Effect of Thermal Aging on the Corrosion and Microstructure of Friction-Stir Welded Alloy 22
- PDF / 1,346,832 Bytes
- 10 Pages / 593.972 x 792 pts Page_size
- 94 Downloads / 253 Views
INTRODUCTION
ALLOY 22 (UNS NO6022), which is also known as Inconel 622 and C 22, is a nickel-chromium-molybdenum tungsten alloy. It has an excellent corrosion resistance and was proposed as the outer barrier material of the waste package for the nuclear waste disposal at Yucca Mountain, Nevada.[1] The waste package design called for a 2cm-thick outer barrier made of alloy 22 and a 5-cm-thick 316 stainless steel (UNS S31600) inner vessel.[1] The inner barrier offers the necessary structural support and mechanical strength, whereas the outer barrier provides the necessary corrosion resistance. The excellent corrosion resistance of alloy 22 results from the presence of molybdenum and chromium. This alloy has a singlephase, face-centered-cubic structure in the mill-annealed condition. Other phases may precipitate with time and degrade both mechanical and corrosion resistance of the alloy.[2,3] One concern for the waste package integrity is the long-term corrosion resistance of the closure welds. During the initial several hundreds to thousand years of service in nuclear waste disposal, the temperature of the waste container is expected to reach a maximum temperature between 523 K (250 C) and 573 K (300 C). BHARAT K. JASTHI, Research Scientist-III, with the Advanced Materials Processing (AMP) Center, South Dakota School of Mines and Technology, 501 East Saint Joseph Street, Rapid City, SD 57701, is also a Adjunct Faculty with the Department of Materials and Metallurgical Engineering, South Dakota School of Mines and Technology. Contact e-mail: [email protected] WILLIAM J. ARBEGAST (deceased), formerly Director, with the NSF Center for Friction Stir Processing (CFSP), South Dakota School of Mines and Technology and also with Advanced Materials Processing (AMP) Center, South Dakota School of Mines and Technology. STANLEY M. HOWARD, Professor, is with the Department of Materials and Metallurgical Engineering, South Dakota School of Mines and Technology. Manuscript submitted July 21, 2011. Article published online March 29, 2012 3192—VOLUME 43A, SEPTEMBER 2012
Therefore, the impact of the long exposure time to these temperatures has to be considered. Previous work on alloy 22 suggests the formation of intermetallic tetrahedral (or topologically) closedpacked (TCP) phases during solidification.[4] Table I shows the nominal compositions of alloy 22 and the TCP ‘‘P,’’ ‘‘l,’’ and ‘‘r’’ phases.[4] These molybdenum-rich phases are formed during solidification. The precipitation of these TCP phases could decrease the corrosion resistance of alloy 22 and could have an impact on the mechanical properties. Gas tungsten arc welding (GTAW) is employed for the fabrication of the containers. GTAW involves melting of the base materials and produces a cast and segregated microstructure in the weld nugget. This segregated microstructure in the weld nugget could affect both the mechanical and corrosion properties.[2,3] Each container is fabricated by rolling plates into cylinders closed using a longitudinal weld. After welding the bott
Data Loading...
