2006 Fiscal Year Final Research Report Summary
Research on the Stress Accelerated Grain Boundary Oxidation of Super Alloys for Advanced Ultra Supercritical Turbine Systems
| Project/Area Number |
17560060
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| Research Category |
Grant-in-Aid for Scientific Research (C)
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| Allocation Type | Single-year Grants |
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| Section | 一般 |
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| Research Field |
Materials/Mechanics of materials
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| Research Institution | Tohoku University |
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Principal Investigator |
AKIO Ohji Tohoku university, Graduate school of engineering, Professor, 大学院工学研究科, 教授 (30374953)
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| Co-Investigator(Kenkyū-buntansha) |
SHOJI Tetsuo Tohoku University, Graduate school of engineering, Professor, 大学院工学研究科, 教授 (80091700)
TAKEDA Yoichi Tohoku University, Graduate school of engineering, Research associate, 大学院工学研究科, 助手 (40374970)
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| Project Period (FY) |
2005 – 2006
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| Keywords | Stress accelerated grain boundary oxidation / Nickel base super alloys / Oxidation characteristics evaluation / purvelized coal fired power plant / material strength in the environment |
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| Research Abstract |
In order for the clarification of material degradation and fracture mechanisms in high temperature steam environments, observation of cracking morphology and evaluation of oxide distribution around the crack tip were carried our by using stress accelerated grain boundary oxidation evaluation facility. The materials concerned is nickel base super alloys that are considered to be used for the components used at high temperature section in the advanced ultra supercritical power plant. The stress accelerated grain boundary oxidation evaluation facility in which the four-point bending testing fixtures were installed was fabricated. Nickel base superalloys, Inconel 718, 625 and 617 are subjected to the oxidation tests under bending stress at 750oC in both air and steam environments. Chromium enriched oxide films were observed with chromium depletion zone beneath the film for all the cases. Such the oxide film formation was exhibited to be accelerated by the stress according to the formed oxi
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de film thickness. Especially, the influence of the stress was more significant and resulted delamination of the surface film was recognized in Inconel 718. The thickness of formed oxide film with and without stress was similar in Inconel 617. Significant amount of the void was observed in the alloy beneath the oxide film and thought to be formed due to selective oxidation of the chromium. This revealed that the diffusion of the chromium towards the surface is dominant in the environment.
Selective oxidation along the grain boundaries and wedge separations were observed in Inconel 718. Cross sectional analysis were performed. Aluminum and titanium was increased with increasing exposure time at the region adjacent to the wedge separations. Furthermore, these became dominant when the stress was applied. Similar tendency was also observed in the chromium concentration in the oxide film formed on the surface above wedge separation.
Uniaxial constant load testing in the environment using a bar specimen, in which the various cross sectional area had been machined, was performed to clarify above mentioned diffusion behavior of alloying element quantitatively. Selective oxidation of the grain boundary was not evident in the section where the applied stress was bellow 35MPa, however, it was observed as well as wedge separation at the section to which the 78MPa of the stress applied. These observations reveal that degradation of Inconel 718 in high temperature steam environment is induced by the localization of the oxidation into grain boundaries and suhseouent oxidation acceleration due to the applied stress. Less
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