Austenitic heat resistant alloy and method for producing the same
Provided is an austenitic heat resistant alloy capable of exhibiting sufficient molten-salt corrosion resistance even when exposed to a molten salt of 600° C., and a production method thereof. The austenitic heat resistant alloy includes a base metal and a Fe—Cr—Ni—W film on the surface of the base metal. The base metal has a chemical composition consisting of: C: 0.030 to 0.120%, Si: 0.02 to 1.00%, Mn: 0.10 to 2.00%, Cr: 20.0% or more to less than 28.0%, Ni: more than 35.0% to 50.0% or less, W: 4.0 to 10.0%, Ti: 0.01 to 0.30%, Nb: 0.01 to 1.00%, sol. Al: 0.0005 to 0.0400%, and B: 0.0005 to 0.0100%, with the balance being Fe and impurities. The Fe—Cr—Ni—W film contains, as oxides, Fe: 15.0 to 35.0 at %, Cr: 15.0 to 35.0 at %, Ni: 10.0 to 45.0 at %, and W: 0.5 to 16.5 at %.
1. An austenitic heat resistant alloy, comprising:
a base metal having a chemical composition consisting of: in mass %,
C: 0.030 to 0.120%,
Si: 0.02 to 1.00%,
Mn: 0.10 to 2.00%,
Cr: 20.0% or more to less than 28.0%,
Ni: more than 35.0% to 50.0% or less,
W: 4.0 to 10.0%,
Ti: 0.01 to 0.30%,
Nb: 0.01 to 1.00%,
sol. Al: 0.0005 to 0.0400%,
B: 0.0005 to 0.0100%,
Zr: 0 to 0.1000%,
Ca: 0 to 0.0500%,
REM: 0 to 0.2000%,
Hf: 0 to 0.2000%,
Pd: 0 to 0.2000%,
P: 0.040% or less,
S: 0.010% or less,
N: less than 0.020%,
O: 0.0050% or less,
Mo: less than 0.5%,
Co: 0 to 0.80%, and
Cu: 0 to 0.50%,
with the balance being Fe and impurities; and
an Fe—Cr—Ni—W film on a surface of the base metal, the Fe—Cr—Ni—W film containing, as oxides, Fe: 15.0 to 35.0 at %, Cr: 15.0 to 35.0 at %, Ni: 10.0 to 45.0 at %, and W: 0.5 to 16.5 at %.
2. The austenitic heat resistant alloy according to claim 1 , wherein
the chemical composition of the base metal contains, in mass %, Zr: 0.0005 to 0.1000%.
3. The austenitic heat resistant alloy according to claim 1 , wherein
the chemical composition of the base metal contains, in mass %, Ca: 0.0005 to 0.0500%.
4. The austenitic heat resistant alloy according to claim 2 , wherein
the chemical composition of the base metal contains, in mass %, Ca: 0.0005 to 0.0500%.
5. The austenitic heat resistant alloy according to claim 1 , wherein
the chemical composition of the base metal contains at least one kind selected from the group consisting of, in mass %, REM: 0.0005 to 0.2000%, Hf: 0.0005 to 0.2000%, and Pd: 0.0005 to 0.2000%.
6. The austenitic heat resistant alloy according to claim 2 , wherein
the chemical composition of the base metal contains at least one kind selected from the group consisting of, in mass %, REM: 0.0005 to 0.2000%, Hf: 0.0005 to 0.2000%, and Pd: 0.0005 to 0.2000%.
7. The austenitic heat resistant alloy according to claim 3 , wherein
the chemical composition of the base metal contains at least one kind selected from the group consisting of, in mass %, REM: 0.0005 to 0.2000%, Hf: 0.0005 to 0.2000%, and Pd: 0.0005 to 0.2000%.
8. The austenitic heat resistant alloy according to claim 4 , wherein
the chemical composition of the base metal contains at least one kind selected from the group consisting of, in mass %, REM: 0.0005 to 0.2000%, Hf: 0.0005 to 0.2000%, and Pd: 0.0005 to 0.2000%.
9. A method for producing an austenitic heat resistant alloy, comprising the steps of:
preparing a starting material having the chemical composition according to claim 1 ; and
forming an Fe—Cr—Ni—W film on a surface of the starting material by immersing the starting material in a solution which has a hydrofluoric acid concentration satisfying Formula (1) and a nitric acid concentration satisfying Formula (2):
35.6≤hydrofluoric acid concentration(mass %)+(0.15Ni+0.85Cr+0.18Fe+0.23W)≤37.7 (1), and
23.5≤nitric acid concentration(mass %)+(0.25Ni+0.31Cr−0.18Fe+0.24W)≤26.5 (2),
where each element symbol in Formulae (1) and (2) is substituted by the content (mass %) of each element.