Microstructural Evolution of Inconel 625 and Inconel 686CPT Weld Metal for Clad Carbon Steel Linepipe Joints: A Comparat
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CARBON steel linepipe internally clad with a corrosion resistant alloy (CRA) is used in the offshore oil and gas industry as a cost effective means for the transport of corrosive product. To avoid localized corrosion within the weld zone of the CRA clad linepipe, a highly alloyed nickel based filler metal is utilized. However, in addition to providing a superior corrosion resistance to that of the CRA layer, the nickel-based filler metal selected must demonstrate overmatching properties in terms of yield strength and toughness. A recently developed nickel based superalloy being utilized for the dissimilar fusion welding of bi-metallic offshore pipelines is INCO-WELD 686CPT (IN686CPT). IN686CPT is manufactured for its superior corrosion resistance and has sufficiently high yield strength to overmatch high grade carbon steel linepipe. However, the microstructural evolution of the weld metal upon solidification is not well documented, and there has been no investigation into the segregation behavior of alloying additions when used for dissimilar CHARLES A. MALTIN, Research Student, and ALEXANDER M. GALLOWAY, Associate Dean (International), are with the Department of Mechanical and Aerospace Engineering, University of Strathclyde, Level 8, James Weir Building, 75 Montrose Street, Glasgow G1 1XJ, U.K. Contact e-mail: charles.maltin.2013@uni. strath.ac.uk MARTIN MWEEMBA, Specialist Engineer, is with the Welding and Materials Department, Subsea 7, East Campus, Prospect Road, Arnhall Business Park, Westhill, Aberdeen AB32 6FE, Scotland, U.K. Manuscript submitted May 25, 2013. Article published online April 23, 2014 METALLURGICAL AND MATERIALS TRANSACTIONS A
weld applications. In the present study, the microstructural evolution of weld metal produced by IN686CPT is compared to that of Inconel 625 (IN625), whose solidification behavior has been widely reported.[1–10] It is worth noting that a majority of solid solution strengthened nickel base alloys are developed to be single phase, face-centered cubic (FCC) austenite (c).[11] However, due to solidification mechanisms and certain alloys exhibiting elemental compositions that are beyond the solubility limit, for the metal matrix in which they are solutioned, brittle secondary phases may be formed.[11] Welding presents an additional metallurgical challenge associated with nickel base superalloys; due to the variable parameters which may be employed and an elevated cooling rate (relative to equilibrium cooling), the microstructure is highly changeable and therefore difficult to predict. Given that both the mechanical and corrosion properties of the weld metal are governed by the microstructure and its evolution upon solidification, there have been extensive studies into the behavior of nickel-based weld metals and the effect of alloying additions on their solidification behavior.[1–5,11–14] In terms of predicting the likely phases to be precipitated, it has been reported[4,5,12,15] that the tendency for certain alloying additions to segregate to either the dendrite core or the interde
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