Corrosion pitting resistant martensitic stainless steel

Abstract

A forged, martensitic, stainless steel alloy is disclosed. The alloy comprises, by weight: about 12.0 to about 16.0 percent chromium; greater than 16.0 to about 20.0 percent cobalt, about 6.0 to about 8.0 percent molybdenum, about 1.0 to about 3.0 percent nickel, about 0.020 to about 0.040 percent carbon; and the balance iron and incidental impurities. The forged, martensitic, stainless steel alloys are highly resistant to pitting corrosion and provide a combination of tensile strength, ductility, and fracture toughness suitable for use as turbine compressor airfoils.

1 . A forged, martensitic, stainless steel alloy comprising, by weight: about 12.0 to about 16.0 percent chromium; greater than 16.0 to about 20.0 percent cobalt, about 6.0 to about 8.0 percent molybdenum, about 1.0 to about 3.0 percent nickel, about 0.020 to about 0.040 percent carbon; and the balance iron and incidental impurities. 
     
     
         2 . The alloy of  claim 1 , wherein the alloy comprises about 16.5 to about 20.0 percent cobalt. 
     
     
         3 . The alloy of  claim 1 , wherein the alloy has a microstructure that contains substantially no laves phase. 
     
     
         4 . The alloy of  claim 1 , wherein the alloy has a microstructure that comprises substantially no chi phase. 
     
     
         5 . The alloy of  claim 1 , wherein the alloy has a microstructure that comprises substantially no delta ferrite phase. 
     
     
         6 . The alloy of  claim 1 , wherein the alloy has a microstructure that comprises substantially no laves phase, chi phase and delta ferrite phase. 
     
     
         7 . The alloy of  claim 1 , wherein the alloy has a microstructure that comprises a retained austenite phase. 
     
     
         8 . The alloy of  claim 7 , wherein the retained austenite phase comprises at least about 15 percent by volume of the microstructure. 
     
     
         9 . The alloy of  claim 8 , wherein the retained austenite phase comprises about 15 percent to about 25 percent by volume of the microstructure. 
     
     
         10 . The alloy of  claim 1 , wherein the alloy is configured to provide a tensile elongation of at least about 14 percent. 
     
     
         11 . The alloy of  claim 10 , wherein the elongation is about 14 to about 24 percent. 
     
     
         12 . The alloy of  claim 1 , wherein the alloy is configured to provide a tensile reduction in area of at least about 41 percent. 
     
     
         13 . The alloy of  claim 12 , wherein the reduction in area is about 41 to about 49 percent. 
     
     
         14 . The alloy of  claim 1 , wherein the alloy has a pitting resistance equivalence number of about 31.8 or more. 
     
     
         15 . The alloy of  claim 1 , wherein the alloy has an ultimate tensile strength of about 150 ksi or more. 
     
     
         16 . The alloy of  claim 1 , wherein the alloy comprises a turbine compressor airfoil. 
     
     
         17 . A method of making a forged, martensitic, stainless steel alloy, comprising:
 providing a forged preform of martensitic, pitting corrosion resistant stainless steel alloy comprising, by weight: about 12.0 to about 16.0 percent chromium; greater than 16.0 to about 20.0 percent cobalt, about 6.0 to about 8.0 percent molybdenum, about 1.0 to about 3.0 percent nickel, about 0.020 to about 0.040 percent carbon; and the balance iron and incidental impurities;   heating the forged preform to a solutionizing temperature for a time sufficient to form a solutionized microstructure;   cooling the forged preform and solutionized microstructure to room temperature to form a martensitic microstructure;   heating the forged preform to a tempering temperature of about 600° F. for a tempering time sufficient to form a tempered forged preform comprising a tempered martensitic microstructure; and   cooling the tempered forged preform to room temperature.   
     
     
         18 . The method of  claim 17 , wherein the solutionizing temperature comprises about 2,000 to about 2,100° F. and the time comprises about 1 to about 3 hours. 
     
     
         19 . The method of  claim 17 , wherein the tempering time is about 3 to about 6 hours. 
     
     
         20 . The method of  claim 17 , wherein the tempered forged preform comprises a turbine airfoil preform.