Patent Publication Number: US-6908519-B2

Title: Isothermal forging of nickel-base superalloys in air

Description:
This invention relates to the forging of nickel-base superalloys and, more particularly, to such forging performed in air. 
     BACKGROUND OF THE INVENTION 
     Nickel-base superalloys are used in the portions of aircraft gas turbine engines which have the most demanding performance requirements and are subjected to the most adverse environmental conditions. Cast nickel-base superalloys are employed, for example, as turbine blades and turbine vanes. Wrought nickel-base superalloys are employed, for example, as rotor disks and shafts. The present invention is concerned with the wrought nickel-base superalloys. 
     The wrought nickel-base superalloys are initially supplied as cast ingots, which are cast from the melt, or as consolidated-powder billets, which are consolidated from powders. The consolidated-powder billets are preferred as the starting material for many applications because they have a uniform, well-controlled initial microstructure and a fine grain size. In either case, the billet is reduced in size in a series of steps by metal working procedures such as forging or extrusion, and is thereafter machined. In one form of forging, the billet is placed between two forging dies in a forging press. The forging dies are forced together by the forging press to reduce the thickness of the billet. 
     The selection of the forging conditions depends upon several factors, including the properties and metallurgical characteristics of the nickel-base superalloy and the properties of the materials of the forging dies. The forging dies must be sufficiently strong to deform the material being forged, and the forged superalloy must exhibit the required properties at the completion of the forging operation. 
     At the present time, nickel-base superalloys, such as Rene™ 88DT and ME3 alloys, are isothermally forged at or above a temperature of about 1900° F. using TZM molybdenum dies. This combination of the superalloy being forged and the die material allows the forging to be performed, and the superalloy has the required properties at the completion of the forging. However, this combination of temperature, the superalloy being forged, and the die material requires that the forging be performed in vacuum or in an inert-gas atmosphere. The requirement of a vacuum or an inert-gas atmosphere greatly increases the complexity and cost of the forging process. 
     There is a need for an improved approach to the forging of nickel-base superalloys that achieves the required properties and also reduces the forging cost. The present invention fulfills this need, and further provides related advantages. 
     BRIEF SUMMARY OF THE INVENTION 
     The present invention provides a method for forging nickel-base superalloys such as Rene™ 88DT and ME3. The method allows the forging to be performed isothermally in air, resulting in a substantial cost saving. The final microstructure has the desired grain structure, and is consistent with and permits additional processing such as supersolvus final annealing. 
     The present invention provides a method for forging a superalloy comprising the steps of providing a forging blank of a forging nickel-base superalloy, and providing a forging press having forging dies made of a die nickel-base superalloy. The die nickel-base superalloy desirably has a creep strength of not less than the flow stress of the forging nickel-base superalloy at a forging temperature of from about 1700° F. to about 1850° F. and a forging nominal strain rate. The method further includes heating the forging blank and the forging dies to the forging temperature of from about 1700° F. to about 1850° F., and forging the forging blank using the forging dies at the forging temperature of from about 1750° F. to about 1850° F. and at the forging nominal strain rate. 
     The forging blank is made of the forging nickel-base superalloy, preferably Rene™ 88DT, having a nominal composition, in weight percent, of 13 percent cobalt, 16 percent chromium, 4 percent molybdenum, 3.7 percent titanium, 2.1 percent aluminum, 4 percent tungsten, 0.75 percent niobium, 0.015 percent boron, 0.03 percent zirconium, 0.03 percent carbon, up to about 0.5 percent iron, balance nickel and minor impurity elements; or alloy ME3, having a nominal composition, in weight percent, of about 20.6 percent cobalt, about 13.0 percent chromium, about 3.4 percent aluminum, about 3.70 percent titanium, about 2.4 percent tantalum, about 0.90 percent niobium, about 2.10 percent tungsten, about 3.80 percent molybdenum, about 0.05 percent carbon, about 0.025 percent boron, about 0.05 percent zirconium, up to about 0.5 percent iron, balance nickel and minor impurity elements. These forging nickel-base superalloys exhibit superplasticity over a respective superplastic temperature range at elevated temperature. The forging deformation is desirably accomplished in the superplastic temperature range to avoid critical grain growth in the subsequent supersolvus anneal The nickel-base superalloys may be furnished in any operable form, such as cast-wrought or consolidated-powder billets. Preferably, however, the nickel-base superalloys are furnished as extruded billet with a grain size of not less than ASTM 12 (i.e., grain sizes of ASTM 12 or smaller). 
     The forging dies may be made of any operable die nickel-base superalloy, but preferably have a nominal composition, in weight percent, of from about 5 to about 7 percent aluminum, from about 8 to about 15 percent molybdenum, from about 5 to about 15 percent tungsten, up to about 140 parts per million magnesium (preferably about 140 parts per million magnesium), no rare earths, balance nickel and impurities. 
     The selections of the isothermal forging temperature and forging nominal strain rate are based upon consideration of the physical properties of the forging nickel-base superalloy and of the die nickel-base superalloy, and also of the temperature requirement to achieve the required structure in the forging nickel-base superalloy at the completion of the processing. The die nickel-base superalloy has sufficient creep strength to deform the forging nickel-base superalloy. With increasing temperature, the compressive strength and the creep strength of both the forging nickel-base superalloy and the die nickel-base superalloy fall, but at different rates. Additionally, for the preferred forging nickel-base superalloy, the selected forging temperature must be within the superplastic range of the alloy to ensure the proper final microstructure. Further, to accomplish the preferred forging in air, the forging temperature must not be so high that the forging nickel-base superalloy and the die nickel-base superalloy excessively oxidize. 
     With these considerations in mind, the isothermal forging temperature range of from about 1700° F. to about 1850° F. was selected. More preferably, the isothermal forging temperature is from about 1750° F. to about 1800° F. The forging nominal strain rate was selected to be not greater than about 0.010 per second. Testing showed that higher strain rates within the forging temperature range result in critical grain growth in the final processed article. 
     The heating and isothermal forging steps are preferably performed in air, at the indicated temperatures. Forging in air, rather than in inert gas or vacuum as required when TZM molybdenum dies are used, saves on the costs of special heating and forging equipment. 
     After the forging processing according to the present approach, the forging may be used in the as-forged state, or post processed by any operable approach, such as cleaning, heat treating, additional metalworking, machining, and the like. In one further processing of interest, the forging is heat treated by annealing at an annealing temperature above the gamma prime solvus temperature, or typically about 2100° F. for Rene™ 88DT alloy and 2160° F. for ME3 alloy. 
     The present approach provides a technique for forging nickel-base superalloys that results in fully acceptable metallurgical structures and properties in the final forging, while significantly reducing the cost of the forging operation by permitting the isothermal forging to be accomplished in air. 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. The scope of the invention is not, however, limited to this preferred embodiment. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block flow diagram of an approach for practicing the invention; 
         FIG. 2  is a schematic elevational view of a forging press and an article being forged; and 
         FIG. 3  is a schematic perspective view of a forging. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  depicts a preferred approach for practicing the invention. A forging blank is provided, step  20 . The forging blank is made of a forging nickel-base superalloy. As used herein, an alloy is nickel-base when it has more nickel than any other element, and is further a nickel-base superalloy when it is strengthened by the precipitation of gamma prime or related phases. Two nickel-base superalloys of particular interest are Rene™ 88DT, having a nominal composition, in weight percent, of 13 percent cobalt, 16 percent chromium, 4 percent molybdenum, 3.7 percent titanium, 2.1 percent aluminum, 4 percent tungsten, 0.75 percent niobium, 0.015 percent boron, 0.03 percent zirconium, 0.03 percent carbon, up to about 0.5 percent iron, balance nickel and minor impurity elements; and alloy ME3, having a nominal composition, in weight percent, of about 20.6 percent cobalt, about 13.0 percent chromium, about 3.4 percent aluminum, about 3.70 percent titanium, about 2.4 percent tantalum, about 0.90 percent niobium, about 2.10 percent tungsten, about 3.80 percent molybdenum, about 0.05 percent carbon, about 0.025 percent boron, about 0.05 percent zirconium, up to about 0.5 percent iron, balance nickel and minor impurity elements. 
     The nickel-base superalloys are furnished in any operable form, but preferably are furnished as consolidated-powder billets. These billets are made by consolidating powders of the selected superalloy by extrusion, producing a billet having a uniform grain size of ASTM 12 or higher (that is, ASTM 12 or finer grains, inasmuch as the grain size decreases with increasing ASTM grain size number). Consolidated-powder billets have the advantage over cast billets in having a more-uniform fine-grain microstructure and are therefore preferred for achieving good chemical uniformity, good deformation homogeneity, and minimal sites for crack initiation. 
     The forging blank has a size and shape selected so that, after forging, the forged article is of the desired size and shape. Procedures are known in the art for selecting the size and shape of the starting forging blank so as to yield the required finished size and shape. 
     A forging press and forging dies are provided, step  22 . Any operable forging press may be used, and  FIG. 2  schematically depicts a basic forging press  40 . The forging press  40  has a stationary lower platen  42 , a stationary upper plate  44 , and stationary columns  46  that support the upper plate  44  from the lower platen  42 . A movable upper platen  48  slides on the columns  46 , and is driven upwardly and downwardly by a drive motor  50  on the upper plate  44 . A lower forging die  52  is stationary and sits on the lower platen  42 . An upper forging die  54  is movable and is affixed to the upper platen  48  so that it rides upwardly and downwardly with the upper platen  48 . A workpiece  56  is positioned between the upper forging die  54  and the lower forging die  52 . A heater  57 , here illustrated as an induction heating coil, is positioned around the forging dies  52  and  54 , and the workpiece  56 , to maintain the forging dies and the workpiece at a selected approximately constant isothermal forging temperature during the forging stroke, thereby achieving isothermal forging. Some minor variation in temperature is permitted during the forging stroke, but in general the forging dies  52  and  54  and the workpiece  56  remain at approximately the constant isothermal forging temperature. 
     The workpiece  56  is initially the forging blank of the forging nickel-base superalloy. The workpiece  56  is positioned between the upper forging die  54  and the lower forging die  52  and is compressively deformed at a nominal strain rate by the movement of the upper forging die  54  in the downward direction. The upper forging die  54  and the lower forging die  52  may be flat plates, or they may be patterned so that the final forging has that pattern impressed thereon.  FIG. 3  is an exemplary forging  58  with a patterned face  60  produced using patterned forging dies. 
     The forging dies  52  and  54  are made of a die nickel-base superalloy, wherein the die nickel-base superalloy has a creep strength of not less than the flow stress of the forging nickel-base superalloy at an isothermal forging temperature of from about 1700° F. to about 1850° F. and a forging nominal strain rate. The forging dies  52  and  54  are preferably made with a nominal composition, in weight percent, of from about 5 to about 7 percent aluminum, from about 8 to about 15 percent molybdenum, from about 5 to about 15 percent tungsten, up to about 140 parts per million magnesium (preferably 140 parts per million magnesium), balance nickel and impurities. 
     A forging temperature and forging nominal strain rate are selected, step  24 . The forging nickel-base superalloys exhibit superplasticity over a respective superplastic temperature range and strain-rate range at elevated temperature. The forging deformation is desirably accomplished in the superplastic temperature range to avoid critical grain growth in the subsequent supersolvus anneal. 
     The acceptable range of temperatures and strain rates may be determined from the plastic deformation properties of the forging nickel-base superalloy. The following Tables I and II respectively present the results of laboratory tests on Rene™ 88DT and ME3 alloys to determine the operable isothermal forging temperatures and strain rates: 
     
       
         
           
               
             
               
                 TABLE I 
               
             
            
               
                   
               
               
                 (Rene ™ 88DT alloy) 
               
            
           
           
               
               
               
               
               
            
               
                   
                 Temperature ° F. 
                 Strain Rate (/sec) 
                 Stress (ksi) 
                 “m” 
               
               
                   
                   
               
            
           
           
               
               
               
               
               
            
               
                   
                 1800 
                 0.0001 
                 3.03 
                 0.512 
               
               
                   
                 1800 
                 0.0003 
                 5.15 
                 0.459 
               
               
                   
                 1800 
                 0.001 
                 8.44 
                 0.406 
               
               
                   
                 1800 
                 0.003 
                 13.62 
                 0.352 
               
               
                   
                 1800 
                 0.01 
                 19.69 
                 0.299 
               
               
                   
                 1800 
                 0.03 
                 25.79 
                 0.249 
               
               
                   
                 1750 
                 0.0001 
                 4.43 
                 0.497 
               
               
                   
                 1750 
                 0.0003 
                 7.48 
                 0.440 
               
               
                   
                 1750 
                 0.001 
                 12.03 
                 0.385 
               
               
                   
                 1750 
                 0.003 
                 18.65 
                 0.329 
               
               
                   
                 1750 
                 0.01 
                 25.91 
                 0.274 
               
               
                   
                 1750 
                 0.03 
                 33.83 
                 0.220 
               
               
                   
                 1700 
                 0.0001 
                 6.85 
                 0.453 
               
               
                   
                 1700 
                 0.0003 
                 10.95 
                 0.400 
               
               
                   
                 1700 
                 0.001 
                 17.14 
                 0.348 
               
               
                   
                 1700 
                 0.003 
                 24.97 
                 0.295 
               
               
                   
                 1700 
                 0.01 
                 33.94 
                 0.243 
               
               
                   
                 1700 
                 0.03 
                 42.56 
                 0.192 
               
               
                   
                   
               
            
           
         
       
     
     
       
         
           
               
             
               
                 TABLE II 
               
             
            
               
                   
               
               
                 (ME3 alloy) 
               
            
           
           
               
               
               
               
               
            
               
                   
                 Temperature ° F. 
                 Strain Rate (/sec) 
                 Stress (ksi) 
                 “m” 
               
               
                   
                   
               
            
           
           
               
               
               
               
               
            
               
                   
                 1800 
                 0.0001 
                 3.07 
                 0.738 
               
               
                   
                 1800 
                 0.0003 
                 5.49 
                 0.677 
               
               
                   
                 1800 
                 0.001 
                 9.59 
                 0.612 
               
               
                   
                 1800 
                 0.003 
                 15.94 
                 0.538 
               
               
                   
                 1800 
                 0.01 
                 23.62 
                 0.458 
               
               
                   
                 1800 
                 0.03 
                 29.76 
                 0.371 
               
               
                   
                 1750 
                 0.0001 
                 4.87 
                 0.747 
               
               
                   
                 1750 
                 0.0003 
                 9.02 
                 0.669 
               
               
                   
                 1750 
                 0.001 
                 15.14 
                 0.582 
               
               
                   
                 1750 
                 0.003 
                 24.00 
                 0.481 
               
               
                   
                 1750 
                 0.01 
                 31.98 
                 0.367 
               
               
                   
                 1750 
                 0.03 
                 38.67 
                 0.240 
               
               
                   
                 1700 
                 0.0001 
                 8.92 
                 0.672 
               
               
                   
                 1700 
                 0.0003 
                 14.54 
                 0.594 
               
               
                   
                 1700 
                 0.001 
                 23.02 
                 0.508 
               
               
                   
                 1700 
                 0.003 
                 33.2 
                 0.408 
               
               
                   
                 1700 
                 0.01 
                 42.89 
                 0.297 
               
               
                   
                 1700 
                 0.03 
                 47.77 
                 0.174 
               
               
                   
                   
               
            
           
         
       
     
     From this information, processing parameters were selected to achieve the required value of “m” of about 0.3 or greater, where “m” is an index of the extent of superplastic deformation of the material. The forging temperature is preferably from about 1700° F. to about 1850° F., and more preferably from about 1750° F. to about 1800° F. to reduce the risks of excessive oxidation of the workpiece at higher temperatures. The forging nominal strain rate is not greater than about 0.01 per second. The “nominal” strain rate is that determined from the gross rate of movement of the upper platen  48 , normalized to the height of the workpiece  56  measured parallel to the direction of movement of the upper platen  48 . Locally within the forging dies  52  and  54 , the actual strain rate may be higher or lower. 
     The forging blank and the forging dies are heated to the isothermal forging temperature of from about 1700° F. to about 1850° F., step  26 . 
     The forging blank is forged using the forging dies at the isothermal forging temperature of from about 1700° F. to about 1850° F. and at the forging nominal strain rate, step  28 , using a forging apparatus such as the forging press  40  of FIG.  2 . 
     The heating step  26  and the forging step  28  are preferably performed in air. The forging in air greatly reduces the cost of the forging operation as compared with forging in vacuum or an inert atmosphere, as required in prior processes for forging the nickel-base superalloys. The determination to forge in air is not an arbitrary one, and air forging may be performed only where the die material does not excessively oxidize in air at the forging temperature and also retains sufficient strength at the forging temperature. The conventional die material, TZM molybdenum, cannot be used at these temperatures in air because of its excessive oxidation. 
     After the forging operation of step  28  is complete, the forging  58  is removed from the forging press  40 . The forging  58  may be used in the as-forged state, or it may be post processed, step  30 . In the preferred case, the forgings of Rene™ 88DT or ME3 nickel-base superalloys are annealed at an annealing temperature above the gamma-prime solvus temperature. The supersolvus annealing is preferably at a temperature of from about 2080° F. to about 2100° F. for the Rene™ 88DT alloy and from about 2120° F. to about 2160° F. for the ME3 alloy, for a time of from about 1 to about 2 hours. Other types of post-processing  30  may include, for example, cleaning, other types of heat treating, additional metalworking, machining, and the like. 
     Although a particular embodiment of the invention has been described in detail for purposes of illustration, various modifications and enhancements may be made without departing from the spirit and scope of the invention. Accordingly, the invention is not to be limited except as by the appended claims.