In 1999 and 2000, three bimetallic welded joints failed in our Kellogg Brown & Root, Inc., (KBR) ammonia processing plant designs. One was at an inlet and others were in the shells of Ammonia Converter Effluent Heat Exchangers (identified as 123Cs). These failures were in high-pressure hydrogen services at two plant locations and occurred within about 5 and 10 years of operation. The exchanger operates with ammonia converter effluent on the shellside and boiler feed water on the tubeside. This paper presents a portion of the metallurgical failure investigation completed for one of the failures. In addition, failure avoidance of the dissimilar weldment (for repairs) and within current designs is noted. The particular failed weld joints were bimetallic, where nickel alloy filler metal “buttering” was applied to low chrome alloy base material. The “buttered” low alloy material had subsequently been postweld heat-treated. The weld connecting the parts together was then completed using common nickel alloy filler materials without additional treatment in order to provide safe repairs for return to service. From these failures, the owners have learned what action steps are required. KBR also issued an “Advisory” to other operators in order to avoid failures of similar weld joints in other plants.