Effects of Long-Term Thermal Exposure on the Microstructure and Properties of a Cast Ni-Base Superalloy
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UCTION
THE K452 alloy is a newly developed high Cr content cast Ni-base superalloy for advanced gas turbine vane applications in marine and industrial fields. It performs well under laboratory conditions with good fatigue resistance, hot-corrosion resistance, and tensileand stress-rupture properties, in addition to being completely oxidation resistant up to 900 C.[1] Compared to aircraft engines, gas turbines for industrial applications, which are exposed to corrosive environment for prolonged periods of time, have more stringent requirements for the hot-end superalloy components to accommodate for their formidable operating conditions. Accordingly, for Ni-base superalloys such as K452, microstructural stability is a very important consideration, which must be carefully examined and assured before putting them to use. Superalloys generally experience various microstructural changes during their service life, including c¢ coarsening, formation of a continuous GB carbide network, topologically close-packed (TCP) phase formation, and MC carbide degeneration.[2–6] These processes remove much of strengthening elements from the c matrix and significantly degrade the properties of the alloys, such as mechanical properties, corrosion resisX.Z. QIN, Student, J.T. GUO, Professor, and C. YUAN, Associate Professor, Superalloys Division, Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, P.R. China and C.L. CHEN, Student, and H.Q. YE, Academician, are with Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, P.R. China. Contact e-mail: [email protected] Manuscript submitted December 18, 2006. Article published online November 7, 2007 3014—VOLUME 38A, DECEMBER 2007
tance and service life. In this article, the microstructural evolution during long-term thermal exposure in the K452 alloy is examined in detail, and the microstructure-property relationships are explored carefully.
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EXPERIMENTAL PROCEDURE
K452 has the composition (wt pct) of 0.105C, 20.9Cr, 11.15Co, 3.5W, 0.6Mo, 0.25Nb, 2.5Al, 3.5Ti, 0.04Zr, 0.015B, and rest Ni. Specimens were subjected to a homogenization for 4 hours at 1170 C followed by furnace cooling to 900 C and then air cooling to room temperature; subsequently, two annealing treatments (4 hours at 1050 C and 16 hours at 850 C), producing the secondary and tertiary c¢ precipitates, were carried out. After the heat treatment, specimens were exposed at temperatures of 800 C, 850 C, and 900 C for times of 1000, 3000, 5000, and up to 10,000 hours, respectively. For each exposure condition at least four specimens were prepared for tensile- or stress-rupture tests (two for tensile-rupture tests and two for stress-rupture tests). Tensile- and stress-rupture tests (gage length 50 mm, gage diameter 5 mm) were performed at 900 C and 900 C/201 MPa, respectively. Each rupture value, including strength and elongation, represents an average of at least two test results. The microstructures were examined using optica
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