Cast heat-resistant austenitic steel with improved temperature creep properties and balanced alloying element additions and methodology for development of the same

Abstract

The present invention addresses the need for new austenitic steel compositions with higher creep strength and higher upper temperatures. The present invention also discloses a methodology for the development of new austenitic steel compositions with higher creep strength and higher upper temperatures.

1 . A cast heat-resistant austenitic steel alloy with improved temperature creep properties and balanced alloyed element additions, said alloy comprising:
 about 0.4 to about 0.7 wt. % carbon;   about 20 to about 35 wt. % chromium;   about 30 to about 45 wt. % nickel;   about 0.5 to about 1.5 wt. % manganese;   about 0.6 to about 2.0 wt. % silicon;   up to about 1.5 wt. % niobium;   up to about 1.5 wt. % tungsten;   up to about 1.5 wt. % molybdenum; and,   the remainder being iron.   
   
   
       2 . The cast heat-resistant austenitic steel alloy of  claim 1 , further comprising about 0.01 to about 1.0 wt. % titanium. 
   
   
       3 . The cast heat-resistant austenitic steel alloy of  claim 1 , further comprising about 0.01 to about 1.0 wt. % zirconium. 
   
   
       4 . The cast heat-resistant austenitic steel alloy of  claim 2 , further comprising about 0.01 to about 1.0 wt. % zirconium. 
   
   
       5 . The cast heat-resistant austenitic steel alloy of  claim 1 , further comprising about 0.02 wt. % cobalt. 
   
   
       6 . The cast heat-resistant austenitic steel alloy of  claim 1 , wherein the creep life of said alloy at about 1200° C. and about 500 psi is in excess of about 600 hours. 
   
   
       7 . The cast heat-resistant austenitic steel alloy of  claim 1 , wherein the total wt. % of carbides in said alloy at about 1200° C. is between about 3.18 and about 7.84. 
   
   
       8 . The cast heat-resistant austenitic steel alloy of  claim 1 , wherein M 23 C 6  and MC carbides are present in the microstructure of said alloy at high temperatures up to about 1204° C. 
   
   
       9 . The cast heat-resistant austenitic steel alloy of  claim 1 , wherein said alloy has the following composition:
 about 0.41 wt. % carbon;   about 23.6 wt. % chromium;   about 34.6 wt. % nickel;   about 1.0 wt. % manganese:   about 0.7 wt. % silicon;   about 0.05 wt. % molybdenum;   about 0.08 wt. % tungsten;   about 0.33 wt. % niobium;   about 0.02 wt. % cobalt;   about 0.1 wt. % titanium; and   the remainder being iron;   said alloy has a maximum temperature of stability of M 23 C 6  carbide of about 1173° C.; a maximum wt. % of M 23 C 6  between about 600° C. and about 1500° C. of about 6.4; a maximum wt. % of MC between about 600° C. and about 1500° C. of about 0.43; and a creep life at about 1204° C. and about 500 psi of about 675 hours.   
   
   
       10 . The cast heat-resistant austenitic steel alloy of  claim 1 , said alloy having the following composition:
 about 0.42 wt. % carbon;   about 23.7 wt. % chromium;   about 35.1 wt. % nickel;   about 1.1 wt. % manganese;   about 0.7 wt. % silicon;   about 0.28 wt. % molybdenum;   about 0.07 wt. % tungsten;   about 0.34 wt. % niobium;   about 0.02 wt. % cobalt;   about 0.4 wt. % titanium;   about 0.1 wt. % zirconium; and   the remainder being iron;   said alloy having a maximum temperature of stability of M 23 C 6  carbide of about 1227° C.; a maximum wt. % of M 23 C 6  between about 600° C. and austenitic 1500° C. of about 6.7; a maximum wt. % of MC between about 600° C. and austenitic 1500° C. of about 0.74; and a creep life at about 1204° C. and about 500 psi of about 1251 hours.   
   
   
       11 . The cast heat-resistant austenitic steel alloy of  claim 1 , said alloy having the following composition:
 about 0.41 wt. % carbon;   about 23.4 wt. % chromium;   about 34.34 wt. % nickel;   about 1.0 wt. % manganese;   about 0.7 wt. % silicon;   about 0.5 wt. % molybdenum;   about 0.34 wt. % niobium;   about 0.1 wt. % tungsten;   about 0.3 wt. % titanium; and   the remainder being iron;   said alloy having a maximum temperature of stability of M 23 C 6  carbide of about 1253° C.; a maximum wt. % of M 23 C 6  between about 600° C. and about 1500° C. of about 6.6; a maximum wt. % of MC between about 600° C. and about 1500° C. of about 0.63; and a creep life at about 1204° C. and about 500 psi of about 1293 hours.   
   
   
       12 . The cast heat-resistant austenitic steel alloy of  claim 1 , said alloy having the following composition:
 about 0.6 wt. % carbon;   about 23.6 wt. % chromium;   about 35.5 wt. % nickel;   about 1.0 wt. % manganese;   about 0.7 wt. % silicon;   about 0.85 wt. % molybdenum;   about 0.34 wt. % niobium;   about 0.07 wt. % tungsten;   about 0.4 wt. % titanium;   about 0.1 wt. % zirconium;   the remainder being iron; and   said alloy having a maximum temperature of stability of M 23 C 6  carbide of about 1267° C.; a maximum wt. % of M 23 C 6  between about 600° C. and about 1500° C. of about 10.1; a maximum wt. % of MC between about 600° C. and about 1500° C. of about 0.75; and a creep life at about 1204° C. and about 500 psi of about 1558 hours.   
   
   
       13 . A cast heat-resistant austenitic steel alloy with improved temperature creep properties and balanced alloyed elements additions, said alloy comprising:
 about 0.4 wt. % carbon;   about 24 wt. % chromium;   about 35 wt. % nickel;   about 1.0 wt. % manganese;   about 0.7 wt. % silicon;   about 0.3 wt. % niobium;   about 0.08 wt. % tungsten;   about 0.5 to about 1.5 wt. % molybdenum;   about 0.1 to about 0.4 wt. % titanium; and,   the rest being iron.   
   
   
       14 . The cast heat resistant austenitic steel alloy of  claim 13 , further comprising less than about 0.1 wt. % zirconium. 
   
   
       15 . The cast heat-resistant austenitic steel alloy of  claim 13 , said alloy having a total calculated wt. % of carbides in the range of about 3.18 to about 5.06. 
   
   
       16 . A method for evaluating compositions of HP and HK alloys comprising the steps of:
 calculating the effect of an addition of alloying elements on the stability of M 23 C 6  and MC utilizing thermodynamic models;   using said HP and HK alloys as a base reference for said calculations; and,   wherein said addition of alloying elements are used to stabilize strengthening phases at a required temperature as indicated by the results of said thermodynamic calculations to provide improved creep-resistance.