GB1579349A - Components resistant to corrosion at high temperatures - Google Patents

Abstract

In order to protect components of gas turbines against high-temperature corrosion, especially sulphidation, without substantially reducing the mechanical properties of the base material as a result of excessive temperatures during heat treatments, the components, which consist of austenitic base material on the basis of nickel, cobalt or iron, are provided with a protective layer which consists of two components. One of these components is a material resistant to said corrosions, while the second component is a metal alloy which melts at a lower temperature than the first component and whose melting range is between 950 and 1300 DEG C. In the novel protective layer, the hard particles of the first component are uniformly dispersed in the softer material of the second component and are anchored to the base material so as to adhere strongly thereto. The preparation of the protective layer involves melting the second component at least partially by heat treatment; the maximum temperature of the heat treatment in the process is at most 50 DEG C above the standard solution annealing temperature of the base material. Consequently, temperatures which are unacceptably high for the base material are avoided during the heat treatment.

Description

(54) COMPONENTS RESISTANT TO CORROSION AT HIGH TEMPERATURES (71) We, SULZER BROTHERS LIMITED, a Company organised under the laws of Switzerland, of Winterthur, Switzerland, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement:- This invention relates to metal components, more particularly but not exclusively for gas turbines, which are resistant to corrosion at high temperatures, more particularly to sulphidation, the components being given a thm protective layer at least over some surface regions of their metal substrate material, which is a nickel-based, cobalt-based or iron-based austenitic material.

In recent years there have been continuous increases in the temperatures of gasturbine components, with the result that it is increasingly demanded that the materials, particularly blade materials, must be both proof against high temperatures and highly resistant to corrosion. Developments in alloys have also shown that it is usually impossible to satisfy the requirement for high strength and simultaneously satisfy the requirement for high corrosion resistance.

Accordingly, it has been proposed to coat a material having above-average mechanical properties but insufficient resistance to corrosion with a protective layer of another material.

The mechanical properties of a gas turbine blade are mainly contributed by the substrate material, whereas the function of a suitable surface layer is to protect the substrate material from corrosion.

In modern gas turbines, where the temperature of the front blade stages is up to about 900"C, the prevailing corrosion mechanism is mainly "alkaline sulphate corrosion" or "sulphidation". The actual corrosive media are sulphates and other sulphur compounds which, at the operating temperature, form at least partly liquid deposits on the turbine blades and chemically corrode them. The commonest and most well-known sulphidation agent is sodium sulphate, but a number of other compounds, particularly mixtures, have the same corrosive effect.

Sodium occurs either as a constituent of fuel ash or in the combustion air in the turbine, particularly in installations near the sea or in salty deserts. In the flame, these sodium compounds react with sulphur, which is present in practically all fossil fuels or is in the air in the form of sulphur oxides, particularly in industrial districts, and form corrosive sodium sulphate. Usually the sulphur content of the fuel cannot economically be reduced to an extent preventing the formation of corrosive sulphates. There is therefore not much point in limiting the fuel sulphur content.

Even "clean" fuels such as diesel oil and other distillates contain some corrosive ash constituents; in addition some fuels have a sodium content above the permissible limits, even if only for short periods.

In addition to corrosion resistance, protective layers for gas turbine blades must satisfy a number of other requirements, i.e.

resistance to erosion, resistance to impact, thermal stability, mechanical stability, adhesive strength, resistance to change of temperature, cheapness, little or no negative effect on the properties of the substrate material and little or no negative effect on blade operation.

Proposals have recently been made (see German Offenlegungsschrift 24 18 607) to satisfy these requirements by using nickelbased protective layers containing chromium and silicon. Such layers, however, cannot always provide the required corrosion resistance, and they are not suitable for all substrate materials in question.

There are, however, a number of other known substances and complex alloys having excellent resistance to corrosion, but they have the disadvantage that they cannot be applied as thin, uniform, dense layers to components such as turbine blades without great expense and difficulty. The reasons are as follows. Apart from expensive evaporation processes, the only methods of preparing protective layers of these highly corrosion-resistant substances or alloys are slurry methods (in which the powdered coating substances mixed with binding agents are applied to the substrate material and subsequently sintered, with decomposition of the binding agent) and thermal spraying methods, i.e. flame or plasma spraying.In order, however, to satisfy the aforementioned requirements in optimum manner, slurry or sprayed layers must be subsequently heat-treated, whereby the typical porosity of such layers is substantially reduced by sintering and the weak mechanical bond is replaced by a strong diffusion bond to the substrate material. In the case of most materials or complex alloys which are resistant to high-temperature corrosion, these effects can be obtained only at high temperatures over 1200"C, owing to their high melting-points or their strong tendency to form diffusion-inhibiting oxide skins.

Such heat treatment, however, impairs the mechanical properties of the substrate materials; more particularly it unacceptably reduces the long-time creep strength of the substrate materials.

An object of the invention is to reduce or obviate the aforementioned disadvantages and provide a highly corrosion-resistant protective layer in the form of a thin, uniform, dense coating without substantially impairing the mechanical properties of the substrate material during manufacture.

Accordingly the present invention provides a metal component which is resistant to corrosion at high temperatures, the component comprising a substrate of nickelbased, cobalt-based or iron-based austenitic material and, over at least part of the surface of the substrate, a sintered protective layer which is not more than 0.8 mm thick and comprises two constituents, the first constituent comprising an intermetallic compound or an alloy containing an intermetallic compound having high resistance to high temperature corrosion and the second constituent being a metal alloy having a melting range which is lower than that of the first constituent but is between 950 and 1300 C and being present in the proportion of 2-30% Volume of the layer, the layer having been heat treated with the exclusion of oxygen so that the second constituent at least partly melts at a temperature not exceeding 50"C above the standard solution annealing temperature of the substrate.

The "standard solution annealing temperature" means the annealing temperature given by the manufacturer of the substrate material and recommended for optimum mechanical properties.

In manufacturing the component, a high-melting and corrosion-resistant material is mixed with a lower-melting alloy and is first mechanically secured to the substrate to be protected, preferably by the slurry pro- cess or by thermal spraying. In the subsequent heat-treatment, the constituents of the layer are brought to a temperature in which the second or low-melting constituent is at least partly liquefied at relatively low temperatures which do not affect the mechanical properties of the substrate and fills the pores in the layer. Owing to the high rate of diffusion in melts, the higher-melting and corrosion-resistant first constituent is partly dissolved in the liquid portion of the second constituent, thus homogenizing the layer, whereupon the melting point of the liquid part rises and the entire layer solidifies.During the entire process, considerable diffusion occurs between the layer and the substrate resulting in the desired adhesion of the layer to the substrate. After cooling, the blade or other component will be found to be coated with a smooth, nonporous, firmly-secured protective layer, the highly corrosion-resistant material being uniformly distributed and embedded in the second constituent. Consequently, interaction between the two constituents results in an anti-corrosion layer having the desirable properties previously mentioned, without the substrate being subjected to unduly high temperatures during heat treatment.

The following are examples of suitable intermetallic compounds for the first constituent, e.g. chromium or zirconium silicides and nickel or zirconium aluminides, containing at least one of the following elements: chromium, silicon, aluminium, yttrium, zirconium and rear earths. Preferably, the first constituent has the following composition by weight percent: Cr 10 to 50 SiOto30 Al0to20 TiOto20 With at least one of: Rare earths: 0 to 10 Zr0 to 10 Balance Ni, Fe or Co The second constituent, which makes up 2-30% by volume of the coating material, may consist of nickel, cobalt or iron-based alloys containing at least one of boron, carbon and silicon as the alloying element.Preferably, the composition of the second component is by weight percent: Boron 0 to 5 Carbon 0to2 Silicon 0to15 Chromium 0 to 30 Balance Iron, nickel or cobalt.

The composition, structure and quality of a coating on a workpiece can be judged and tested by metallurgical methods, such as quantitative analysis and microsection and structural investigations.

The invention will now be described in greater detail with respect to examples, which describe the manufacture of coated gas turbine blades.

Example 1: In this Example those parts of a turbine blade which in use, will be subjected to hot gases was coated by plasma spraying.

The substrate material of the blade was the nickel super-alloy IN 738 LC, which has the following composition in wt.%: C 0.17, Fe 0.5, Ca 8.5, Ta 1.75, Mo 1.75, Ti 3.4, Al 3.4, W 2.6, balance Ni. A mixture of (wt.%) 75% chromium silicide (Cr3Si containing 85% Cr) and 25% of a nickel-based alloy having the composition Cr 7.5, Si 4, B 1.5, Fe 1.5, balance nickel, having a layer thickness of between 0.2 and 0.3 mm was sprayed on to the substrate using a conventional plasma spraying torch, the mixture being in powder form with a particle size of 50-100/ m and the rate of delivery of mixture being 35 g per minute.

The plasma gas was argon+ 10 vol.% hydrogen, the rate of flow being approximately 3.2 Nm3/h. The carrier gas was likewise argon, the rate of flow being approximately 0.3 Nm3/h. The plasma current was 300 amps and the applied voltage was 60 V.

The layer was produced as follows.

First, a blade made of the substrate material was cleaned and degreased, using chemical and/or mechanical agents; next, all parts of the blade which were not to be coated (e.g. cooling ducts, if any, the foot of the blade and the platform) were masked.

Sheet-metal masks were used for this purpose; alternatively these regions could be masked with graphite masks or with plastics, e.g. commercial silicone rubber, which harden at room temperature. In the latter case, of course, the hardening of the plastics can be accelerated by raising the temperature.

Next, the regions to be coated were mechanically roughened by sand-blasting using electrocorundum sand having a particle size of 0.8 - 1.2 mm. at a blast pressure of 6-7 atm. gauge.

If the masks had been made of non-heatresist ant material, they would have now been removed and replaced by a new mask, e.g. a special dye, to prevent the subsequently-applied sprayed layer from sticking to surface regions which must not be coated.

After the actual plasma spraying, using the aforementioned torch device and the aforementioned operating conditions, and after cooling, the mask was removed, thus also removing the sprayed layer over those parts which were not to be coated.

The subsequent heat treatment was carried out in a high-vacuum annealing furnace in which a pressure of oft3.10~4 Torr was maintained. After this pressure was reached, the furnace was heated linearly from room temperature to 11200C. At this temperature the second constituent at least partly melted, so that the pores closed and the first constituent was embedded and bonded, e.g. by incipient dissolving, in the coated surface. The temperature was maintained for about 2 hours, after which the furnace heating was switched off and the coated component was rapidly cooled by flushing with argon.

During heat treatment the coating material, which had been only mechanically bonded to the base material by the plasma spraying diffused into the substrate material. The result was a protective layer resistant to high-temperature corrosion and consisting of a chromium-rich nickel phase with inclusions of chromium silicides which slowly dissolve and make up the chromium and silicon lost from the protective layer surface owing to corrosion.

The coated blade surface was subsequently smoothed by slurry blasting (a smoothing process for reducing roughness using a mixture of air and water containing sand particles, the process being similar to sand-blasting). Alternatively, smoothing could have been by grinding.

Finally, the coated blade was agehardened by heating. This second heat treatment was to improve the mechanical properties of the substrate material and, in the present example, consisted of 24 hours of tempering at 850 0C and subsequent cooling in air. In this final treatment, neither the heating-up nor the cooling rates are very important.

Example 2 The steps for preparing the coating and the steps after coating, including the heat treatment, were similar to the steps described in Example 1, insofar as they were necessary; in Example 2, for example, there was no need for masking with a heatresist ant special dye.

In contrast to the preceding Example, the substrate material was a nickel-based alloy having the following composition in wt.% C 0.08, Cr 19, Co 18, Mo 4, Ti 2.9, Al 2.9, balance nickel.

In this Example, the coating was applied by immersing the aerofoil part of the blade in a suspension consisting of a mixture of the following powders (in wt.%): 12% of a first nickel-based alloy having the composition Cr 16, Si 4.15, B 3, Fe 4, balance nickel, and 88% of a second nickel-based alloy having the composition: Cr 49.4, Mo 2.5, Ti 1.7, Al 0.95, Zr 0.26, balance nickel.

The powder mixture was suspended in a solution of paraffin in chloroform.

After the suspension adhering to the base material had dried, the previouslymentioned heat treatment was performed, producing a corrosion-resistant coating substantially consisting of the second nickel alloy bonded by a first Ni-Cr-Si alloy.

WHAT WE CLAIM IS: 1. A metal component which is resistant to corrosion at high temperatures, the component comprising a substrate of nickelbased, cobalt-based or iron-based austenitic material and, over at least part of the surface of the substrate, a sintered protective layer which is not more than 0.8 mm thick and comprises two constituents, the first constituent comprising an intermetallic compound or an alloy containing an intermetallic compound having high resistance to high temperature corrosion and the second constituent being a metal alloy having a melting range which is lower than that of the first constituent but is between 950 and 1300"C and being present in the proportion of 2-30% Volume of the layer, the layer having been heat treated with the exclusion of oxygen so that the second constituent at least partly melts, at a temperature not exceeding 50"C above the standard solution annealing temperature of the substrate.

2. A component as claimed in Claim 1 in which the first constituent comprises an intermetallic compound containing at least one of the following elements: chromium, silicon, aluminium, yttrium, zirconium and rare earths.

3. A component as claimed in Claim 1 in which the first constituent has the following composition by weight percent: Cr 10 to 50 with at least one of Rare earths: 0 to 10 SiOto30 Zr O to 10 Al 0 to 20 Balance Ni, Fe or Co TiOto20 4. A component as claimed in any of the preceding Claims in which second constituent is a nickel-based, cobalt-based or iron-based alloy containing at least one of boron, carbon and silicon as the alloying elements.

5. A component as claimed in Claim 4 in which the composition of the second component is by weight percent: Boron 0 to 5 Carbon 0 to 2 Silicon 0 to 15 Chromium 0 to 30 Balance Iron, nickel or cobalt.

6. A component as claimed in any of the preceding Claims, the component being a gas turbine blade.

7. A method of manufacturing a gas turbine blade substantially as described herein with reference to either of the foregoing Examples.

8. A gas turbine blade when made by a method as claimed in Claim 7.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (8)

**WARNING** start of CLMS field may overlap end of DESC **. 88% of a second nickel-based alloy having the composition: Cr 49.4, Mo 2.5, Ti 1.7, Al 0.95, Zr 0.26, balance nickel. The powder mixture was suspended in a solution of paraffin in chloroform. After the suspension adhering to the base material had dried, the previouslymentioned heat treatment was performed, producing a corrosion-resistant coating substantially consisting of the second nickel alloy bonded by a first Ni-Cr-Si alloy. WHAT WE CLAIM IS:

1. A metal component which is resistant to corrosion at high temperatures, the component comprising a substrate of nickelbased, cobalt-based or iron-based austenitic material and, over at least part of the surface of the substrate, a sintered protective layer which is not more than 0.8 mm thick and comprises two constituents, the first constituent comprising an intermetallic compound or an alloy containing an intermetallic compound having high resistance to high temperature corrosion and the second constituent being a metal alloy having a melting range which is lower than that of the first constituent but is between 950 and 1300"C and being present in the proportion of 2-30% Volume of the layer, the layer having been heat treated with the exclusion of oxygen so that the second constituent at least partly melts, at a temperature not exceeding 50"C above the standard solution annealing temperature of the substrate.

2. A component as claimed in Claim 1 in which the first constituent comprises an intermetallic compound containing at least one of the following elements: chromium, silicon, aluminium, yttrium, zirconium and rare earths.

3. A component as claimed in Claim 1 in which the first constituent has the following composition by weight percent: Cr 10 to 50 with at least one of Rare earths: 0 to 10 SiOto30 Zr O to 10 Al 0 to 20 Balance Ni, Fe or Co TiOto20

4. A component as claimed in any of the preceding Claims in which second constituent is a nickel-based, cobalt-based or iron-based alloy containing at least one of boron, carbon and silicon as the alloying elements.

5. A component as claimed in Claim 4 in which the composition of the second component is by weight percent: Boron 0 to 5 Carbon 0 to 2 Silicon 0 to 15 Chromium 0 to 30 Balance Iron, nickel or cobalt.

6. A component as claimed in any of the preceding Claims, the component being a gas turbine blade.

7. A method of manufacturing a gas turbine blade substantially as described herein with reference to either of the foregoing Examples.

8. A gas turbine blade when made by a method as claimed in Claim 7.