Patent Application
20110142712
The present invention relates to a nickel-base gamma prime strengthened superalloy. It further relates to its use in hot components such as, but not restricted to, vanes, heat shields, sealings and combustor parts in gas turbines. It further relates to its use as filler alloy for cladding and repair of hot components such as, but not restricted to, blades, vanes, heat shields and combustor parts in gas turbines. It further relates to its use in equiaxed, directionally solidified or monocrystalline form.
Gamma prime strengthened nickel-base superalloys are essential for critical components in aero and land based gas turbines, but are used also in other applications. The difference between said superalloys depend on the level of knowledge and production technology available at the time they were developed, and, on different relative emphasis on properties such as hot corrosion resistance, oxidation resistance, coating compatibility, weldability, phase stability and creep strength. Most alloy development has emphasized high creep strength as needed for gas turbine blades which are subject to high loadings from the rotation of the rotor to which they are attached.
Nickel-base gamma prime strengthened superalloys are used in monocrystalline (SX), directionally solidified (DS) or equiaxed form. In each crystal there is a matrix of the phase gamma which is essentially Ni with elements like Co, Cr, Mo, W and Re in solid solution, and, particles of the phase gamma prime which is essentially Ni3Al with elements like Ti, Ta and Nb in solid solution. Grain boundaries, if present, are usually decorated by carbides and/or borides which provide cohesive strength. Zr also contributes to grain boundary cohesion.
Creep strength is provided by elements like Mo, W and Re which provide solution strengthening of the gamma matrix, and, Ti, Ta and Nb which provide solution strengthening of the gamma prime particles. Ta has a particularly high strengthening effect per at %. Al provides creep strength as it increases the amount of gamma prime particles, and as it concentrates the levels of Mo, W and Re in the matrix.
If the concentration of Cr, Mo, W and Re in the matrix is too high, so called topologically close packed (TCP) phases will form in service. Therefore, an increased gamma prime content, or, increased Mo, W or Re levels, must be accompanied by a reduction in Cr if TCP formation is to be avoided. One particular effect of TCP precipitation is a reduction in creep strength.
[Caron 1] teaches that the risk for TCP precipitation for an alloy can be estimated through comparison of the Md value for this alloy and a relatively similar alloy with known TCP risks. For the alloys analyzed here, the Md value can be calculated from
Md=0.717aNi+0.787aCo+1.142aCr+1.55aMo+1.655aW+1.9aAl+2.271aTi+2.224aTa wherein aCo is the content of Co in atom % etc.
Hot corrosion resistance is provided by Cr. It is also important to allow at most moderate Mo levels.
[Goldschmidt] teaches that the hot corrosion resistance of the alloy SC16 with 16 wt % Cr and 3 wt % Mo is significantly inferior to that of IN738LC with 16 wt % Cr and 1.8 wt % Mo. IN738LC is generally recognized as having high hot corrosion resistance. Therefore a restriction to at most 2 wt % Mo in a new alloy seems prudent.
In the context of high firing temperature gas turbines it is generally accepted that high oxidation resistance require the ability to form an adherent continuous AL2O3 scale, as needed for metal temperatures on the 1000 degree Celcius level.
[Barrett] teaches that this ability is provided by Al, enhanced by Cr and Ta, somewhat reduced by Mo and W, and significantly reduced by Ti and Nb. This implies that less Al is needed to form such an Al2O3 scale if the levels of Cr and Ta are increased, or, the levels of Ti and Nb are reduced.
[Sarioglu] teaches that the scale adherence is severely reduced by tramp elements such as S, but, that this effect can be neutralized by a combination of clean casting and addition of small measured levels of reactive elements (RE).
[Pint 1] underlines the importance of S, and further teaches the beneficial RE effects when small levels of Hf and the rare earth Y are combined.
[Caron 2] teaches the beneficial RE effects when small levels of Hf and Si are combined.
[Pint 2] teaches that optimal RE effects can be obtained when multiple RE are used, one example being the excellent cyclic oxidation resistance seen in tests on Haynes-214 which contained small levels of Zr, Si and Y.
One important aspect of coating compatibility particularly relevant for modern high firing temperature gas turbines is the cyclic life of an applied thermal barrier coating.
[Wahl&Harris] teaches that the spallation life of a TBC increased significantly when rare earths were added to the base alloy.
[Wu] teaches that the spallation life of a TBC can be correlated to the oxidation resistance of the base alloys. In particular, the lowest spallation life was obtained for the base alloy with the highest Ti content.
It is generally accepted that the risk for weld cracking during solidification, or, subsequent heat treatment, increases with increasing gamma prime content of the base and filler alloys.
The used references can be found as follows:
Early alloy development resulted in alloys such as U-700 which has a composition, in wt %, given by Ni-17Co-15Cr-4.5Mo-4.3Al-3.5Ti-0.07Zr-0.08C. U-700 can form a continuous Al2O3 scale with 4.3 wt % Al thanks to 15 wt % Cr and despite 3.5 wt % Ti. These alloys have less hot corrosion resistance than suggested by their Cr levels due to their high levels of Mo, the main strengthening element in use at the time they were developed. Their gamma prime contents are in the 45 to 55 vol % range implying that they do not provide good weldability. They have a tendency to precipitate TCP, and typically provide moderate creep strength.
One line of subsequent alloy development led to alloys such as the commercially available alloys IN939, IN738LC and IN792, and this class of alloys remains the norm in land based gas turbines to this day. IN939, an alloy typically used for vanes, has a composition, in wt %, given by Ni-19Co-22Cr-2W-2Al-3.7Ti-1.4Ta-1Nb-0.1Zr-0.15C-0.01B. That of IN738LC is Ni-8.5Co-16Cr-1.8Mo-2.6W-3.4Al-3.4Ti-1.8Ta-0.9Nb-0.09Zr-0.08C-0.01B. That of IN792 is Ni-9Co-12.5Cr-1.8Mo-4.2W-3.4Al-4.2Ti-4.2Ta-0.08C-0.015B.
Compared to alloys such as U-700, Mo is partly replaced by W for improved hot corrosion resistance, and, Al is partly replaced by Ti, Nb and Ta for improved creep strength through increased solution strengthening of the gamma prime. In particular, these alloys contain high Ti levels.
Their hot corrosion resistance range from very high in alloys like IN939 to moderate in IN792 depending on their Cr contents. These alloy do not have the capability to form a continuous Al2O3 scale, thus they do not provide high oxidation resistance. Their coating compatibility is at best moderate due to their high Ti levels. The gamma prime particle contents are typically in the 30-55 vol % range, and alloys like IN939 at the lower end of this interval have good weldability by the standards of blade and vane alloys. Improved knowledge allowed alloy designers to handle phase stability better than for the old alloys above. The creep strength varies from moderate in alloys like IN939 to high in alloys like IN792.
Derivatives such as SCA425 and SCA425Hf with reduced Ti and increased Al and Ta levels have recently been developed, see [Caron 3]. SCA425 has a composition, in wt %, given by Ni-5Co-16Cr-1Mo-4W-4Al-2Ti-5Ta, and SCA425Hf also includes 0.4Hf for RE effect and coating compatibility. In-house oxidation tests on SCA425 showed that it is almost, but not quite, capable of forming a continuous Al2O3 scale, see
Another line of subsequent alloy development led to alloys such as Mar M-247 which has a composition, in wt %, given by Ni-10Co-8Cr-0.7Mo-10W-5.65Al-1Ti-3Ta-1.5Hf-0.15C. The introduction of DS and SX casting led to derivative alloys such as CM247DS and Rene N5. In Rene N5, some W has been replaced by Re, and Hf has been replaced by Ta. The composition in wt % is Ni-7.5Co-7Cr-1.5Mo-5W-3Re-6.1Al-6.5Ta-0.1Hf-0.05C.
The hot corrosion resistance in this class of alloys is poor due to their very low Cr levels. Their high Al levels supported by moderate to high Ta levels, allow them to form continuous Al2O3 scales despite their very low Cr levels. Their coating compatibility is high thanks to low Ti levels. Recently, significant work has been done to enhance their oxidation resistance and coating compatibility via clean casting and use of RE effects. Their weldability is poor due to their very high gamma prime contents in the 60 to 75 vol % range. Care has been taken to achieve phase stability to avoid reduction of creep strength due to TCP precipitation. Their creep strengths range from high in alloys such as Mar M-247 to very high in alloys such as Rene N5.
Outside these three main classes, there are some specialized alloys of interest. Haynes-214 has a basic composition in wt % given by Ni-3Fe-16Cr-4.5Al and also contains small levels of Zr, Si and Y to provide RE effects. It combines high hot corrosion resistance, excellent oxidation resistance and high weldability. The creep strength is poor because it contains no strengthening elements.
Document U.S. Pat. No. 5,240,491 discloses a brazing alloy powder of similar composition as defined in the claims with regard to tungsten, rhenium, molybdenum, aluminum. However, this document does not consider zirconium to be of relevance to obtain a high oxidation resistance.
None of the alloys above provide a blend of high hot corrosion resistance, high oxidation resistance, high coating compatibility, good weldability, high phase stability and at least moderate creep resistance, and it is an objective of this invention to provide such a blend.
This blend of properties will be useful for design of moderately stressed gas turbine components such as vanes and heat shields, and, for a filler used for cladding and weld-repair of moderately stressed parts of gas turbine components such as blade tips and platforms. Further, this blend of properties will be useful in components protected by thermal barrier coatings.
Consider a component and a hot spot on said component. A base alloy with this blend of properties would provide a higher cyclic life for an applied thermal barrier coating than an Ti rich alloy such as IN939, and, would provide a longer residual life if spallation occurs.
Consider further a component cast in a Ti rich alloy such as IN939 and protected by a thermal barrier coating, and that said component has suffered local spallation of this thermal barrier coating at a hot spot, leading to oxidation damage. Weld-repair with a filler alloy having the said blend of properties would result in locally improved resistance to spallation of the thermal barrier coating when re-applied after weld-repair, and, increased residual life if spallation occurs.
Base and filler alloys with this blend of properties will also provide better robustness in corrosive environments than high Mo content alloys such as U-700, or low Cr content alloys such as Mar M-247. Further, they will provide better dimensional stability than a weak alloy such as Haynes-214.
Consider further that in a weld, mixing during welding and subsequent interdiffusion in service will occur. In particular, Ti may diffuse into the filler alloy in the case of Ti rich base alloys such as IN939. It would be an advantage to have a filler alloy which retained adequate phase stability also while having absorbed some Ti, to avoid TCP precipitation. TCP phases are, after all, also known as brittle phases.
In the present invention, the requirements on high hot corrosion resistance, high oxidation resistance, high coating compatibility, good weldability, high phase stability and at least moderate creep resistance are met through the use of moderate levels of Al and Ta, high levels of Cr, multiple reactive elements, moderate levels of matrix strengthening elements like Mo, W and Re, a particular restriction on the level of Mo, and by avoiding detrimental gamma prime strengthening elements like Ti and Nb.
The idea is that moderate levels of Al and Ta are sufficient to enable formation of an adherent continuous AL2O3 scale thanks to support from high levels of Cr, multiple RE effects and that we avoid detrimental elements like Ti and Nb. Therefore, the gamma prime content is kept sufficiently low to allow for good weldability. Further, moderate levels of matrix and gamma prime strengthening elements enable moderate creep strength. Further, high levels of Cr and a strict limit on Mo enable high hot corrosion resistance. Further, high levels of Cr can be tolerated from a phase stability point of view because the gamma prime content and the amount of matrix strengthening elements are moderate. Further, high coating compatibility is enabled by avoiding detrimental elements like Ti and Nb, and, by a combination of clean casting and the use of RE to neutralize any remaining S.
According to one embodiment of the invention the alloy may include, measured in wt %, up to 20 wt % Co, between 17 and 21 wt % Cr, between 2 and 5 wt % of Mo+W+Re, at most 2 wt % Mo, between 4 and 4.7 wt % Al, between 3 and 7 wt % Ta, between 0.01 and 0.3 wt % of C+Zr+B, between 0.05 and 1 wt % Hf, between 0.05 and 1 wt % Si, and between 0.01 and 0.2 wt % of the sum of rare earths such as Sc, Y, the actinides and the lanthanides.
Additionally, the alloy may include, measured in wt %, between 2 and 12 wt % Co, between 17 and 21 wt % Cr, up to 1 wt % Mo, between 1 and 4 wt % W, between 4 and 4.7 wt % Al, between 3 and 7 wt % Ta, between 0.01 and 0.1 wt % C, between 0.01 and 0.15 wt % Zr, between 0.002 and 0.02 wt % B, between 0.05 and 1 wt % Hf, between 0.05 and 1 wt % Si, and between 0.01 and 0.2 wt % of Ce+La+Y+Dy.
Additionally, the alloy may include between 3 and 7 wt % Co, between 17 and 19 wt % Cr, between 0.5 and 1 wt % Mo, between 2 and 3 wt % W, between 4.2 and 4.6 wt % Al, between 3.5 and 5 wt % Ta, between 0.01 and 0.07 wt % C, between 0.01 and 0.1 wt % Zr, between 0.002 and 0.01 wt % B, between 0.05 and 0.5 wt % Hf, between 0.05 and 0.5 wt % Si, and between 0.01 and 0.1 wt % of the sum of Ce+La+Y+Dy.
Additionally, the alloy may include between 4.5 and 5.5 wt % Co, between 17.7 and 18.3 wt % Cr, between 0.7 and 0.9 wt % Mo, between 2.3 and 2.7 wt % W, between 4.3 and 4.5 wt % Al, between 4.2 and 4.6 wt % Ta, between 0.02 and 0.04 wt % C, between 0.02 and 0.05 wt % Zr, between 0.002 and 0.008 wt % B, between 0.07 and 0.13 wt % Hf, between 0.07 and 0.13 W. % Si, and between 0.01 and 0.03 wt % of the sum of Ce+La+Y+Dy.
In a preferred embodiment called STAL18, the alloy may include about 5 wt % Co, about 18 wt % Cr, about 0.8 wt % Mo, about 2.5 wt % W, about 4.4 wt % Al, about 4.4 wt % Ta, about 0.03 wt % C, about 0.03 wt % Zr, about 0.005 wt % B, about 0.1 wt % Hf, about 0.1 wt % Si, and about 0.02 wt % Ce.
Alternatively, the alloy can include between 3 and 7 wt % Co, between 19 and 21 wt % Cr, between 2 and 4 wt % W, between 4.3 and 4.7 wt % Al, between 4 and 7 wt % Ta, between 0.01 and 0.07 wt % C, between 0.01 and 0.1 wt % Zr, between 0.002 and 0.01 wt % B, between 0.05 and 0.5 wt % Hf, between 0.05 and 0.5 wt % Si, and between 0.01 and 0.1 wt % of the sum of Ce+La+Y+Dy.
Additionally, the alloy may include between 4.5 and 5.5 wt % Co, between 19.7 and 20.3 wt % Cr, between 2.7 and 3.3 wt % W, between 4.4 and 4.6 wt % Al, between 5.3 and 5.7 wt % Ta, between 0.02 and 0.04 wt % C, between 0.02 and 0.05 wt % Zr, between 0.002 and 0.008 wt % B, between 0.07 and 0.13 wt % Hf, between 0.07 and 0.13 wt % Si, and between 0.01 and 0.03 wt % of the sum of Ce+La+Y+Dy.
In a preferred embodiment called STAL20, the alloy may include about 5 wt % Co, about 20 wt % Cr, about 3 wt % W, about 4.5 wt % Al, about 5.5 wt % Ta, about 0.03 wt % C, about 0.03 wt % Zr, about 0.005 wt % B, about 0.1 wt % Hf, about 0.1 wt % Si, and about 0.02 wt % Ce.
The superalloy according to the invention is preferably processed with a clean production process. To guarantee best results, the superalloy should contain less than 2 ppmw S.
Alternatively, further embodiments can be designed to optimize compatibility with specific coatings when the alloy is used as a base alloy. Alternatively, further embodiments can be designed to optimize compatibility with specific base alloys and coatings when the alloy is used as filler alloy for cladding and weld repair. Re may for example be included for compatibility with Re containing base alloys, and Co levels can be matched.
For the widest embodiment, the lower bound on Cr and the highest allowable level of Mo, i.e. the worst possible combination, is most likely to provide high hot corrosion resistance.
Further, the ability to form an adherent continuous AL2O3 scale is best illustrated by comparison with the alloy SCA425Hf which was able to form a continuous AL2O3 scale on most of the surface, but, failed to do so in some spots, after 600 h at 1000 degree Celcius as seen in
Further, high coating compatibility is provided by having no Ti or Nb, and by clean casting and the use of RE to neutralize S.
The conclusions above are applicable to any preferred embodiment.
The ThermoCalc simulation of the composition Ni-5Co-18Cr-0.8Mo-2.5W-4.4Al-4.4Ta predict a gamma prime content of 30% at an equilibrium temperature of 900 degree Celcius for the embodiment called STAL18, see
The calculated Md values based on the Caron formula were 0.9477 and 0.9657 for Ni-5Co-18Cr-0.8Mo-2.5W-4.4Al-4.4Ta and Ni-5Co-18Cr-0.8Mo-2.5W-4.4Al-1Ti-4.4Ta respectively, which can be compared with an Md value of 0.9771 for STAL425, an alloy which is known to be adequately stable w.r.t. TCP precipitation. This means that 1 wt % Ti can be absorbed due to mixing or interdiffusion during welding or in service without risk for TCP precipitation when this alloy is used as filler alloy for welding of Ti rich base alloys like IN939. The implication is that the risk for poor fatigue properties in the interdiffusion zone due to TCP precipitation is significantly reduced. After all, TCP phases are also known as brittle phases.
The large heat treatment window seen in the ThermoCalc simulation also suggests that it will respond with full solutioning without incipient melting for the solutioning and HIP procedures associated with most base alloys when it is used as filler alloy for welding.
The level of matrix strengthening elements is higher than for IN939. The high strengthening effect of Ta implies at least moderate solution strengthening of the gamma prime particles despite the fact that the amount of strengthening elements in at % is relatively small. The Ta/Al ratio is e.g. similar to the ratio in Rene N5, another alloy in which Ti and Nb are avoided. Together with a gamma prime content of 30% this strongly suggests at least moderate creep strength for the embodiment above.
The embodiment called STAL20 will have higher hot corrosion resistance and creep strength than STAL18 thanks to higher Cr and Ta levels. The calculated Md value is 0.9621 which indicates good phase stability based on comparison with SCA425. Unlike STAL18 it will not have a large margin w.r.t. TCP precipitation against absorbation of Ti and is therefore less useful as filler alloy for Ti rich base alloys.
| Number | Date | Country | Kind |
|---|---|---|---|
| 08003904.3 | Mar 2008 | EP | regional |
This application is the US National Stage of International Application No. PCT/EP2009/052341, filed Feb. 27, 2009 and claims the benefit thereof. The International Application claims the benefits of European Patent Office application No. 08003904.3 EP filed Mar. 3, 2008. All of the applications are incorporated by reference herein in their entirety.
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/EP09/52341 | 2/27/2009 | WO | 00 | 3/2/2011 |
