Metal treatment – Stock – Nickel base
Reexamination Certificate
1999-09-01
2001-04-24
King, Roy V. (Department: 1742)
Metal treatment
Stock
Nickel base
C148S442000, C148S410000, C420S445000, C428S680000
Reexamination Certificate
active
06221181
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to an improved class of protective coatings for use on superalloy articles, such as gas turbine rotating blades and stationary vanes.
Wide use of single crystal (SX) and directionally solidified (DS) hot-stage components has allowed increased turbine inlet temperature and therefore turbine efficiency. The improvement in high-temperature strength of these new superalloys involved an increased susceptibility of the alloy to sulfidation and oxidation. To restore environmental resistance to engine parts made from DS and SX alloys requires a new generation of high-temperature resistant coatings. Historically, aluminide or MCrAlY coatings (where M represents a transition element such as Ni, Co, Fe or mixtures thereof) have been applied by engine manufacturers to extend the useful life of hot section components.
Due to their limited thickness (typically around 50 ~&mgr;m) aluminide coatings do not offer sufficient oxidation and corrosion protection for the long exposure times in stationary gas turbines (20000-50000 hours). Present MCrAlY coatings, in particular when the Al reservoir phase consists of &bgr; (NiAl) phase demonstrate much greater environmental resistance compared to aluminide coatings. However, since a coated turbine blade undergoes complicated stress states during engine operation (especially during start up and shut down) advanced high temperature coatings must not only provide environmental protection but must also have specifically tailored physical and mechanical properties to provide high thermo-mechanical fatigue resistance. In summary, high-temperature resistant coatings must meet the following requirements:
high oxidation resistance
slowly growing oxide scale and good oxide scale adherence
hot corrosion resistance, superior to SX/DS superalloys
low interdiffusion of Al and Cr into the substrate to prevent the precipitation of brittle needle-like phases under the coating
high thermo-mechanical fatigue resistance
U.S. Pat. Nos. 5,273,712 and 5,154,885 disclose coatings with significant additions of Re which simultaneously improves creep and oxidation resistance at high temperatures. However, the combination of Re with high Cr levels, typical for traditional coatings, results in an undesirable phase structure of the coating and interdiffusion layer. At intermediate temperatures (below 950-900° C.), &agr;-Cr phase is more stable in the coating than the &ggr;-matrix. This results in low toughness and low ductility. In addition, a significant excess of Cr in the coating compared to the substrate results in diffusion of Cr to the base alloy, which enhances precipitation of needle-like Cr-, W- and Re-rich phases.
U.S. Pat. No. 4,447,503 discloses a superalloy coating composition with high temperature oxidation resistance. The coatings consist essentially of, by weight, 5-50% Cr, 3-30% Al, 0.01-15% Ta, up to 10% Mn, up to 5% W, up to 12% Si, up to 10% Hf, up to 5% reactive metal from the group consisting of La, Y, and other rare earth (RE) elements, up to 5% of RE and/or refractory metal oxide particles, and the balance selected from the group consisting of Ni, Co and Fe, and combinations thereof. Additions of up to 5% Ti and up to 15% noble metals are also contemplated. However, the coatings are only intended for applications where the need for improved high temperature oxidation is paramount and where the coating ductility is relatively unimportant.
OBJECT OF THE INVENTION
It is an object of the present invention to provide an improved coating for structural parts of gas turbines which exhibits improved-mechanical behavior.
It is another object of the present invention to provide an improved coating for structural hot-stage components of gas turbines that operate in high temperature oxidizing and sulfidizing environments.
It is a further object of the present invention to provide an improved coating with sufficient oxidation and corrosion resistance and diffusional stability for the long exposure times customary in stationary gas turbines.
SUMMARY OF THE INVENTION
Briefly, the present invention discloses a nickel base alloy which provides simultaneously excellent environmental resistance, phase stability during diffusion heat treatment and during service, and highly improved thermomechanical behavior, and hence is particularly adapted for use as coating for advanced gas turbine blading. The alloy according to the present invention is prepared with the elements in an amount to provide an alloy composition as shown in Table 1 (a).
TABLE 1(a)
Range of Preferred Coating Compositions of Present Invention
Elements in wt % of composition
Ni
Co
Cr
Al
Y
Si
Ta
Re
Ca
Mg
Ru
La*
Coating
Bal.
28-35
11-15
10-13
0.005-0.5
1-2
0.2-1
0-1
0-1
0-1
0-5
0-0.5
La* = La + elements from Lanthanide series
Y + La (+ La-series) ≦ 0.3-2.0 wt %
Si + Ta ≦ 2.5 wt %
Hf, C < 0.1 wt %
Ca + Mg > 2 × (S + 0)
It was found that the preferred alloys of Table 1 (b) exhibit a dramatically and unexpectedly high TMF resistance, while providing excellent environmental protection and phase stability during high temperature exposure.
TABLE 1(b)
Preferred Coating Compositions
Elements in wt % of composition
Coating
Ni
Co
Cr
Al
Re
Y
Si
Ta
Ca
Mg
PC1
Bal.
29.7
12.9
11.5
0
0.27
1.2
0.5
0.003
PC2
Bal.
30.2
11.9
12.1
0.1
0.1
1.1
0.4
0.001
PC3
Bal.
32
13.1
10.9
0.2
0.25
1.3
0.5
0.005
0.001
Preferably, the alloy of the desired composition can be produced by the vacuum melt process in which powder particles are formed by inert gas atomization. The powder can then be deposited on a substrate using, for example, thermal spray methods. However, other methods of application may also be used. Heat treatment of the coating using appropriate times and temperatures is recommended to achieve a high sintered density of the coating and to promote bonding to the substrate.
Prior art coatings, such as EC0 in table 2 (a), are known to exhibit excellent oxidation/sulfidation resistance and good thermomechanical fatigue properties. However, as turbine inlet temperatures increase and turbine operating cycles become more severe (e.g. higher strain ranges, higher cooling rates, higher number of cycles), the cyclic life of protective coatings needs to be further improved.
In an effort to develop a coating with improved mechanical properties—without sacrificing too much oxidation resistance—a variety of alloy compositions was evaluated. In order to prove the advantage of the preferred compositions of table 1 (b) a number of additional alloys whose compositions are given in Table 2 have also been tested. Compared with the preferred compositions, alloys EC0-EC6 were found to have either reduced oxidation resistance or inferior mechanical properties. Only the alloy according to the invention provides simultaneously high oxidation resistance and thermomechanical fatigue resistance and phase stability.
TABLE 2(a)
Prior Art Coating Composition
Elements in wt % of composition
Coating
Ni
Co
Cr
Al
Re
Y
Si
Ta
EC0
Bal.
24
13
11
3
0.3
1.2
0.5
TABLE 2(b)
Additional Experimental Coating Compositions
Elements in wt % of composition
Coating
Ni
Co
Cr
Al
Re
Y
Si
Ta
EC1
Bal.
24
13
11
—
0.3
1.2
0.5
EC2
Bal.
30
13
11
3
0.3
1.2
0.5
EC3
Bal.
30
13
11
1.5
0.3
1.2
0.5
EC4
Bal.
24
15
11
3
0.3
1.2
0.5
EC5
Bal.
24
17
11
—
0.3
0.2
—
EC6
Bal.
35
22
11
—
0.3
—
—
REFERENCES:
patent: 4124737 (1978-11-01), Wolfla et al.
patent: 4339509 (1982-07-01), Dardi et al.
patent: 4419416 (1983-12-01), Gupta et al.
patent: 4447503 (1984-05-01), Dardi et al.
patent: 4758480 (1988-07-01), Hecht et al.
Bossmann Hans-Peter
Holmes Peter
Konter Maxim
Schmutzler Hans J.
Sommer Christoph
ABB Research Ltd.
Coy Nicole
King Roy V.
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