties one seeks to obtain in the methods and alloy articles according to the present disclosure. At the very least, and not as an attempt to limit the application of the doctrine of equivalents to the scope of the claims, each numerical parameter should at least be construed in light of the number of reported significant digits and by applying ordinary rounding techniques. [0012] The present disclosure, in part, is directed to methods and alloy articles that address certain of the limitations of conventional approaches for heat treating powder metallurgy nickel-base alloy articles. Referring to Figure 1 , a non-limiting embodiment of a method according to the present disclosure for heat treating powder metallurgy nickel-base alloy articles is illustrated. The method includes placing the article in a furnace at a start temperature in the furnace that is 80°C to 200°C below a gamma prime solvus temperature (block 100), increasing the temperature in the furnace to a solution temperature at a ramp rate in the range of 30°C per hour to 70°C per hour (block 1 10), solution treating the article for a predetermined time (block 120), and cooling the article to ambient temperature (block 130). The solution heat treatment may be followed by a lower temperature aging heat treatment to relieve residual stresses that develop as a result of the quench, and/or to produce a distribution of γ' precipitates in a gamma γ matrix.

[0013] According to certain non-limiting embodiments, the nickel-base alloy comprises, in weight percentages, 8 to 20.6 cobalt, 13,0 to 16.0 chromium, 3.5 to 5.0 molybdenum, 2.1 to 3.4 aluminum, 3.6 to 3.7 titanium, 2.0 to 2,4 tantalum, up to 0,5 hafnium, 0.04 to 0.06 zirconium, 0.027 to 0.06 carbon, up to 0.025 boron, up to 0.9 niobium, up to 4 tungsten, up to 0.5 iron, nickel, and incidental impurities. In certain non-limiting embodiments, the alloy includes 0.5 hafnium. More generally, the methods described herein may be used in connection with the heat treatment of powder metallurgy nickel- base alloys, in certain non-limiting embodiments, the alloy includes 0.5 hafnium. Non- limiting examples of powder metallurgy nickel-base alloys that can be processed in accordance with various non-limiting embodiments disclosed herein include the alloys in Table 1. It will be appreciated by those skilled in the art that the alloy compositions in Table 1 refer only to the major alloying elements contained in the nickel-base alloy on a weight percent basis of the total alloy weight, and that these alloys may also include other minor additions of alloying elements.

Table 1

Although the present description references certain specific alloys, the methods

5 and alloy articles described herein are not limited in this regard, provided that they relate to powder metallurgy nickel-base alloys. A "powder metallurgy nickel-base alloy" is a term of art and will be readily understood by those having ordinary skill in the production of nickel-base alloys and articles including such alloys. Typically, a powder metallurgy nickel-base alloy is compacted to densify the loose powder mass. The 10 compacting is conventionally performed by hot isostatic pressing (also referred to as "HiPping") or extrusion, or both.

[0015] Referring to Figures 2-3, in certain non-limiting embodiments, the start temperature in the furnace is 1 10°C to 350°C below the γ' solvus temperature of the particular powder metallurgy nickel-base alloy. For example, if the γ' solvus

1 5 temperature is 1 150°C, the start temperature in the furnace can be 800°C to 1040°C.

Typical γ' solvus temperatures of powder metallurgy nickel-base alloy are 1 120°C to 1 190°C. Therefore, the start temperature in the furnace is generally within the range of 770°C to 1080°C. According to certain non-limiting embodiments, the start temperature in the furnace is 60°C to 200°C below the alloy's γ' solvus temperature. According to

20 certain particular non-limiting embodiments, the start temperature in the furnace is 200°C below the alloy's γ' solvus temperature.

[0016] According to certain non-limiting embodiments, the ramp rate is in the range of 30°C per hour to 70°C per hour. According to certain non-limiting embodiments, the ramp rate is in the range of 50°C per hour to 70°C per hour, or in the range of 50°C per hour to 55°C per hour. For example, if the ramp rate is 55°C per hour, and the furnace is ramped from 927.5°C to 1 120°C, the time required to complete the ramp is 3.5 hours. Depending on the usage requirement or preferences for the particular alloy article, a ramp rate faster than 70°C per hour may not provide the requisite grain structure or other desired properties, as further explained below. On the other hand, a ramp rate slower than 30°C per hour may not be economically feasible due to the increased time required to complete the heat treatment. According to certain non-limiting embodiments, the ramp rate is a constant rate. That is, the instantaneous rate is constrained to be uniform throughout the step of increasing the temperature. According to other embodiments, the ramp rate may have slight variations over the ramp cycle. According to certain non-limiting embodiments, the average ramp rate fails within the range of 50°C per hour to 70°C per hour, wherein the instantaneous ramp rate is always within the range of 50°C per hour to 70°C per hour.

[0017] According to certain non-limiting embodiments, the article is solution treated for 1 hour up to 10 hours such that the material is of uniform composition and properties. For example, the article can be solution treated in the range of 1 hour to 10 hours, 1 hour to 9 hours, 1 hour to 8 hours, 1 hour to 7 hours, 1 hour to 6 hours, 1 hour to 5 hours, 1 hour to 4 hours, 1 hour to 3 hours, or 1 hour to 2 hours. According to certain non-limiting embodiments, the solution temperature is at least 10°C below the γ' solvus. For example, the solution temperature for the RR1000 alloy can be 1 120°C. According to certain non-limiting embodiments, the article is maintained at the solution

temperature with a temperature tolerance of ±14°C. According to other embodiments, the article is maintained at the solution temperature with a temperature tolerance of ±10°C. According to other embodiments, the article is maintained at the solution temperature with a temperature tolerance of ±8°C. According to further embodiments, the temperature tolerance can vary, so long as the article is maintained at a

temperature not exceeding the γ' solvus temperature. As used herein, phrases such as "maintained at" with reference to a temperature, temperature range, or minimum temperature, mean that at least a desired portion of the powder metallurgy nickel-base alloy reaches, and is held at, a temperature at least equal to the referenced temperature or within the referenced temperature range. [0018] According to certain non-limiting embodiments, the article is cooled to ambient temperature after the solution heat treatment. According to certain non-limiting embodiments, the article is quenched in a medium, e.g. , ai or oil, so that a temperature of the entire cross-section of the article (e.g., center to surface of the article) cools at a rate of at least 0.1 °C/second. According to other embodiments, the article is control cooled at other cooling rates.

[0019] According to certain non-limiting embodiments, the powder metallurgy nickel- base alloy produced according to various non-limiting embodiments of the methods disclosed herein comprises an average grain size of 10 micrometers or less,

corresponding to an ASTM grain size number that is approximately equal to or greater than 10 in accordance with ASTM E1 12. According to certain non-limiting embodiments, the powder metallurgy nickel-base alloy produced according to various non-limiting embodiments of the methods disclosed herein comprises a coarse grain population and a fine grain population, and the average grain size of the coarse grain population differs from the average grain size of the fine grain population by two ASTM grain size numbers or less (in accordance with ASTM E1 12). For example, certain non-limiting embodiments of powder metallurgy nickel-base alloy produced according to various non-limiting embodiments of the methods disclosed herein comprises a coarse grain population having an average grain size of ASTM 10 in accordance with ASTM E1 12, corresponding to an average grain size of 1 1 .2 μητι, and a fine grain population having an average grain size of ASTM 12 in accordance with ASTM E1 12, corresponding to an average grain size of 5.6 μητι. According to further non-limiting embodiments, the coarse grain population has an average grain size of ASTM 10 or finer, and the fine grain population has an average grain size of ASTM 12 or finer, in accordance with ASTM E1 12. Although examples of possible grain size populations are given herein, these examples do not encompass all possible grain size populations for powder metallurgy nickel-base alloy articles according to the present disclosure. Rather, the present inventors determined that these grain size populations represent possible grain size populations that can be suitable for certain powder metallurgy nickel-base alloy articles processed according to various non-limiting embodiments of the methods disclosed herein. It is to be understood that the methods and alloy articles of the present disclosure may incorporate other suitable grain size populations.

[0020] Depending on the use requirements or preferences of the particular method or alloy articles, before the step of placing the article in the furnace at the start temperature, the powder metallurgy nickel-base alloy article is forged. According to further

embodiments, additional steps such as, for example, coating, rough, and final

machining and/or surface finishing, may be applied to the article before placing the article in the furnace at the start temperature.

EXAMPLE 1 [0021] Referring to Fig. 2, a disk forging of RR1000 alloy was placed in a furnace at a start temperature in the furnace of 927°C. The temperature in the furnace was increased to 1 120°C at a ramp rate of 55°C per hour. The disk was maintained at 1 120°C for four hours, and then air-cooled to ambient temperature. Subsequently, the disk was milled to remove the oxide layer, and etched to inspect the macro grain structure. The macro inspection revealed a uniform grain structure, with no coarse grain bands at the hub or rim areas. Samples were cut from both the bore hub areas and the rim of the disk, for mounting and micrographic examination. The micrographic examination from the upper hub location did show some grain size banding between the surface and center of the part, with the coarser region at the part surface having an AST grain size number of 1 1.5, and the adjacent matrix having an ASTM grain size number of 12.5. Grain sizes from outer rim and lower hub locations were both uniform with no banding. The outer rim grain size was an ASTM 1 1.5, and the lower hub grain size was an ASTM 12.

EXAMPLE 2 [0022] Referring to Fig. 3, a disk forging of RR1000 alloy was placed in a furnace at a start temperature in the furnace of 1010°C. The temperature in the furnace was increased to 1 120°C at a ramp rate of 55°C per hour. The disk was maintained at 1 120°C for four hours, and then air-cooled to ambient temperature. Samples were cut from both the bore hub areas and the rim of the disk, for mounting and micrographic examination. The micrographic examination from the upper hub location did show some grain size banding between the surface and center of the part, with the coarser region having an ASTM grain size number of 10, and the adjacent matrix having an ASTM grain size number of 12. Grain sizes from outer rim and lower hub locations were both uniform with no banding. The outer rim and the lower hub grain sizes were both an ASTM 12,

EXAMPLE 3

[0023] A disk forging of RR 000 alloy is placed in a furnace at a start temperature in the furnace of 927°C. The temperature in the furnace is increased to 1 1 10°C at a ramp rate of 66°C per hour. The disk is maintained at 1 10°C for four hours, and then air cooled to ambient temperature.

EXAMPLE 4

[0024] A disk forging of RR1000 alloy is placed in a furnace at a start temperature in the furnace of 927°C. The temperature in the furnace is increased to 1 1 10°C at a ramp rate of 50°C per hour. The disk is maintained at 1 1 0°C for four hours, and then air cooled to ambient temperature.

[0025] Non-limiting examples of articles of manufacture that may be fabricated from or include the present powder metallurgy nickel-base alloy produced according to various non-limiting embodiments of the methods disclosed herein are a turbine disc, a turbine rotor, a compressor disc, a turbine cover plate, a compressor cone, and a compressor rotor for aeronautical or land-base turbine engines. Those having ordinary skill can fabricate the articles of manufacture from alloys processed according to the present methods using known manufacturing techniques, without undue effort. [0026] Although the foregoing description has necessarily presented only a limited number of embodiments, those of ordinary skill in the relevant art will appreciate that various changes in the methods and alloy articles and other details of the examples that have been described and illustrated herein may be made by those skilled in the art, and

- g . all such modifications will remain within the principle and scope of the present disclosure as expressed herein and in the appended claims. It is understood, therefore, that the present invention is not limited to the particular embodiments disclosed or incorporated herein, but is intended to cover modifications that are within the principle and scope of the invention, as defined by the claims. It will also be appreciated by those skilled in the art that changes could be made to the embodiments above without departing from the broad inventive concept thereof.