Effect of single aging on microstructure and impact property of INCONEL X-750
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I.
INTRODUCTION
INCONEL X-750 has found wide applications in both PWR and BWR nuclear power plants as bolts, springs, pins, and other structural components ~'2 on the basis of its high strength, age-hardenability, and good oxidation and corrosion resistance. 3'4'5 By far the most recognized and industrially practiced aging treatment for this alloy is a triple heat treatment, comprised of solution treatment at 1149 ~ 2 hours, air cooling to 843 ~ hours, air cooling to 704 ~ hours, and further air cooling. However, a major problem, intergranular stress corrosion cracking (IGSCC), has been encountered in reactor components made from INCONEL X-750 alloy treated in this way. It has been suggested 6 that a high temperature solution quench followed by a single step aging treatment can improve the IGSCC resistance of this alloy due to a more favorable intermetallic and carbide morphology. The carbide morphology, grain boundary Cr-depletion, hardness profiles, and grain boundary chemistry associated with a solution annealing treatment of 1075 ~ for 2 hours followed by a single aging treatment between 704 ~ and 871 ~ under air cooling have been examined by Sinha, Moore, and Hebsur 7'8 using scanning electron and Auger microscopy. Point analyses and fractographs have also been studied to locate the segregation of trace elements. Austenitic INCONEL X-750 constitutes mainly the ~/ matrix, 7 ' (Ni3A1 and Ni3Ti) "intermetallic" precipitation inside the grains together with MC type (TIC, NbC) and M23C6-type (Cr23C6) carbides. 9 The formation of the M23C6 type is believed to occur by the following reaction: MC + y --~ M23C 6 q- "y' *INCONEL is a trademark owned by the INCO family of companies. B. MISHRA, Research Assistant, and J, J. MOORE, Associate Director and Professor, are with Mineral Resources Research Center, University of Minnesota, Minneapolis, MN 55455. A.K. SINHA is Visiting Assistant Professor, Department of Materials Science and Engineering, Cornell University, Ithaca, NY 14853. Manuscript submitted July 16, 1984. METALLURGICALTRANSACTIONS A
Studies of the fracture behavior of these single-aged alloys by means of Charpy V-notch impact testing and intergranular corrosion susceptibility detection with a modified Huey test ~~have been carried out to develop an acceptable aging sequence. Further studies are being conducted on this "optimally" heat treated material to evaluate the influence of reducible sulfur species on stress corrosion cracking under PWR conditions, i.e., 8 x l06 Nt/m 2 pressure and 290 ~ temperature in an automated autoclave system.
II.
EXPERIMENTAL P R O C E D U R E
Samples for the various experiments in this study were prepared from an electroslag refined, hot rolled 38-mm diameter rod. The chemical composition of these samples is given in Table I. The samples were solution annealed at 1075 ~ for 2 hours and water quenched. Half of the samples were aged at 704 ~ 760 ~ and 871 ~ between 2 and 200 hours followed by air cooling, while the remaining samples were furnace cooled under a vacuum of 10 6
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