Advanced reactors, such as the high temperature gas-cooled reactor, are being developed to solve current and future energy production needs. These advanced reactor designs may benefit by transitioning components via the use of two different alloys joined via arc welding processes. Historically, dissimilar metal welds have been used in industrial applications to change mechanical properties, corrosion resistance, and/or reduce cost. From a design perspective, transitions can be greatly beneficial, but the abrupt changes in material composition and mechanical properties can result in metallurgical issues and localized strain accumulation. Some of the metallurgical issues include solidification cracking, detrimental phase formation, and undesirable phase boundary formation. Additionally, coefficient of thermal expansion, strength, and ductility mismatch result in localized stress and strain that will exacerbate cracking mechanisms and lead to premature failure. An example of this is reduction of creep resistance and premature creep cracking as compared to either alloy’s properties. The American Society of Mechanical Engineers (AMSE) Boiler and Pressure Vessel Code (BPVC) Section III Division 5 instructs the high temperature reactor design rules, which does not provide significant guidance on dissimilar metal welds between two different alloys for elevated temperature service. The purpose of this document is to review the issues specific to dissimilar metal welding and how BPVC Section III Division 5 addresses reduced weld metal properties through stress reduction factors. Additionally, how other ASME BPVC sections address and permit dissimilar metal welds is discussed and recommendations are made regarding dissimilar metal welding.
