High-Temperature Tensile Behaviors of Base Metal and Electron Beam-Welded Joints of Ni-20Cr-9Mo-4Nb Superalloy
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-BASED superalloy Ni-20Cr-9Mo-4Nb is widely used in aeronautical, aerospace, chemical, petrochemical and marine applications due to its superior combination of high-temperature mechanical properties,
R.K. GUPTA and V. ANIL KUMAR are with the Materials and Mechanical Entity, Vikram Sarabhai Space Centre, Trivandrum, India. Contact e-mail: [email protected] ARJUN SUKUMARAN is with the National Institute of Technology, Calicut, India. VINOD KUMAR is with the Steel Authority of India Ltd., RDCIS, Ranchi, India. Manuscript submitted December 10, 2017.
METALLURGICAL AND MATERIALS TRANSACTIONS A
weldability and resistance to high-temperature corrosion and oxidation on prolonged exposure to an aggressive environment.[1] The alloy is strengthened by addition of molybdenum and columbium. It contains a high level of chromium which along with molybdenum imparts excellent oxidation and corrosion resistance. It also exhibits high strength at cryogenic temperature and can be joined by welding and brazing.[1,2] Generally, the microstructures of superalloys are highly complex, where numerous dispersed intermetallics and other phases are present, which govern the mechanical behavior of the alloy. It can be further described as fcc structure (c-phase matrix) containing number of secondary phases like face-centered cubic(FCC) carbides, ordered FCC c¢, ordered body-centered tetragonal(BCT) c¢¢, ordered intermetallic
compounds, etc. c¢ is the principal strengthening phase in most of the nickel-based superalloys. Its shape varies from spherical to cubic depending on the exposure time and temperature. In the Inconel-625 alloy, Al and Ti contents are 0.4 pct max. each, whereas in Inconel-718 alloy, Al and Ti contents are 0.8 and 1.15 max., respectively. Al+Ti content in both these alloys is < 5 pct, hence no gamma prime phase is expected to form in these alloys. Further, the Fe is around 5 pct max in Inconel-625 alloy and Fe is around 15 to 16 pct in Inconel-718 alloy. Thus, Inconel-718 is a classified as Ni-Fe-based superalloy where in gamma double prime is the prime strengthening precipitate. Extensive literature is available on microstructural evolution and precipitation in Inconel-718 alloy.[3–5] The Mo content is 8 to 10 pct in Inconel-625 and is 2.8 to 3.3 pct in Inconel-718. Thus Inconel-625 is a predominantly solid strengthened with higher Mo content and gamma double prime strengthened alloy. Both solid solution strengthening as well as c¢¢ contribute to high-temperature strength. Although c¢¢ forms in the Inconel-625 alloy, it is not as a primary strengthening precipitate as other M23C6 and M6C type carbides are also present which render the alloy its creep strength.[6] The Inconel-625 alloy, when tested at 700 C, reveals the formation of c¢¢ precipitates. The c¢¢-phase precipitates are ordered A3B type with a composition of Ni3(Nb, Al, Ti) and a tetragonal crystal structure. They possess generally disk-shaped morphology and are coherent with the matrix. Similarly, when the alloy is tested at 725 C, the d-phase prec
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