Metal treatment – Process of modifying or maintaining internal physical... – Processes of coating utilizing a reactive composition which...
Reexamination Certificate
2002-04-26
2004-04-13
Wyszomierski, George (Department: 1742)
Metal treatment
Process of modifying or maintaining internal physical...
Processes of coating utilizing a reactive composition which...
C148S527000, C148S537000
Reexamination Certificate
active
06719853
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to textured articles and in particular to the restoration of the microstructure of textured articles. The invention is further related to the field of refurbishment of gas turbine airfoils.
BACKGROUND OF THE INVENTION
An example of the background technology of the present invention is U.S. Pat. No. 5,611,670 which shows a gas turbine blade made of a nickel based superalloy. Superalloys are nickel- or cobalt-based alloys, typically comprising chromium, titanium, tantalum, aluminum, tungsten and other elements, with excellent high temperature resistance, thereby maintaining high strength properties. Accordingly, superalloys are widely used in high temperature applications where additionally high mechanical strength is required. A typical application is the casting of airfoils for gas turbines, jet engines as well as stationary gas turbines, e.g. for industrial applications like power generation. Further improvements in mechanical strength is achieved by casting the superalloy as a columnar or as a single crystal. A textured article has no or very few grain boundaries.
As additional background, European Patent Application EP 1 038 982 A1 describes a process for manufacturing single crystal superalloy articles. After casting the article is subjected to a heat treatment in order to further improve the mechanical strength. The heat treatment is a high temperature solution heat treatment which homogenizes the microstructure of the alloy itself formed by different crystal phases. However, this heat treatment may lead to a grain re-crystallization occurrence, initiated by dislocations in the crystal structure. This grain re-crystallization destroys locally the single crystal structure which may lead to a dramatic decrease in the mechanical strength of the article. Accordingly, grain re-crystallization is a cause for rejection of single crystal castings if present beyond a preset maximum for re-crystallized grains and can result in low yields of acceptable heat-treated single crystal castings. By heat treating in a carburizing atmosphere, carbon is introduced into the casting and forms carbides therein that reduce or localize the occurrence of grain re-crystallization.
European Patent EP 0 525 545 B1 describes the refurbishment of corroded superalloy articles. In particular gas turbine airfoils are subjected to corrosion by hot gases. Typically, a corrosion protective coating is provided on the body made from the superalloy. Widely used coatings are of the MCrAlY type, where M is iron, cobalt and/or nickel, and Y, for yttrium or another rare earth element or another element such as lanthanum. This type of coating is usually applied by a plasma spray process. However, despite a corrosion protective coating, the airfoils are still under corrosion and erosion attack which leads to the need for servicing after a certain period of time. Corrosion results from contaminants in the fuel and/or air, and oxidation may also occur at high temperatures. Depending on the conditions of operation, an oxide layer of varying thickness may form on the surface of the airfoil. Also, and very significantly, sulfur can penetrate into the base material to form sulfides. Furthermore, internal oxides and nitrides may form within the metal near the surface.
Instead of completely exchanging airfoils, it is often a cost saving option to refurbish the airfoils, i.e. providing a new protective coating. This requires complete removal of the old coating, which is realized by applying mechanical stripping as well as chemical treatment, e.g. with acid. After removal of a substantial part of the old coating, the surface is aluminized. Subsequently, the aluminide layer is removed, thereby also removing oxidized and corroded regions at the surface.
U.S. Pat. No. 5,413,648 discloses a directionally solidified article with a plastic deformation damage at the surface, which is prone to recrystallization. This problem is overcome by removing a part of the deformed surface region.
European patent No. EP 1 036 850 A1 discloses a single crystal having a surface coating for preventing recrystallization fracture by reinforcing the grain boundaries. After a heat treatment the texture of the article at the surface shows no single crystal structure anymore, because the surface has grain boundaries, which are reinforced by grain boundary strengthening elements like Zr, Hf, B or C.
U.S. Pat. No. 6,271,668 shows the need of a coating during one step of refurbishment of gas turbine components, which is applied on the surface of the component. The coating is heat treated by which a surface region of the article is aluminized. This heat treatment is performed at low temperatures in order to avoid detrimental diffusion of atoms from corrosion products. The coating is removed together with the corroded layers before further heat treatments are performed.
SUMMARY OF THE INVENTION
The present invention provides a method for restoring the microstructure of a textured article, e.g. a single crystal or a directionally solidified article, which comprises creating on the surface of the article a high temperature stable surface coating and subsequently performing a solution heat treatment, thereby maintaining said thermally stable surface coating.
As stated previously, grain recrystallization may occur during a solution heat treatment of the article. The present invention underlies the discovery that grain recrystallization occurs at lower temperatures at the surface of an article compared to bulk regions. The energy needed for forming new grains with grain boundaries is lower at the surface. By applying a coating on the surface and maintaining this surface coating during the solution heat treatment, grain recrystallization is suppressed due to the now provided bulk conditions. Accordingly, an effective solution heat treatment can be processed, thereby restoring the microstructure of the textured article without introducing grain recrystallization. A full description of the effect of suppressing grain recrystallization by surface coating was given by one of the applicants in the publication “Recrystallization In Single Crystals Of Nickel Base Superalloys”, R. Bürgel, P. D. Portella, J. Preuhs, Superalloys 2000, edited by T. M. Pollock, pages 229-238, the teaching of which is incorporated herein by reference, in particular with respect to the alloy compositions disclosed in table I (balance Ni) and heat treatment parameters in table II.
TABLE I
Cr
Co
Mo
W
Ta
Nb
Al
Ti
C
Hf
CMSX-11B
12.5
7
.5
5
5
.1
3.6
4.2
—
.04
PWA 1483
12.2
9
1.9
3.8
5
—
3.6
4.1
.07
—
SRR 99
8.5
5
—
9.5
2.8
—
5.5
2.2
0.2
—
CMSX-6
10
5
3
—
2
—
4.8
4.7
—
.08
TABLE II
CMSX-
solutioning,
1204° C./2 h + 1227° C.2/h + 1249° C./3 h +
11B
SHT*
1260° C./6 h; heating with 1 K/min
age hdn.
1120° C./5 h + 870° C./24 h + 760° C./30 h
PWA
solutioning*
1260° C./1 h
1483
age hdn.
1090° C./4 h
SRR 99
solutioning*
1270° C./0, 5 h + 1280° C./1 h + 1290° C./
2 h + 1300° C./0, 5 h + 1305° C.0, 5 h;
heating with 1 K/min
age hdn.
1080° C./4 h + 870° C./16 h
3 CMSX-
solutioning*
1227° C./2 h + 1238° C./2 h + 1271° C./2 h +
6
1277° C./3 h + 1280° C./2 h; heating
with 1 K/min
age hdn.
1080° C./4 h + 870° C./16 h
*SHT: Solution heat treatment. Additional steps at lower temperatures may precede the solutioning to equilibrate the furnace.
The article is preferably made from a superalloy, which may be nickel-based or cobalt-based. The microstructure in such a nickel-based superalloy is formed by a &ggr;-phase and a &ggr;′-phase. The temperature required for solution heat treatment is considerably high and is at least the solution temperature of the &ggr;′-phase. By maintaining such a temperature during the solution heat treatment, an effective restoring of the microstructure is achieved. This high temperature normally increases the risk of recrystallization. This risk is substantially reduced by applying the surface coating before performing the solution heat treatment. The
Buergel Ralf
Esser Winfried
Ott Michael
Roan Der-Yan Frank
Siemens Aktiengesellschaft
Wyszomierski George
LandOfFree
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