NO139970B - COMPOSITE OBJECT OF A SUPER-ALLOY SUPPORT AND A COATING ALLOY BOUND TO THE SUPER-ALLOY SUPPLY - Google Patents

Description

The invention relates to a composite object of a superalloy substrate coated with a coating alloy resistant to oxidation and hot corrosion consisting of nickel, chromium and silicon and which will hereafter be referred to as AMB-2.

Resistance to hot corrosion is necessary for applications in turbines where natural gas or uncontaminated light distillates are burned, or in polluted environments including burnt diesel oil, heavy distillate oils or residual oils.

A number of protective alloy coatings for superalloy substrates are described in the prior art. These coatings include the usual aluminide coatings which are technically limited in that the coating thickness is limited and lead to a premature breakdown of the coatings and which also do not have the very good resistance to hot corrosion as the alloy coating applied to the coated alloy article according to the invention, especially when used under hot corrosive conditions. The coating alloy used according to the invention is also less expensive to produce than the known alloys. A 'repair' in the field of components taken out of service can be carried out easily and more economically in selected areas

new can be coated without the need to mask or protect areas that do not need to be coated again,

as is the case when applying an aluminide gasket coating or be-

layer deposited from the vapor phase using an electron beam.

When assessing coating alloys for superalloy substrates, it was discovered that the alloy must have the following properties: 1) It must be resistant to oxidation and hot corrosion within the temperature range 760-1093°C.

2) It must melt, wet and flow smoothly at a temperature

below the initial melting point of the superalloy substrate.

Ideally, the vacuum brazing time/temperature cycle used to apply the coating alloy to the substrate should be compatible with the normal heat treatment cycle for the substrate. 3) It must be metallurgically stable and compatible with the base alloy.

The invention relates to a composite object which is resistant to oxidation and hot corrosion and consists of a superalloy substrate and a coating alloy bound to this, and the composite object is characterized by the fact that the coating alloy consists of 40-65% by weight chromium, 5-12% by weight silicon and the rest nickel and has a microstructure containing a mixture of a ^<*->Ni matrix, precipitated <3^-Cr particles and a Ni:Si eutectic.

A preferred coating alloy consists of approx. 45% by weight chrome, approx. 10 wt% silicon and the rest nickel.

It is known that large amounts of Cr are very effective in preventing hot corrosion caused by Na- and S-containing atmospheres within the temperature range 760-982°C. At these temperatures, more than approx. 36% Cr to form <£-Cr sweep. However, it should be noted that the chromium content should not be too high as the coating may thereby become brittle within the aforementioned temperature range. This embrittlement is caused by the formation of excessive amounts of hard, space-centered, cubic «(,-Cr refinement.

Similarly, silicon is beneficial for achieving resistance to oxidation and hot corrosion because it leads to the formation of SiO 2 . Silicon is used to regulate the coating's melting and solidification temperature, as the eutectic temperature for the pure binary Ni-Cr system is 1343°C, which is too high for most nickel-based superalloy substrates.

The coating can be applied to the substrate using different methods, including vacuum wrap soldering which is a well-established industrial method. However, other common methods of applying the coating alloy to the superalloy substrate can be used, such as spraying a slurry, an aerosol or a plasma with heat treatment and using transfer tape methods. However, certain methods should be avoided, such as vapor phase deposition which leads to a coating with a microstructure that is oriented vertically on the surface of the substrate and thereby represents potential short-circuit diffusion paths, e.g. grain boundaries, and growth defects for the introduction of corrosive materials, such as sulfur and oxygen, into the substrate. The alloy structure applied by means of vacuum brazing is unoriented after renewed solidification so that the mentioned potential failure is avoided. As the coating alloy is applied in a liquid state and not from a vapor phase, a greater separation of the coating elements will occur. When using the described coating alloy, this fact is taken advantage of, as a high chromium content leads to precipitated fl^-Cr particles dispersed in a solid solution of a "V\-nickel base material containing a large amount of chromium. The eutectic phase Ni:Si with a lower melting point is likewise well dispersed in the coating when it solidifies.

Examples

A plating alloy was prepared using an alloy comprising 45% chromium, 10% silicon and the balance nickel. Other mixtures may be used within the above range. The substrate was treated by means of mechanical grinding or chemical cleaning followed by electro-application of a 0.051-0.0254 mm thick nickel layer. The alloy used was in the form of a powder and was transferred to a brazing transfer belt. The tape consisted of a powder with particles from -200 to +325 mesh held together with approx. 5% of an organic binder on a plastic carrier sheet. A piece of the desired shape to fit the substrate was cut from the transfer tape. The plastic carrier sheet was removed and the tape applied to the substrate.

Additional liquid brazing cement may be required for the tape to adhere to the site. The coated part is then subjected to a vacuum brazing cycle. This is regulated so that the binder is emitted in the form of a gas at a temperature of 371-538°C to reduce the contamination of the coating and the substrate to a minimum. The optimal vacuum brazing cycle consists of the alloy being heated to a temperature of approx. 1135°C for approx. 5 minutes, followed by a cooling with argon gas. A finishing treatment is not usually necessary as the thus coated surface has a surface finish within the range 889-1524 nm (root-mean-square). The part can be subjected to a final heat treatment to develop the mechanical properties of the substrate. This step should be carried out at a temperature well below the alloy application temperature to prevent a renewed melting of the coating or an excessive diffusion of critical elements across the interface between the coating and the substrate. The coating cycle must therefore be covered by the suitable heat treatment sequence for the particular substrate alloy used.

In the solidified state, the microstructure of the alloy contains a mixture of -Ni matrix, precipitated fif -Cr particles and a Ni:Si eutectic. The exact composition and morphology of these phases are dependent both on the alloy powder and the substrate alloy's starting composition and also on the subsequent coating application and heat treatment cycles. The alloy's corrosion resistance is due to the high Cr content (i.e. 45%) in the coating, and more specifically (the L -Cr particles and the ^-Ni base mass with a high Cr content which make up a very significant proportion of the coated object. When the coating is applied in liquid form and brought to

solidify again, elements from the substrate will easily be incorporated into the coating. The application cycle (time and temperature) can therefore be used to regulate the coating's morphology and composition to a certain extent. Nickel-based superalloys have been coated within the temperature range 1126-1166°C with a time within this temperature range of 2-20 minutes. Lower application temperatures are not preferred due to the melting characteristics of AMB-2. Higher application temperatures may be used depending on the heating and cooling rates, the equipment used and other considerations. Special superalloy substrates may also require higher temperatures. However, the optimal parameters for the described alloys are 5 minutes at 1135°C.

The higher the temperature, the shorter the time will usually be

to prevent excessive movement, reaction and diffusion into the substrate. High temperatures and/or longer application times promote the formation of large Qf -Cr particles and a smaller amount of Ni:Si eutectic. 0q a stronger diffusion between the coatings

and the substrate. Lower temperatures and times lead to the formation of smaller, better dispersed {£-Cr particles and a smaller amount of Ni:Si eutectic and a reduced diffusion between the coating and the substrate. A distinctive feature of the described applied alloy coating in the state in which it exists as applied is that it does not have a complex "diffusion zone" between the coating and the substrate. In contrast to this, ordinary aluminide coatings are characterized by a finger-like diffusion zone containing brittle intermetallic compounds, such as (f and carbides.

The compositions for the superalloy substrates on which the coating is applied are given in Table A below.

The non-oriented structure of the described alloy is due to the species

of the melting process and the renewed solidification process. A separation of the elements and the resulting precipitations depend on the composition, the added amount of heat during the application and the cooling conditions. Vapor-phase deposited line-of-sight coatings, such as the MCrAlY coatings deposited by electron beam evaporation, typically grow perpendicular to the substrate surface. Grain growth therefore occurs vertically in relation to the substrate, and this leads to growth defects also being oriented. Growth defects, when they occur in the described alloy, are unoriented solidification defects.

As mentioned above, the described alloy coating for a superalloy substrate has a superior resistance to hot corrosion compared to known coated superalloys. Investigations of the resistance to oxidation and hot corrosion have been carried out under simulated gas turbine conditions in a small combustor. A regulated atmosphere was produced by burning doped diesel oil containing 1% S to which artificially produced sea salt was mixed to give 8 parts Na per parts per million (ppm) in the combustion products.

The device was maintained at 871°C, the air:fuel ratio was

60:1, and the gas velocity was 21 m/s. The test pieces were removed and blown with air every 50 hours until they had cooled to room temperature, to simulate the run-down of a gas turbine and to promote scaling of oxide and/or coating under severe thermal cycling conditions. This is the . most powerful test condition used to simulate hot corrosion conditions occurring in operation.

In a first attempt, AMB-2 was applied to IN-738 using the methods described above and compared to commercially available aluminide coatings applied to IN-738. The results were obtained by dividing the test pieces into sections and by metallographically at a magnification of 100 times determining the maximum depth of corrosion penetration through the coating and substrate, the average coating surface loss and an approximate calculation of the percentage coating area that remained . The results presented in Table I clearly show that AMB-2 is superior to conventional aluminide coatings.

It is important to note that the technical process used for applying ordinary aluminide coatings and which is known as packing concentration, is subject to technical and economic limitations which limit the aluminide thickness to approx. 76 / Usm and slightly thinner on Co-based superalloys. As gasket cementation is basically a process for deposition from the vapor phase, the applied thickness is time-dependent. However, AMB-2 can be applied with a thickness of up to approx. 254 yu^m without changing the time/temperature vacuum application cycle. These results in Table 1 show that the investigated aluminide coatings were essentially completely penetrated after only 600-1000 hours and that practically no coating remained. In several cases, the IN-738 substrate was out-

set for a significant corrosion due to the destruction of the coating.

The results in table I also show that a significant proportion

of AMB-2 was back after 2000 hours of testing. This is partly due to the fact that AMB-2 can be applied with a thickness of up to 254 µm, as mentioned above, as no change is necessary to the method used or to the time/temperature parameters used for the application of this alloy. AMB-2 therefore provides both a more corrosion-resistant alloy composition and an increased coating thickness, both of which lead to a longer service life.

Another series of tests carried out in a burner device was carried out for the alloy Rene-77 with an applied coating of AMB-2 (see Table II) and compared with conventional aluminide coatings. Part of the experiments were carried out in an undoped natural gas atmosphere, which provides a normal oxidizing environment. Aluminide coatings usually provide excellent resistance in this case, but they are subject to severe attack in the presence of atmospheres contaminated with S and Na. This result, which is reproduced in Table II, confirms the above statement that AMB-2 is at least as protective as aluminide coatings in undoped environments and superior in polluted environments.

Another measure of the coating's properties is its impact

on the mechanical properties of the substrate (see table III). Two different effects are possible: (1) the coating alloy metallurgically reacts with the substrate to form undesirable phases that embrittle the structure, or (2) the heat treatment required to apply and stabilize the coating is incompatible with the heat treatment of the substrate and thereby degrades its mechanical properties.

According to Table III, standard 6.4 mm diameter cast test bars of Rene-77 and IN-738 were brazed with AMB-2, and the tensile strength and fracture strength were examined. The results are in Table III compared with the results for rods of these two alloys coated with aluminide. Each coating/alloy combination was subjected to the full heat treatment previously found to be optimal for this system. The results show that the tensile strength properties at room temperature for both IN-738 and Rene-77 were least affected by AMB-2. The conventional yield strength is 10-15% higher than for the aluminide coatings, and the ductility was only slightly less. The latter is partly due to the greater thickness of AMB-2.

Claims (3)

1. Composite object that is resistant to oxidation and hot corrosion and consists of a superalloy substrate and a coating alloy bound to this, characterized in that the coating alloy consists of 40-65% by weight chromium, 5-12% by weight silicon and the rest nickel and has a microstructure which contains a mixture of a ^"-Ni matrix, precipitated a-Cr particles and a Ni:Si eutectic. 2. Item according to claim 1, characterized in that the coating alloy consists of approx. 45% by weight chrome, approx. 10% by weight silicon and the rest nickel. 3. Item according to claim 1 or 2, characterized in that the thickness of the applied alloy coating is up to 254 µm.