Method for substrate stabilization of diffusion aluminide coated nickel-based superalloys
An article and method for stabilization of a nickel-based superalloy coated with a diffusion aluminide coating. The region below the aluminide coating is first carburized to form refractory carbides. The article is cleaned and masked as required so that regions that will not have an aluminide coating are not carburized. After placing the article into a furnace and heating in a non-oxidizing atmosphere to a carburizing temperature, a carburizing gas is introduced, and the near surface region is carburized to a depth of about 100 microns. Refractory carbides are formed in this region. When a diffusion aluminide coating is formed on the article, the refractory elements, being present as refractory carbides, are not available to form detrimental TCP phases.
1. A method for preparing a coated article, comprising the steps of:
providing a nickel-based superalloy substrate, the nickel-based superalloy substrate including at least one refractory element;
cleaning a surface of the nickel-based superalloy substrate by removing surface oxides;
placing the substrate article in a working zone of a furnace;
heating the substrate article to a carburizing temperature while maintaining a non-oxidizing atmosphere in the working zone of the furnace;
upon reaching the carburizing temperature, introducing into the working zone of the furnace a carburizing gas selected from the group consisting of alkynes, ethylene, propane and combinations thereof;
maintaining sufficient carburizing gas in the working zone of the furnace for a time and at a temperature to carburize a near surface region of the superalloy substrate to a maximum depth of about 100 microns;
then, stopping the flow of carburizing gas into the working zone of the furnace and substantially simultaneously introducing a non-reactive gas into the working zone of the furnace to cool the article to a preselected carburizing temperature; and thereafter
applying an aluminide coating to at least a portion of the surface cleaned by removing surface oxides.
2. The method of claim 1 wherein the step of providing a nickel-based superalloy article includes providing a turbine airfoil.
3. The method of claim 2 wherein the step of providing a turbine airfoil further includes providing an airfoil selected from the group consisting of a blade and a vane.
4. The method of claim 1 further including an additional step of masking a preselected portion of the substrate surface, thereby leaving the remainder of the substrate surface exposed prior to placing the article in the furnace, and wherein the step of applying an aluminide coating includes applying a diffusion aluminide coating over the carburized portion of the substrate.
5. The method of claim 1 wherein the step of cleaning the surface of the substrate further includes grit blasting the substrate surface at a preselected pressure using grit of a preselected size to remove surface oxides.
6. The method of claim 5 wherein the preselected pressure is about 20-90 psi and the preselected grit size is 80-600 mesh grit.
7. The method of claim 1 wherein the step of heating the substrate article while maintaining a non-oxidizing atmosphere in the working zone of the furnace includes maintaining a reducing atmosphere and wherein a reducing gas that provides the reducing atmosphere is hydrogen.
8. The method of claim 1 wherein the step of heating the substrate article while maintaining a non-oxidizing atmosphere in the working zone of the furnace includes maintaining an inert atmosphere and wherein an inert gas that provides the inert atmosphere is selected from the group consisting of argon, helium and combinations thereof.
9. The method of claim 1 wherein the step of heating the substrate article while maintaining the non-oxidizing atmosphere in the working zone of the furnace includes maintaining the non-oxidizing atmosphere at a partial pressure of about 0.0005-10 Torr.
10. The method of claim 9 wherein the step of heating the substrate article while maintaining the non-oxidizing atmosphere in the working zone of the furnace includes maintaining the non-oxidizing atmosphere at a partial pressure of about 0.05-1.0 Torr.
11. The method of claim 1 wherein the step of heating the substrate article to the carburizing temperature includes heating the article to a temperature in the range of about 1800°-2250° F.
12. The method of claim 11 wherein the step of heating the substrate article to the carburizing temperature includes heating the article to a temperature in the range of 1900°-2050° F.
13. The method of claim 1 wherein the step of maintaining sufficient carburizing gas in the working zone of the furnace is accomplished by a method selected from the group consisting of the pulse method and the continuous method.
14. The method of claim 1 wherein the step of introducing into a carburizing gas includes introducing acetylene.
15. The method of claim 13 wherein the step of maintaining sufficient carburizing gas in the working zone of the furnace is accomplished while avoiding the formation of soot on the substrate surface.