Research Methods for Engineers 4
materials in the proximity of the fusion line of the dissimilar metal weld. This causes reduced
strength or overall performance of the weld.
Reference number: 03
Title of article: Review of Dissimilar Metal Welding for the NGNP Helical-Coil Steam
Generator
Authors: John N. DuPont Ronald E. Mizia
Publication: U.S. Department of Energy Office of Nuclear Energy Under DOE Idaho
Operations Office
Type: Journal
Year Published: March 2010 Number of citations: 88
Primary or secondary: Primary
What was the research question? research and development of a new high-temperature gas-
cooled reactor (HTGR) that is capable of providing high-temperature process heat for industry
What themes were discussed in the literature review? Failure of DMWs below the
expected creep life of either base metal and well below the design life of the plant. The failure
time depends on a wide range of factors related to service conditions, welding parameters, and
alloys involved in the DMW
Design: Alloy 800H was expected to be used for the superheater section, and 2.25Cr-1Mo
steel was expected to be used for the evaporator-economizer section. Dissimilar metal welds
(DMW) were needed to join these two materials
What were the findings? The microstructure of DMWs in the as-welded condition consisted
of a sharp chemical concentration gradient across the fusion line that separated the ferritic and
austenitic alloys. Upon cooling from the weld thermal cycle, a band of martensite forms
within this concentration gradient due to high hardenability and the relatively rapid cooling
rates associated with welding. Upon aging, during post-weld heat treatment (PWHT), and/or
during high-temperature service, C diffuses down the chemical potential gradient from the
ferritic 2.25Cr-1Mo steel toward the austenitic alloy. This can lead to the formation of a soft C
denuded zone near the interface on the ferritic steel, and nucleation and growth of carbides on
the austenitic side that is associated with very high hardness.
What were the gaps? A review of available creep-rupture data for DMWs indicates that a
substantial reduction in creep life can be expected relative to the minimum creep-rupture life
of 2.25Cr-1Mo base metal and welds. In addition, the difference in creep life between the
DMWs and 2.25Cr-1Mo steel is expected to increase with increasing time at high temperature.

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