JP3320739B2 - Protective layer for turbine blade - Google Patents

Abstract

The invention relates to a turbine blade with a corrosion-resistant protective coating of MCrAlY. The aim of the invention is to provide a turbine blade on which the top coating is not subject to spallation. This aim is attained in that the surface layer of the protective MCrAlY coating consists, to a depth of 5 - 50 mu m, of a single-phase alloy in a large area spread evenly over the entire surface layer, where the single-phase alloy is obtained by remelting with a pulsed electron beam.

Description

The invention relates to a turbine blade according to the preamble of claim 1.

When operating a hot gas turbine, temperatures up to 900 ° C. are achieved at the surface of the turbine blades. At this high temperature, the main corrosion mechanism is caused by oxidation (diffusion of oxygen). Therefore, the blades are coated with a high temperature superalloy MCrAlY (M = metal base, eg Ni, Co).

The MCrAlY protective layer is generally applied by a plasma spray method. The alloy hardens in two phases. This creates an undesired base for forming an Al ₂ O ₃ protective layer on the surface. The formation of a uniform oxide layer on the surface of the two-phase alloy is prevented. The formed oxide protective layer has a tendency to spall.

R.Sivakumar, Princ.of Solidific.and Mat.Process
It is known from Vol. 2, pages 671-726, that this two-phase alloy can be converted into a single-phase alloy by a remelting method using a laser beam. The disadvantages of this method are on the one hand the small spatial area of the laser beam (at the required power density of 10 ⁵ to 10 ⁶ W / cm ² here) of less than 10 ⁻² cm ² , and on the other hand the laser radiation on the material Less penetration depth.

The ingress of spatially limited energy results in significant thermal stress, which is pronounced by longitudinal and transverse crack formation. Crack formation is the crush resistance of the oxide layer
swiderstand) and therefore the corrosion resistance.

Another consequence of the small beam diameter is that the scanning of the laser beam causes the formation of bulges at the surface and phase separation and recrystallization of the surface layer.

A rather long irradiation time of a few milliseconds for complete melting of a layer thickness of a few tens of μm results in a change in the original stoichiometry in the layer, ie a reduction in the proportion of light elements (Al, Y), which It floats due to convection, and thus is insufficient in the step of regenerating the oxide protective layer.

It is an object of the present invention to provide a turbine blade in which the protective layer has no tendency to shatter.

The object is achieved by the features of claim 1.

Claims 2 and 3 describe advantageous embodiments of the invention.

The present invention will be described in detail with reference to the embodiments shown in the following drawings. The drawing shows a cross section of a conventional two-phase MCrAlY-turbine blade protection layer (a) and the protection layer after the remelting step (b).

Melting the protective layer for a short time and cooling it very fast, i.e. fast so that no time for phase separation remains, results in a single-phase structure that is nanocrystalline or amorphous depending on the cooling rate,
This results in the formation of a uniform, uninterrupted oxide protective layer. Corrosion tests in air at 1000 ° C. for up to 10,000 hours have shown that a uniform, firmly adherent uninterrupted oxide protective layer is formed on the surface of the protective layer according to claim 1, In comparative tests not performed, these layers show a partially fractured, interrupted structure. These flaws in the oxide protective layer are healed by the incorporation of aluminum, but this step results in the depletion of the aluminum in the MCrAlY protective layer and thus a reduction in the pot life.

Another advantage of the protective layer for turbine blades is that the fine roughness of the surface resulting from the production is eliminated by a heat treatment step on the surface, which reduces the heat exchange between the gas and the surface, and thus the higher gas introduction temperature Is possible. Higher gas introduction temperatures result in increased efficiency.

The conditions for forming a uniform oxide protective layer on a uniform single phase alloy are provided. An oxide protective layer that withstands uniform crushing most effectively blocks oxygen ingress, and the new formation of the oxide protective layer delays Al depletion of the protective layer.

In order to produce a corrosion protection layer, a pulsed electron beam having a large radiation cross section is used. Radiation cross section is 25-10
It must be 0cm ^2. A cross section of 50-100 cm ² is optimal. The advantage of a pulsed electron beam is the large emission diameter, the large penetration depth of the electrons into the material, which can be easily adjusted by the energy of the electrons. High power densities of up to 3 × 10 ⁶ W / cm ² can be achieved uniformly on a 50 cm ² plane using a pulsed electron beam. This is a 10,000 times larger cross section than the laser beam. Due to the uniform power density distribution, there is no temperature gradient parallel to the surface in the molten layer and therefore no formation of lateral stress cracks. The formation of a so-called thermal zone at the radiating edge is not a problem because of the very short processing times and high cooling rates.

The depth of the melted layer is adjusted by the energy, pulse time and electron beam power density.

The cooling rate of the self-quenching (Selbstabschreckung) process is important for the formation of stress cracks perpendicular to the surface and for converting a two-phase alloy to a single-phase, amorphous to nanocrystalline structure. .

Cooling rates below 10 ⁵ K ゜ / s do not result in the desired phase formation.

Cooling rates higher than 10 ⁷ K ゜ / s cause thermal stress cracking.

The cooling rate in self-quenching can be affected by electron energy (which adjusts the melt depth), power density, and pulse time. Increasing the electron penetration depth (melting depth) and decreasing the power density result in lower cooling rates.

The electron beam parameters for producing the protective layer according to any one of claims 1 to 3 can have the following configurations.

Electron energy: 50 to 150 keV Output density: 5 × 10 ^{5 to} 3 × 10 ⁶ W / cm ² Pulse time: 10 to 60 μs JGSmeggil Mat. Sci. And Eng. 87 (1987) 261/65 and AMHuntz: Mat. Sci.and Eng. 87 (1987) 251/60
It is known from the page that alloying of the elements according to claim 2 has a positive effect on the fracture resistance, crack formation and high-temperature stability of the layer structure.

These alloys are coated with MCrAlY powder by plasma spraying. In this case, especially high-temperature metals (Ta, Re, M
o, W) melts only poorly due to its high melting point,
It generally condenses in its original powder form. This forms insoluble islands of high-temperature metal, which are only partially effective in this form. With the remelting method according to the invention, these metals dissolve together with the MCrAlY protective layer, and thus only their stabilizing effect can be exerted in the entire alloyed layer range.

The stabilizing action of the alloyed elements is only necessary for the layers close to the surface which are strongly exposed to corrosion, and therefore according to claim 3 the additional elements are applied to the surface by coating (eg PVD),
It is proposed to alloy by a remelting method. This has the economic advantage that a considerable portion of the often expensive elements to be processed can be saved.

──────────────────────────────────────────────────続 き Continued on the front page (72) Inventor Gerd Kraft Germany Karlsruhe Zazigweg 8 (72) Inventor Georg Müller Germany Karlsruhe les Singstrasse 45 (72) Inventor Gustav Schumacher Germany Karlsruhe Overfeldstraße 24 (56) References JP-A-5-78860 (JP, A) JP-A-62-103357 (JP, A) JP-A-61-204372 (JP, A) France Patent application publication 2400568 (FR, A1) West German patent application publication 3905347 (DE, A1) European patent application publication 168868 (EP, A1) (58) Fields investigated (Int. Cl. ⁷ , DB name) F01D 5 / 28 C23C 4/08 C23C 4/18

Claims (3)

(57) [Claims] 1. A turbine blade having a corrosion-resistant MCrAlY protective layer, wherein the surface layer of the MCrAlY protective layer comprises a large-area single-phase alloy uniformly spread over the entire surface layer to a depth of 5 to 50 μm. characterized in that the single-phase alloy is obtained by remelting a pulse electron beam having an emission cross-section of 25～100Cm ^2, the turbine blade. 2. The MCrAlY protective layer according to claim 1, wherein said corrosion-resistant MCrAlY protective layer comprises at least one component consisting of a strong oxide-forming agent such as La, Al and Ce and a high-temperature metal having a melting point higher than 2500 ° C. The turbine blade according to claim 1, wherein the turbine blade is uniformly distributed over the turbine blade. 3. The MCrAlY protective layer is uniformly coated with one or more components comprising a strong oxide forming agent such as La, Al, Ce and a high temperature metal having a melting point above 2500 ° C. as another thin layer; Claim 1 or 2 which is re-melted together with said layer
The described turbine blade.