Dissimilar metal welds between martensitic and advanced austenitic high temperature creep resisting steels: Creep rupture test results and fusion line investigation

Abstract

Modern and future power plants will use more elaborate and complex materials to withstand the higher steam temperatures and pressures as well as thermal cycling. As such, dissimilar metal welds (DMWs) will be much more widespread in new designs than before and will need to demonstrate resistance to combinations of severe creep, corrosion/oxidation, and low-cycle fatigue. The DMWs between different steels or alloys are an underestimated topic. Experienced and documented premature failures reveal the criticality of DMWs. This study focused on the DMWs between martensitic and advanced austenitic stainless steels. Two collaborative projects related to 18% Cr and 25% Cr austenitic creep-resistant stainless steel delivered data concerning base metal characterization and weldability of similar and dissimilar welding. The specific features of dissimilar welding are further approached in more detail in this analysis. Two main methodologies, i.e., direct welding and buttering techniques using different filler metals, were investigated and characterized via uniaxial creep rupture testing and metallographic examination, particularly focusing on the fusion line carbide formation and morphology. For the limited exposure times of 10kh in this project, the results show that aligned and linked-up type I carbide formation is occurring when A617-type filler metals are used. The selection of P87 or A82 type filler metals demonstrated isolated to partly aligned carbides, respectively, at the fusion line. Due to a risk for corrosion and the somewhat better creep rupture strengths at comparable conditions, buttering techniques seem to be recommended above direct welding.