Cast alumina forming austenitic stainless steels

1. An austenitic iron base alloy consisting essentially of, in weight percent ranges:

0.15-0.5C;

8-37Ni;

10-25Cr;

2.5-5Al;

greater than 0.6, up to 2.5 total of at least one element selected from the group consisting of Nb and Ta;

up to 3Mo;

up to 3Co;

up to 1W;

up to 3Cu;

up to 15Mn;

up to 2Si;

0.007-0.15B;

up to 0.05P;

up to 1 total of at least one element selected from the group consisting of Y, La, Ce, Hf, and Zr;

<0.3 Ti+V;

<0.03N; and,

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, and a stable essentially single phase FCC austenitic matrix microstructure, said austenitic matrix being essentially delta-ferrite free and essentially BCC-phase-free.

2. The austenitic iron base alloy of claim 1 , wherein the C weight percent range is 0.2-0.5C.

3. The austenitic iron base alloy of claim 1 , wherein the C weight percent range is 0.2-0.4C.

4. The austenitic iron base alloy of claim 1 , wherein the Cr weight percent range is 10-15Cr.

5. The austenitic iron base alloy of claim 1 , wherein the Cr weight percent range is 14-16Cr.

6. The austenitic iron base alloy of claim 1 , wherein the Ni weight percent range is 15-30Ni.

7. The austenitic iron base alloy of claim 1 , wherein the Ni weight percent range is 20-30 Ni.

8. The austenitic iron base alloy of claim 1 , wherein the Mn weight percent range is 0-5Mn.

9. The austenitic iron base alloy of claim 1 , wherein the Ni weight percent range is 8-12 Ni and the Mn weight percent range is 5-15.

10. The austenitic iron base alloy of claim 1 , wherein the Si weight percent range is up to 1 Si.

11. The austenitic iron base alloy of claim 1 , wherein the Nb/Ta weight percent range is greater than 0.9, up to 2.5 total of at least one element selected from the group consisting of Nb and Ta.

12. An austenitic iron base alloy consisting essentially of in weight percent ranges:

0.4C;

23-27Ni;

13-15Cr;

3.0-4Al;

greater than 0.9, up to 1 total of at least one element selected from the group consisting of Nb and Ta;

up to 3Mo;

up to 3Co;

up to 1W;

up to 3Cu;

up to 5Mn;

up to 2Si;

0.007-0.15B;

up to 0.05P;

up to 1 total of at least one element selected from the group consisting of Y, La, Ce, Hf, and Zr;

<0.3 Ti+V;

<0.03N; and,

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, and a stable essentially single phase FCC austenitic matrix microstructure, said austenitic matrix being essentially delta-ferrite free and essentially BCC-phase-free.

13. The austenitic iron base alloy of claim 1 , wherein the Ni weight percentage is about 2.5Ni, the Cr weight percentage is about 14Cr, the Al weight percentage is about 3.5Al, and the Nb/Ta weight percentage is about 0.95 total of at least one element selected from the group consisting of Nb and Ta.

14. A method of making iron base alloy articles, comprising the steps of:

providing, in weight percent ranges:

0.15-0.5C;

8-37Ni;

10-25Cr;

2.5-5Al;

greater than 0.6, up to 2.5 total of at least one element selected from the group consisting of Nb and Ta;

up to 3Mo;

up to 3Co;

up to 1W;

up to 3Cu;

up to 15Mn;

up to 2Si;

0.007-0.15B;

up to 0.05P;

up to 1 total of at least one element selected from the grou consisting of Y, La, Ce, Hf, and Zr;

<0.3 Ti+V;

<0.03N; and,

balance Fe, wherein the weight percent Fe is greater than the weigh percent Ni, and wherein said alloy forms an external continuous scale comprising alumina, and a stable essentially single phase FCC austenitic matrix microstructure, said austenitic matrix being essentially delta-ferrite free and essentially BCC-phase-free;

heating said mixture;

cooling said mixture to solidify the mixture to form a solid alloy.

15. The method of claim 14 , wherein said heated mixture is cast prior to cooling.