Abstract:
For a group of nickel-based superalloys, improved properties have been obtained by stabilizing at increased temperature for a reduced time relative to prior art specifications. In particular, improved creep properties have been obtained with a one-hour 1800° F. stabilization relative to a prior art four-hour 1500° F. stabilization.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     The invention relates to heat treatment of superalloys. More particularly, the invention relates to stabilization of nickel-based alloys for disks and other gas turbine engine rotating parts.  
         [0002]     The combustion, turbine, and exhaust sections of gas turbine engines are subject to extreme heating as are latter portions of the compressor section. This heating imposes substantial material constraints on components of these sections. One area of particular importance involves structural rotating parts such as blade-bearing turbine disks and shafts. The disks are subject to extreme mechanical stresses, in addition to the thermal stresses, for significant periods of time during engine operation. Shafts are subject to somewhat similar stresses and variant alloys have been developed for shaft use.  
         [0003]     Exotic materials have been developed to address the demands of turbine disk use. Shafts are subject to somewhat similar stresses and variant alloys have been developed for shaft use. Separately, other materials have been proposed to address the demands of turbine blade use. Turbine section blades are typically cast and some blades include complex internal features.  
         [0004]     U.S. Pat. Nos. 5,120,373 and 5,938,863 disclose advanced nickel-base superalloys. One commercial disk alloy embodiment of such an alloy has a nominal composition of 16.0 Cr, 13.5 Co, 4.15 Mo, 4.6 Ti, 2.2 Al, 0.07 Zr, 0.006 B, 0.0025 Mg, balance Ni, by weight percent. For reference, this alloy is identified as alloy “A” hereafter. A commercial shaft alloy variant has a nominal composition of 15.75 Cr, 13.5 Co, 4.15 Mo, 4.6 Ti, 2.2 Al, 0.07 Zr, 0.006 B, 0.0025 Mg, balance Ni, by weight percent. For reference, this alloy is identified as alloy “B” hereafter. Alloy “B” is a higher tensile strength alloy. Both are used in a conventionally processed (not powder metallurgical) form.  
         [0005]     U.S. Pat. No. 6,521,175 discloses an advanced nickel-base superalloy for powder metallurgical manufacture of turbine disks. The &#39;175 patent discloses disk alloys optimized for short-time engine cycles, with disk temperatures approaching temperatures of about 1500° F. (816° C.). Other disk alloys are disclosed in U.S. Pat. No. 5,104,614, U.S. Pat. No. 2004221927, EP1201777, and EP1195446.  
         [0006]     An exemplary processing of a forging includes: solution treatment; stabilization; and age hardening stages. Exemplary solution treatment comprises heating to a high temperature effective to remove prior precipitate phases (principally gamma prime (γ′)). An exemplary temperature is in excess of 1900° F. (e.g., 1910-2015° F. in standard alloy “A” processing with an upper limit reflecting a desired control of grain size). Such a temperature is maintained for an interval effective to achieve desired precipitate phase removal (e.g., two hours in standard (prior art) alloy “A” processing). Air cooling or a faster cooling rate is then performed to rapidly decrease temperature to avoid precipitate formation at undesirable intermediate temperatures. An exemplary cooling is to a temperature near or below 1000° F.  
         [0007]     Stabilization serves to form carbides at grain boundaries. Exemplary stabilization comprises heating at an intermediate temperature effective to form sufficient carbides to stabilize the grain boundaries (e.g., 1500+/−25° F. in standard alloy “A” processing). Such a temperature is maintained for an interval effective to achieve the desired carbide formation (e.g., four hours in standard alloy “A” processing). Fan air cooling or an equivalent is then performed to similarly avoid any precipitate formation at undesirable intermediate temperatures. An exemplary cooling is to a temperature near or below 1000° F.  
         [0008]     Age hardening (precipitation heat treatment) serves to grow desired γ′ within the γ matrix. Exemplary age hardening comprises heating at a lower temperature and for a time effective to grow a desired size and volume fraction of γ′ (e.g., 1350+/−25° F. for eight hours in standard alloy “A” processing). Air cooling or fan air cooling is then performed to rapidly terminate γ′ formation.  
       SUMMARY OF THE INVENTION  
       [0009]     For a group of nickel-based superalloys, improved properties have been obtained by stabilizing at increased temperature for a reduced time relative to prior art specifications.  
         [0010]     Experimentally, for alloys whose standard prior art stabilization is four hours at 1500° F., improved creep properties have been obtained with a one-hour 1800° F. stabilization.  
         [0011]     The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]      FIG. 1  is a photomicrograph of alloy “A” after a prior art heat treatment.  
         [0013]      FIG. 2  is a photomicrograph of alloy “A” after heat treatment with an inventive modified stabilization.  
         [0014]      FIG. 3  is a table of stress-rupture properties of powder metal alloy “A”.  
         [0015]      FIG. 4  is a table of 1200° F. tensile properties of powder metal alloy “A”.  
         [0016]      FIG. 5  is a table of creep properties of powder metal alloy “A”.  
         [0017]      FIG. 6  are Larson-Miller curves for alloy “A”.  
         [0018]      FIG. 7  is a table of tensile properties of conventional alloy “A”.  
         [0019]      FIG. 8  is a table of creep properties of conventional alloy “A”.  
         [0020]      FIG. 9  is lognormal plot of creep for conventional alloy “A”.  
         [0021]      FIG. 10  is a table of creep properties of conventional alloy “C”.  
         [0022]      FIG. 11  is a Larson-Miller curve for alloy “C”.  
         [0023]      FIG. 12  is a table of creep properties of conventional alloy “B”.  
         [0024]      FIG. 13  is a Larson-Miller curve for alloy “B”. 
     
    
       [0025]     Like reference numbers and designations in the various drawings indicate like elements.  
       DETAILED DESCRIPTION  
       [0026]     A relatively short duration, high temperature stabilization cycle has been found to provide improved properties. In a specific example, substituting an 1800° F., one-hour stabilization cycle for the standard 1500° F., four-hour cycle has been demonstrated to substantially improve creep and stress-rupture properties of both cast/wrought and powder metal (PM) versions of several nickel-base superalloys.  
         [0027]     As discussed below, tested alloys include production alloys “A” and “B” and an experimental alloy “C”. Alloy “C” was derived from alloy “A” as an improved low cycle fatigue (LCF) variant principally through reduced Mo content. With prior art heat treatment, Alloy “C” has improved smooth and notched LCF properties. However, those improvements came at the expense of lower stress-rupture (SR) and creep properties. Alloy “C” has a composition within U.S. Pat. No. 5,938,863. Nominal alloy “C” composition is 2.2Al, 4.6Ti, 15.5Cr, 3.0Mo, 13.5Co, 0.015C, 0.015B, 0.04Zr, 0.002Mg, balance essentially Ni, by weight percent.  
         [0028]     Other strong superalloys may also benefit from the present modified heat treatment. This may be particularly relevant for alloys whose prior art stabilization cycles are in the 1500-1600° F. range. For example, Udimet 700 and 720LI alloys (Special Metals Corp., New Hartford, N.Y., referenced in U.S. Pat. No. 6,132,527), Astroloy (UNS N13017) and standard Waspaloy (UNS N07001 and Werkstoff Number 2.4654), all typically used in non-PM wrought form, and alloy IN 738, typically used in cast form (e.g., a TOBI duct, turbine exhaust case, and the like), have specified prior art stabilization in the 1500-1600° F. range. The nominal, composition of Udimet 720LI alloy is 16Cr, 14.7Co, 3.0Mo, 1.25W, 5.0Ti, 2.5Al, 0.010C, 0.015B, 0.03Zr, balance essentially Ni, by weight percent. Among differences relative to alloys “A” and “B”, Udimet 720LI has a tungsten content whereas the others have essentially none. Udimet 720LI also has a relatively low molybdenum content and a relatively high titanium content.  
         [0029]     Specifically, the modified stabilization had no detrimental effect on dwell da/dN (fracture mechanics) behavior of PM alloy “A” which was the only material so tested. Further testing demonstrated that the microstructural damage caused by prior art stabilization at 1500-1600° F. cannot be reversed without a re-solution treatment. The modified stabilization also improved the properties of non-PM alloy “C”, with significant improvements in SR and creep behavior.  
       Powder Metallurgical (PM) Testing  
       [0030]     In a prior art treatment, PM alloy “A” forgings were solutioned at 2030° F. for two hours followed by an oil quench. The forgings were then stabilized at 1500° F. for four hours followed by a four hour fan air cool (FAC). The forgings were then aged at 1350° F. for eight hours followed by FAC. Similar forgings were prepared using the inventive (“modified”) heat treatment substituting an 1800° F., one-hour stabilization cycle for the standard 1500° F., four-hour cycle.  
         [0031]      FIG. 1  shows the exemplary prior art microstrucure with light areas representing matrix, including γ′ phases  20 . Dark spots represent carbides (including M 23 C 6 ) and/or borides  22 .  FIG. 2  shows microstructure produced by the exemplary modified heat treatment. It appears that the 1800° F. stabilization cycle spheroidizes the carbides and/or borides  22 ′ relative to those of the prior art and may reduce their size.  
         [0032]     The initial SR testing of extruded powder material with the standard alloy “A” stabilization cycle demonstrated properties that failed the conventional (non-PM) alloy “A” specification minima ( FIG. 3 ).  
         [0033]     The standard stabilization cycle of PM alloy “A” material encountered low lives/ductilities and notch failures.  
         [0034]     Several PM alloy “A” finish machined specimens with the prior art heat treatment were re-solutioned and then stabilized according to the modified stabilization. Re-solutioning was in vacuum at 1975° F. for two hours then fan air cooled (the low solution temperature avoided grain growth). Stabilization was at 1800° F. for one hour followed by a forced argon cool (FArC). Age hardening was at 1350° F. for eight hours followed by FArC. This procedure produced no dimensional distortion. Rupture lives were increased by a factor of two to three ( FIG. 3 ) while notch failures were eliminated and no grain coarsening occurred. Thus, at least in the tested alloy, the improvement changed a notch-weakened condition to a notch-strengthened condition.  
         [0035]     Tensile testing at 1200° F. ( FIG. 4 ) showed a very minor decrease in ultimate tensile strength for material that received the modified stabilization relative to the prior art. However, all tensile data well exceeded the alloy “A” specification minima on a −2σ statistical basis. The modified stabilization cycle was found to eliminate unusual “double shear lip” failures encountered in some PM alloy “A” tensile specimens.  
         [0036]     The modified stabilization cycle also improved creep properties ( FIGS. 5 and 6 ). The modified stabilization cycle had no impact on dwell crack growth behavior. It appears from  FIG. 2  that M 23 C 6  carbides and/or borides are spheroidized by the 1800° F. stabilization cycle. This may have decreased the minimum creep rate, resulting in an overall improvement in creep performance with the majority of creep in Stage III.  
       Conventional (Non-PM) Testing  
       [0037]     In the past, conventional wrought alloy “A” with prior art heat treatment occasionally did not meet specification creep requirements. Coarsening the grain size by increasing the solution temperature typically improves creep capability. However, the alloy&#39;s γ′ solvus temperature is too low to allow this without encountering excessive grain growth. Grain growth would benefit creep, stress-rupture, and da/dN properties. However, grain growth has a negative effect on tensile strength and fatigue properties. These countervailing factors have restricted attempts to achieve an advantageous balance of these properties.  
         [0038]     A slower cooling rate during the superoverage (SOA) cycle (e.g., U.S. Pat. No. 4,574,015) used in billet manufacturing possibly could increase the primary γ′ particle spacing and produce a somewhat coarser, controllable grain size. However, this approach was not tested.  
         [0039]     In a different approach, conventional alloy “A” was re-solutioned (1975° F. for two hours followed by FAC). It was then stabilized/aged using the modified stabilization cycle discussed above. This allowed evaluation of the benefit of the modified stabilization cycle while avoiding the possibility of grain growth similar to that used for the PM version of alloy “A”.  FIG. 7  shows that the 1200° F. tensile properties of conventionally processed (non-PM) alloy “A” experienced only a minor decrease in tensile yield/UTS with no effect on ductility. Specification tensile property requirements were well satisfied. Creep testing conducted at 1300° F./40 ksi and 1300° F./70 ksi showed improvements ranging from 45-75% at least through 1300° F. ( FIGS. 8 and 9 ). Thus, the modified stabilization cycle produced creep lives which substantially exceeded the specification requirements.  
         [0040]     For alloy “C”, initially, creep properties were determined using the standard alloy “A” heat treatment. Additional creep specimens were machined from material processed through the modified 1800° F. stabilization cycle. Test data ( FIGS. 10 and 11 ) showed a substantial improvement.  
         [0041]     It was theorized that a “yo-yo” heat treatment might provide an improved balance between nucleation and growth of the carbides and/or borides in the alloy and thus improve creep behavior.  
         [0042]     Alloy “B” was used in the following test as an expedient because available alloy “C” material had been consumed and these two alloys have similar compositions with the principal exception of molybdenum. The material was re-solutioned at 1975° F. and given either the modified stabilization cycle or an alternative prior art “yo-yo” heat treatment (see, e.g., U.S. Pat. No. 4,907,947). The solution temperature was at the high end of the alloy “B” specification range to be compatible with the prior alloy “A” work. It is noted that 1975° F. is the upper end of a specification solution temperature of 1900-1975° F. The remainder of the alloy “B” specification heat treatment coincides with that of alloy “A”.  
         [0043]     The “yo-yo” stabilization involved a 40-minute 1600° F. interval, then FAC, then a 45-minute 1800° F. interval, then FAC. The “yo-yo” aging followed with a 24-hour 1200° F. interval, then ambient air cooling (AC), then a 4-hour 1400° F., then AC.  
         [0044]      FIGS. 12 and 13  show alloy “B” creep results from 1250-1400° F. The modified heat treatment increased typical creep properties by an order of magnitude relative to the standard. This may have been caused by grain coarsening. However, the data shows that the “yo-yo” heat treatment produced properties that were inferior to the 1800° F. stabilization cycle over the range tested. Both sets of alloy “B” material were observed to have the same grain size after these heat treatment. Thus, the microstructural damage encountered at 1500-1600° F. apparently cannot be recovered in this alloy without re-solutioning.  
         [0045]     Typical shaft applications for alloy “B” involve temperatures below where creep is a concern. However, the improve creep performance indicates that the modified stabilization cycle may be useful for similar alloys in higher temperature applications.  
       CONCLUSIONS  
       [0046]     In conclusion, lower than desired creep properties in alloy “A” and derivative/similar alloys have been significantly improved by changing the four-hour 1500° F. stabilization cycle to a one-hour 1800° F. cycle. This temperature increase and duration decrease produced a substantial improvement in both creep and stress-rupture properties for both conventional and PM forms of alloy “A”. The alloy “C” compositional modification of alloy “A” as well as alloy “B” also benefited from this stabilization cycle change.  
         [0047]     Ultimate tensile strength at 1200° F. showed a slight decrease but remained well above the specification requirements. The slightness of the decrease may provide an indication that further refinement could produce at least a slight increase.  
         [0048]     The tests across several compositions provide an indication of broader applicability.  
         [0049]     One or more embodiments of the present invention have been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the invention. Accordingly, other embodiments are within the scope of the following claims.