Heat and corrosion resistant cast austenitic stainless steel alloy with improved high temperature strength

Abstract

A heat and corrosion resistant cast austenitic stainless steel alloy which contains less than about 15% nickel. The alloy has a completely austenitic microstructure in an as-cast state, and a creep rupture life exceeding 20,000 hrs at a stress of 35 MPa and a temperature of 850° C., when creep tested in the as-cast state under ASTM E139 test conditions.

1 . A heat and corrosion resistant cast austenitic stainless steel alloy comprising; 
 less than about 15% nickel;    completely austenitic microstructure in the as-cast state; and,    creep rupture life exceeding 20,000 hrs at a stress of 35 MPa and a temperature of 850° C., when creep tested in the as-cast state under ASTM El 39 test conditions.    
     
     
         2 . The cast austenitic stainless steel alloy of  claim 1  further including, 
 creep rupture life exceeding 2000 hours, when creep tested in the as-cast state under ASTM E139 test conditions at a stress of 100 MPa and a temperature of 750° C.    
     
     
         3 . The cast austenitic stainless steel alloy of  claim 2  further including, 
 minimum creep rate lower than 5×10 −5 , when creep tested in the as-cast state under ASTM E139 test conditions at a stress of 100 MPa and a temperature of 750° C.    
     
     
         4 . The cast austenitic stainless steel alloy of  claim 1  further including, 
 about 0.07 weight percent to about 0.15 weight percent carbon,    about 0.2 weight percent to about 1 weight percent silicon,    about 0.1 weight percent to about 1 weight percent molybdenum, and    about 0.6 weight percent to about 1.25 weight percent niobium.    
     
     
         5 . The cast austenitic stainless steel alloy of  claim 4  further including, 
 about 0.5 weight percent to about 10 weight percent manganese,    about 18 weight percent to about 25 weight percent chromium, and    about 0.05 weight percent to 0.5 weight percent nitrogen.    
     
     
         6 . The cast austenitic stainless steel alloy of  claim 1  further including, 
 completely austenitic microstructure after high temperature aging for 3000 hrs at 750° C.    
     
     
         7 . The cast austenitic stainless steel alloy of  claim 1  further including, 
 0.2% yield strength at 700° C. exceeding 125 MPa in the as-cast state, and a decrease in 0.2% yield strength from 700 to 900° C. of less than 20%.    
     
     
         8 . The cast austenitic stainless steel alloy of  claim 7  further including, 
 an ultimate tensile strength in the as-cast state exceeding 300 MPa at 700° C. and 125 MPa at 900° C.    
     
     
         9 . The cast austenitic stainless steel alloy of  claim 1  further including, 
 a creep ductility, expressed as a percentage change in elongation at rupture, exceeding 5%, when creep tested in the as-cast state at 35 MPa and 850° C., and    a creep ductility, expressed as a percentage reduction in area at rupture, exceeding 10%, when creep tested in the as-cast state at 35 MPa and 850° C.    
     
     
         10 . The cast austenitic stainless steel alloy of  claim 1  further including, 
 a low cycle fatigue life at a strain range of 0.3% exceeding 200,000 cycles at 650° C., and exceeding 20,000 cycles at 800° C., when tested in the as-cast state under fully reversed (R-ratio=−1) strain controlled tests under constant frequency.    
     
     
         11 . An article made from the cast austenitic stainless steel alloy of  claim 1 .  
     
     
         12 . A heat and corrosion resistant cast austenitic stainless steel alloy comprising; 
 less than about 15% nickel;    a creep rupture life exceeding 3,000 hrs and a minimum creep rate less than 1×10 −3  at a stress of 100 MPa and a temperature of 750° C., when creep tested in the as-cast state under ASTM E139 test conditions;    a 0.2% yield strength exceeding 130 MPa at 750° C. in the as-cast state;    less than 20% decrease in 0.2% yield strength from 750 to 900° C.; and    completely austenitic microstructure after casting.    
     
     
         13 . The cast austenitic stainless steel alloy of  claim 12  further including, 
 niobium carbide precipitates in the microstructure that are less than or equal to about 50 nanometers after 20,000 hours of creep test at a stress of 35 MPa and a temperature of 850° C. under ASTM E139 test conditions.    
     
     
         14 . The cast austenitic stainless steel alloy of  claim 12  further including, 
 about 0.05 weight percent to about 0.15 weight percent carbon,    about 1.5 weight percent to about 3.5 weight percent copper, and    about 0.2 weight percent to about 1 weight percent silicon.    
     
     
         15 . The cast austenitic stainless steel alloy of  claim 14  further including, 
 about 0.1 weight percent to about 1 weight percent molybdenum,    about 0.1 weight percent to about 1.5 weight percent niobium, and    about 0.25 weight percent to about 1.0 weight percent tungsten    
     
     
         16 . The cast austenitic stainless steel alloy of  claim 12  further including, 
 a creep rupture life exceeding 200 hours and a minimum creep rate less than 5×10 −3  at a stress of 140 MPa and a temperature of 750° C., when creep tested in the as-cast state under ASTM E139 test conditions.    
     
     
         17 . An article made from the cast austenitic stainless steel alloy of  claim 12 .  
     
     
         18 . An article made of a heat and corrosion resistant cast austenitic stainless steel alloy comprising; 
 less than about 15% nickel;    completely austenitic microstructure;    no detectable ferromagnetic behavior when measured with a measurement device after casting and after high temperature aging for 3000 hrs at 750° C.;    a creep rupture life exceeding 20,000 hrs at a stress of 35 MPa and a temperature of 850° C., when creep tested in the as-cast state under ASTM E139 test conditions; and,    a creep rupture life exceeding 2000 hours and a minimum creep rate less than 5×10 −3  at a stress of 100 MPa and a temperature of 750° C., when creep tested in the as-cast state under ASTM E139 test conditions.    
     
     
         19 . The article of  claim 18  further including, 
 a creep rupture life exceeding 3,000 hrs and a minimum creep rate less than 1×10 −3  at a stress of 100 MPa and a temperature of 750° C., when creep tested in the as-cast state under ASTM E139 test conditions,    a 0.2% yield strength at 750° C. exceeding 130 MPa in the as-cast state, and    less than 20% decrease in 0.2% yield strength from 750 to 900° C.    
     
     
         20 . The article of  claim 18  further including, 
 about 0.07 weight percent to about 0.15 weight percent carbon,    about 18 weight percent to about 25 weight percent chromium,    about 0.65 weight percent to about 1.5 weight percent niobium,    about 0.2 weight percent to 0.5 weight percent nitrogen,    about 0.5 weight percent to about 10 weight percent manganese,    less than or equal to about 3.5 weight percent copper,    about 0.1 weight percent to about 0.45 weight percent molybdenum,    less than or equal to about 2 weight percent tungsten, and    about 0.1 weight percent to about 1.0 weight percent silicon.    
     
     
         21 . A heat and corrosion resistant cast austenitic stainless steel alloy comprising; 
 a completely austenitic microstructure in the as-cast state;    about 0.05 weight percent to about 0.15 weight percent of carbon;    about 1.5 weight percent to about 3.5 weight percent copper;    about 0.25 weight percent to about 1.0 weight percent tungsten; and    about 0.6 weight percent to about 1.5 weight percent of niobium.    
     
     
         22 . The cast austenitic stainless steel alloy of  claim 21  further including, 
 about 0.1 weight percent to about 1.0 weight percent silicon, and    about 0.2 weight percent to about 0.5 weight percent nitrogen.