By:  Terry Totemeier

Introduction
A dissimilar metal weld (DMW) is created whenever alloys with substantially different chemical compositions are welded together – for example, when a low-alloy steel such as Grade 22 (2¼ Cr-1Mo) is welded to an austenitic stainless steel such as TP304H (18Cr-8Ni).  Many DMWs are commonly present in fossil-fired power plants, examples being material transitions in boiler furnace tubes, stainless steel attachments welded onto ferritic steel tubes or pipes, and stainless steel thermowells or steam sampling lines in ferritic steel pipes.  The chemical composition gradients associated with DMWs present unique issues relative to their design, in-service behavior, and life management, particularly for those DMWs operating at elevated temperatures where solid-state diffusion and cyclic thermal stresses are factors, which was previously presented in News and Views (Volume 43, page 19).

With the now widespread use of Grade 91 steel (9Cr-1Mo-V-Nb) for elevated-temperature applications in modern power plants, DMWs involving this material have become common, and increasing service experience has revealed some unique characteristics and failure mechanisms, especially in thicker-section DMWs with austenitic materials.  This article presents a short overview of Grade 91 DMWs:  their design, fabrication, and failure, with emphasis on current industry issues.

There are two basic classes of DMWs in Grade 91 steel:  ferritic-to-ferritic and ferritic-to-austenitic.  The first type corresponds to Grade 91 welded to another ferritic steel with a lower chromium content, such as Grade 22; the second type corresponds to Grade 91 welded to an austenitic stainless steel such as TP304H.  Each of these types has unique concerns and considerations.

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