The welding metallurgy of HASTELLOY alloys C-4, C-22, and C-276
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I.
INTRODUCTION
HASTELLOY Alloys C-4, C-22, and C-276 are highly corrosion-resistant nickel-base alloys derived from the NiCr-Mo ternary system. In addition to the expected impurities (Table I), Alloy C-4 has an intentional Ti alloying addition, and Alloys C-22 and C-276 have W and Fe as additional alloying components. These materials are nominally singlephase, solid-solution strengthened alloys not hardenable by conventional aging treatments. The thermal stability of these and similar (e.g., H A S T ELLOY S) alloys in the mill-annealed condition has been investigated in some detail. ~-s Long range ordering has been observed t 4 in which the disordered face-centered-cubic matrix transforms to an ordered orthorhombic superlattice (isomorphous with Pt2Mo). This phenomenon generally occurs only after at least several hundred hours in the 600 ~ temperature range and hence is not important in the consideration of the fusion zone microstructure generated by cooling rates characteristic of arc welding. 9 Precipitation of intermetallic phases (/x, P) and carbides has also been observed. 1.5-8 The intermetallic compounds are the most relevant to the present study. Leonard s has shown that P phase can precipitate in Alloy C-276 within a few minutes of exposure at 875 ~ He also suggests that/x phase is the long-time transformation product of P phase. Hodge and Kirchner 6 found no evidence o f / z phase formation in Alloy C-4 during isothermal heat treatments in the temperature range 650 ~ to 1090 ~ for times up to 100 hours. Matthews ~ reported that /z phase precipitation occurs in Alloy C-4, but only after extended heat treatment (-> 1000 hours) in the 800 ~ temperature regime. Recently, Cieslak et al. ~o reported on the occurrence of topologically-close-packed phases in Alloy C-22 and Alloy C-276 weld metal. Alloy C-276 contained both the P and/z *HASTELLOY is a trademark of Cabot Corporation.
M. J. CIESLAK, Div. 1833, Process Metallurgy, T. J. HEADLEY, Div. 1822, Electron Optics and X-Ray Analysis, and A. D. ROMIG, Jr.. Div. 1832, Physical Metallurgy, are with Sandia National Laboratories, P.O. Box 5800, Albuquerque, NM 87185. Manuscript submitted January 2, 1986. METALLURGICALTRANSACTIONS A
phases. Alloy C-22 contained P, /x, and o- phases. All of these phases are possible equilibrium structures in the NiCr-Mo ternary system. ~,z2.~3Raghavan et al. ~3observed that P and cr phases were present in the near-solidus (1250 ~ isothermal section of the Ni-Cr-Mo system; and P, ~r, and /x phases were present in the 850 ~ isothermal section. Bloom and Grant ~2speculated on the high temperature invariant reactions above 1250 ~ in the Ni-Cr-Mo system. In the composition range of importance relative to commercial alloys, the possible equilibrium phases were liquid, austenite (y), P, and o-. In this paper we report the results of hot-cracking susceptibility tests on Alloys C-4, C-22, and C-276, and the identification of the minor phases (P, o-,/x, or MC carbide) in the solidified weld microstructures. An equivalent chemistry model
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