Cast iron-base, high-strength, oxidation-resistant alloy

Abstract

A cast AFA alloy composition comprising, in weight percent: 0.4 to 0.59 Nb+Ta; 0.4 to 0.6 C; 16 to 18 Cr; 18-23 Ni; 3.5-5.5 Al; 0.005 to 0.15 B; up to 1.5 Mo; up to 2 Co; up to 1 W; up to 3 Cu; up to 4 Mn; up to 2 Si; up to 0.5 wt. % total of at least one element selected from the group consisting of Ti and V; up to 0.06 N; up to 1 wt. % total of at least one element selected from the group consisting of Y, La, Ce, Hf, and Zr; balance Fe, wherein the weight percent Fe is greater than the weight percent Ni, and wherein the alloy forms an external continuous scale comprising alumina to at least 900° C. in air with 10% H 2 O, and a stable essentially single-phase FCC austenitic matrix microstructure, the austenitic matrix being essentially delta-ferrite free and essentially BCC-phase-free, with creep rupture life in excess of 500 h at 900° C. and 50 MPa.

We claim: 
     
         1 . An AFA alloy composition comprising:
 0.4 to 0.59 Nb+Ta;   0.4 to 0.6 C;   16 to 18 Cr;   18-23 Ni;   3.5-5.5 Al;   0.005 to 0.15 B;   up to 1.5 Mo;   up to 2 Co;   up to 1 W;   up to 3 Cu;   up to 4 Mn;   up to 2 Si;   up to 0.5 wt. % total of at least one element selected from the group consisting of Ti and V;   up to 0.06 N;   up to 1 wt. % total of at least one element selected from the group consisting of Y, La, Ce, Hf, and Zr;   balance Fe, wherein the weight percent Fe is greater than the weight percent Ni, and wherein said alloy forms an external continuous scale comprising alumina to at least 900° C. in air with 10% H 2 O, and a stable essentially single phase FCC austenitic matrix microstructure, said austenitic matrix being essentially delta-ferrite free and essentially BCC-phase-free, with creep rupture life in excess of 500 h at 900° C. and 50 MPa.   
     
     
         2 . The alloy of  claim 1 , wherein the alloy has a creep rupture life greater than 1000 h at 900° C. and 50 MPa. 
     
     
         3 . The alloy of  claim 1 , wherein the alloy has a yield strength at 900° C. that is greater than 18.75 ksi. 
     
     
         4 . The alloy of  claim 1 , wherein the alloy has a yield strength at 900° C. that is greater than 21 ksi. 
     
     
         5 . The alloy of  claim 1 , wherein the alloy has a ultimate tensile strength is greater than 22.75 ksi. 
     
     
         6 . The alloy of  claim 1 , wherein the alloy has a ultimate tensile strength is greater than 26 ksi. 
     
     
         7 . The alloy of  claim 1 , wherein the mass gain after exposure to air with 10 vol % H 2 O at 950° C. in 1 h cycles for 1000 h is less than 1 mg/cm 2 . 
     
     
         8 . The alloy of  claim 1 , wherein the mass gain after exposure to air with 10 vol % H 2 O at 1000° C. in 1 h cycles for 500 h is less than 1 mg/cm 2 . 
     
     
         9 . The alloy of  claim 1 , wherein the atomic ratio R can be from 3.77 to 4.77. 
     
     
         10 . The alloy of  claim 1 , wherein the atomic ratio R can be from 4.42 to 4.77. 
     
     
         11 . The alloy of  claim 1 , wherein the calculated equilibrium mole % at 900° C. of MC carbides is from 0.31 to 0.67. 
     
     
         12 . The alloy of  claim 1 , wherein the calculated equilibrium mole % at 900° C. of M 23 C 6  carbides is from 8.56 to 9.86. 
     
     
         13 . The alloy of  claim 1 , wherein the calculated equilibrium mole % at 900° C. of MC and M 23 C 6  carbides is from 9.19 to 10.24. 
     
     
         14 . The alloy of  claim 1 , wherein the calculated change in mole % of total carbides after exposure to 900° C. is from 5.46 to 6.33. 
     
     
         15 . The alloy of  claim 1 , wherein the calculated change in mole % of MC and M 23 C 6  after exposure to 900° C. is from 6.86 to 8.61. 
     
     
         16 . The alloy of  claim 1 , wherein the alloy consists essentially of:
 0.4 to 0.59 Nb+Ta;   0.4 to 0.6 C;   16 to 18 Cr;   18-23 Ni;   3.5-5.5 Al;   0.005 to 0.15 B;   up to 1.5 Mo;   up to 2 Co;   up to 1 W;   up to 3 Cu;   up to 4 Mn;   up to 2 Si;   up to 0.5 wt. % total of at least one element selected from the group consisting of Ti and V;   up to 0.06 N;   up to 1 wt. % total of at least one element selected from the group consisting of Y, La, Ce, Hf, and Zr;   balance Fe, wherein the weight percent Fe is greater than the weight percent Ni, and wherein said alloy forms an external continuous scale comprising alumina to at least 900° C. in air with 10% H 2 O, and a stable essentially single-phase FCC austenitic matrix microstructure, said austenitic matrix being essentially delta-ferrite free and essentially BCC-phase-free, with creep rupture life in excess of 500 h at 900° C. and 50 MPa.   
     
     
         17 . The alloy of  claim 1 , wherein the alloy consists of:
 0.4 to 0.59 Nb+Ta;   0.4 to 0.6 C;   16 to 18 Cr;   18-23 Ni;   3.5-5.5 Al;   0.005 to 0.15 B   up to 1.5 Mo   up to 2 Co;   up to 1 W;   up to 3 Cu;   up to 4 Mn;   up to 2 Si;   up to 0.5 wt. % total of at least one element selected from the group consisting of Ti and V;   up to 0.06 N;   up to 1 wt. % total of at least one element selected from the group consisting of Y, La, Ce, Hf, and Zr;   balance Fe, wherein the weight percent Fe is greater than the weight percent Ni, and wherein said alloy forms an external continuous scale comprising alumina to at least 900° C. in air with 10% H 2 O, and a stable essentially single-phase FCC austenitic matrix microstructure, said austenitic matrix being essentially delta-ferrite free and essentially BCC-phase-free, with creep rupture life in excess of 500 h at 900° C. and 50 MPa.