Strip process for superalloys

Abstract

A process for forming a coated substrate comprises providing a nickel base alloy substrate, depositing a chromium coating onto the nickel base alloy substrate and diffusing chromium from said coating into the substrate, applying a MCrAlY coating onto the nickel base alloy substrate and heat treating the substrate with the deposited chromium and the MCrAlY coating so that chromium diffuses into an outer region of the substrate. Further, in accordance with the present invention, a strip process for removing a coating from a substrate broadly comprises the steps of providing a nickel base alloy substrate having chromium diffused into an outer region and a MCrAlY coating deposited over the substrate with the diffused chromium and removing the MCrAlY coating by immersing the nickel base alloy substrate in an acid solution containing a sulfuric acid-hydrochloric acid mixture in water.

1 . A process for forming a coated substrate comprising the steps of: 
 providing a nickel base alloy substrate;    depositing a chromium coating onto said nickel base alloy substrate;    diffusing chromium in said coating into an outer region of said nickel base alloy substrate; and    applying a MCrAlY coating onto said nickel base alloy substrate having said deposited chromium coating.    
   
   
       2 . The process according to  claim 1 , further comprising heat treating said substrate after said MCrAlY coating step.  
   
   
       3 . The process according to  claim 2 , wherein said heat treating step comprises heating said substrate to a temperature in the range of about 1950 to 2000 degrees Fahrenheit for a time period in the range of about 1 to 5 hours.  
   
   
       4 . The process according to  claim 3 , wherein said heat treating step comprises performing said heat treating step in an inert gas atmosphere at a partial pressure of at least about 1000μ.  
   
   
       5 . The process according to  claim 3 , wherein said heat treating step comprises performing said heat treating step in an argon atmosphere at a partial pressure in the range of from about 1000 to 5000μ.  
   
   
       6 . The process according to  claim 1 , wherein said nickel base alloy has an outer surface and said diffusing step comprises diffusing said chromium into said substrate to a depth in the range of from 0.2 to 1.5 mils from said outer surface.  
   
   
       7 . The process according to  claim 1 , wherein said nickel base alloy has an outer surface and said diffusing step comprises diffusing said chromium into said substrate to a depth in the range of from 0.2 to 0.8 mils from said outer surface.  
   
   
       8 . The process according to  claim 1 , wherein said depositing and diffusing step comprises depositing said chromium using a gas phase deposition process.  
   
   
       9 . The process according to  claim 8 , wherein said chromium depositing step comprises providing a chromium source and an activator, suspending the substrate over said source and said activator, and heating said part, said source, and said activator to a temperature in the range of from about 1700 to 2150 degrees Fahrenheit for a time period in the range of from about 5 to 20 hours.  
   
   
       10 . The process according to  claim 9 , wherein said activator providing step comprises providing an activator containing a halide selected from the group consisting of a chloride, a fluoride, iodide, or bromide.  
   
   
       11 . The process according to  claim 9 , wherein said activator providing step comprises providing less than 20 grams of an ammonium chloride activator.  
   
   
       12 . The process according to  claim 1 , further comprising heat treating said substrate after said MCrAlY coating applying step and maintaining said chromium level in said outer region at a level greater than 20 wt %.  
   
   
       13 . The process according to  claim 12 , wherein said chromium level maintaining step comprises maintaining said chromium level in the range of from about 20 to 30 wt %.  
   
   
       14 . The process according to  claim 1 , wherein said nickel-based alloy substrate providing step comprises providing a substrate formed from a nickel-based alloy having a high volume fractions of γ/γ′ eutectic phase.  
   
   
       15 . A strip process for removing a coating from a substrate comprising the steps of: 
 providing a nickel base alloy substrate having chromium diffused into an outer region and a MCrAlY coating deposited over said substrate with said diffused chromium; and    removing said MCrAlY coating by immersing said nickel base alloy substrate in an acid solution containing a sulfuric acid-hydrochloric acid mixture in water.    
   
   
       16 . The strip process according to  claim 15 , wherein said immersing step comprises maintaining said bath at a temperature in the range of from about 120 to 180° F. and maintaining said substrate with said coating being stripped in said solution for a time period less than about 1 hour.  
   
   
       17 . The strip process according to  claim 16 , wherein said immersing step comprises immersing said substrate in a solution containing from about 20 to 30 vol % sulfuric acid and from about 3.0 to 8.0 vol % hydrochloric acid.  
   
   
       18 . The strip process according to  claim 16 , wherein said immersing step comprises immersing said substrate in a solution containing from about 23 to 27 vol % sulfuric acid and from about 4.0 to 6.0 vol % hydrochloric acid.  
   
   
       19 . A turbine engine component comprising: 
 a nickel based alloy substrate having an outer region with diffused chromium;    a chromizing coating over portions of said substrate; and    a MCrAlY coating over said chromizing coating.    
   
   
       20 . The turbine engine component according to  claim 19 , wherein said substrate is formed from a nickel based alloy having high volume fractions of a γ/γ′ eutectic phase.  
   
   
       21 . The turbine engine component according to  claim 19 , wherein said chromium in said outer region is at a level greater than about 20 wt %.  
   
   
       22 . The turbine engine component according to  claim 19 , wherein said chromium in said outer region is at a level in the range of from about 20 to 30 wt %.  
   
   
       23 . The turbine engine component according to  claim 19 , wherein said chromium is diffused into said substrate to a depth in the range of from 0.2 to 1.5 mils from an outer surface of said substrate.  
   
   
       24 . The turbine engine component according to  claim 19 , wherein said chromium is diffused into said substrate to a depth in the range of from 0.2 to 0.8 mils from an outer surface of said substrate.  
   
   
       25 . The turbine engine component according to  claim 19 , wherein said MCrAlY coating contains from about 8 to 15 wt % aluminum.  
   
   
       26 . A solution from stripping a coating from a nickel based alloy substrate, said solution comprising a hydrochloric acid and sulfuric acid mixture in water.  
   
   
       27 . The solution according to  claim 26 , wherein said solution contains from about 20 to 30 vol % sulfuric acid and from about 3.0 to 8.0 vol % hydrochloric acid.  
   
   
       28 . The solution according to  claim 26 , wherein said solution contains from about 23 to 27 vol % sulfuric acid and from about 4.0 to 6.0 vol % hydrochloric acid.  
   
   
       29 . A strip process for removing a coating from a substrate comprising the steps of: 
 providing a nickel base alloy substrate having a MCrAlY coating deposited over the substrate; and    removing said MCrAlY coating by immersing said nickel base alloy substrate in an acid solution containing a sulfuric acid hydrochloric acid mixture in water.    
   
   
       30 . The strip process according to  claim 29 , wherein said immersing step comprises maintaining said bath at a temperature in the range of from about 120 to 180° F. and maintaining said substrate with said coating being stripped in said solution for a time period less than about 1 hour.  
   
   
       31 . The strip process according to  claim 29 , wherein said immersing step comprises immersing said substrate in a solution containing from about 20 to 30 vol % sulfuric acid and from about 3.0 to 8.0 vol % hydrochloric acid.  
   
   
       32 . The strip process according to  claim 29 , wherein said immersing step comprises immersing said substrate in a solution containing from about 23 to 27 vol % sulfuric acid and from about 4.0 to 6.0 vol % hydrochloric acid.