Pre-Oxidation Heat-Treatment Method to Yield an Aluminum Oxide Scale

Abstract

A pre-oxidation heat-treatment method to heat treat nickel-base alloys containing, in weight percent, 1.8 wt. % minimum aluminum and 10 wt. % minimum chromium is disclosed that can provide an aluminum oxide scale having a thickness of at least 100 nanometers (nm) that contains little or no chromium oxide (Cr2O3), nickel oxide (NiO), titanium oxide (TiO2), silicon oxide (SiO2) or mixed oxides. The method involves heating a workpiece made from an alloy in a furnace containing a flowing argon atmosphere at a pressure of at least 500 microns, and at a temperature of 1400° F. to 2150° F. until a continuous scale of aluminum oxide (Al2O3) is formed on at least one surface of the workpiece. The flowing argon atmosphere in the pre-oxidation heat-treatment method should contain less than 0.1 vol. % oxygen content, and the gas flux is at least 1 mL/min·cm2.

What is claimed is: 
     
         1 . A method for forming a continuous aluminum oxide scale on a product made of a nickel-base alloy, comprising:
 a. providing a metal workpiece having surfaces and comprised of a nickel-base alloy containing at least 1.8 weight percent of aluminum and at least 10 weight percent of chromium;   b. placing the metal workpiece in a furnace containing a flowing argon atmosphere containing less than 0.1 vol. % oxygen content at a pressure of at least 500 microns; and   c. heating the metal workpiece at a temperature of 1400° F. to 2150° F. until a continuous aluminum oxide scale is formed on at least one surface of the workpiece.   
     
     
         2 . The method of  claim 1  wherein the metal workpiece is heated at a temperature of 1700° F. to 2100° F. 
     
     
         3 . The method of  claim 1  wherein the metal workpiece is heated for at least 0.5 hours. 
     
     
         4 . The method of  claim 1  wherein the metal workpiece is heated for 24 hours at 1700° F. 
     
     
         5 . The method of  claim 1  wherein the metal workpiece is heated for 8 hours at 2000° F. 
     
     
         6 . The method of  claim 1  wherein the metal workpiece is heated for 1 hour at 2100° F. 
     
     
         7 . The method of  claim 1  wherein the continuous aluminum oxide scale has an average thickness of at least 100 nanometers. 
     
     
         8 . The method of  claim 1  wherein the continuous aluminum oxide scale has a minimum thickness of 100 nanometers at all points on the at least one surface on which the continuous aluminum oxide scale has been formed. 
     
     
         9 . The method of  claim 1  wherein the furnace atmosphere contains a flowing argon atmosphere at a pressure of at least 5000 microns. 
     
     
         10 . The method of  claim 1  wherein the furnace atmosphere contains 1 atm flowing argon. 
     
     
         11 . The method of  claim 1  wherein the gas flux in the method is at least 1 mL/min·cm 2 . 
     
     
         12 . The method of  claim 1  wherein the preferred gas flux in the method is 5 to 25 mL/min·cm 2 . 
     
     
         13 . The method of  claim 1  wherein the metal workpiece contains 1.8-5.0 wt. % aluminum. 
     
     
         14 . The method of  claim 1  wherein the metal workpiece contains 10-35 wt. % chromium. 
     
     
         15 . The method of  claim 1  wherein the metal workpiece contains 15-26 wt. % chromium. 
     
     
         16 . The method of  claim 1  wherein the metal workpiece contains 3.0-3.5 wt. % aluminum and 18-20 wt. % chromium. 
     
     
         17 . The method of  claim 1  wherein the metal workpiece contains 3.5-4.1 wt. % aluminum and 18-22 wt. % chromium. 
     
     
         18 . The method of  claim 1  wherein the metal workpiece has an as-fabricated surface before being placed in the furnace. 
     
     
         19 . The method of  claim 1  wherein the metal workpiece has an oxidation-annealed and pickled surface before being placed in the furnace. 
     
     
         20 . The method of  claim 1  wherein the metal workpiece has a bright annealed surface before being placed in the furnace. 
     
     
         21 . The method of  claim 1  also comprising machining at least one surface of the metal workpiece before placing the metal workpiece in the furnace. 
     
     
         22 . The method of  claim 1  also comprising grinding or polishing at least one surface of the metal workpiece before placing the metal workpiece in the furnace. 
     
     
         23 . The method of  claim 1  also comprising abrasive blasting at least one of the surfaces of the metal workpiece before placing the metal workpiece in the furnace.