2001 Fiscal Year Final Research Report Summary
Degradation of alloy properties by super purification and optimum design of advanced steels by the best use of impurity elements
| Project/Area Number |
11305050
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| Research Category |
Grant-in-Aid for Scientific Research (A)
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| Allocation Type | Single-year Grants |
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| Section | 一般 |
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| Research Field |
Structural/Functional materials
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| Research Institution | Nagoya University |
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Principal Investigator |
MORINAGA Masahiko Nagoya Univ., Materials Science and Eng., Professor, 工学研究科, 教授 (50126950)
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| Co-Investigator(Kenkyū-buntansha) |
YUKAWA Hiroshi Nagoya Univ., Materials Science and Eng., Research Associate, 工学研究科, 助手 (50293676)
MURATA Yoshinori Nagoya Univ., Materials Science and Eng., Associate Professor, 工学研究科, 助教授 (10144213)
TANAKA Nobuo Nagoya Univ., Applied Physics, Professor, 工学研究科, 教授 (40126876)
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| Project Period (FY) |
1999 – 2001
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| Keywords | impurity / sulfur / heat-resisting steel / steam oxidation |
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| Research Abstract |
Some of alloy properties degrade by super purification. The best use of impurity elements such as S and B is needed for the optimum design of advanced heat-resisting steels. The objective of this research is to investigate the effect of impurity elements on the steam oxidation resistance. The results on the S impurity are summarized as follows ;
( 1 ) It was observed that a mass loss occurred in the advanced high Cr ferntic steels m the early stage of steam oxidation at 650℃. However, this mass loss decreased significantly in the S-containing steels.
( 2 ) As S was an impurity element to segregate on the surface of the stee at 650℃ it worked to assist the Cr enrichment on the surface owing to the strong affinity of these two elements As a result, Cr_2O_3 could form easily on the steel surface in the early stage of steam oSon a far as tne steel contained S.
( 3 ) S existed under the Cr_2O_3 oxide layer or at the interface between the oxide and the base steel. The existence of S at the inte
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rface suppressed severe Cr depletion under the oxide, resulting in good oxidation resistance even for a long time exposure to steam.
( 4 ) It was stressed here that even Cr_2O_3 oxide specimen itself showed a mass loss after steam oxidation at 650℃. However, the S doping into Cr_2O_3 decreased such a mass loss.
( 5 ) As might be expected from these results, this S effect was not remarkable in the low Cr steels (e.g., 2.25%Cr steel).
The mass loss obseryed ferritic steels and Cr_2O_3 oxide during steam oxidation was interpreted as due to the formation of a volatile hydroxide, CrO_2(OH)_2, as reported previously in austltic steels. The presence of S at an impurity level probably suppressed the formation of he volatile compound. This is probably the reason why S is effective in increasing the oxidation resistance to steam but not to air. The S effect became dominant if the steels contained 50ppm S. This is an allowable amount of S since it does not degrade mechanical properties of the steels. Therefore, for the optimum design, 50〜100ppmS should remain in advanced high Cr ferritic steels. Less
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