Influence of welding on low cycle fatigue properties of Co-based superalloy FSX-414

Co-based superalloys such as FSX-414 have been recently used in gas turbine first stage nozzles. During service, nozzles are exposed to low cycle fatigue, which can lead to cracking of these components. The cracks on these nozzles are usually welded with tungsten arc welding (TIG) using Co-based filler metals. In this paper, the effect of TIG on the tensile and low cycle fatigue properties of Co-based superalloy FSX-414 was studied at 950 °C. The experimental results show that the yield and ultimate tensile stresses of welded and unwelded specimens are comparable to each other. But toughness of welded specimens is lower than that of unwelded ones. The low cycle fatigue properties of FSX-414 were studied at a strain rate of 3.3×10−4 s−1, strain ratio R=−1 (R=εmin/εmax) and Δεt (total strain change) from 0.8% to 2%. In welded specimens, at high strain cycling, the nucleation and growth of cracks occur in the welded zone. But at Δεt=0.8%, fracture occurs in the same zones of unwelded specimens. The results show that the total fatigue lives of the welded specimens are shorter than those of unwelded ones. In all of the low cycle fatigue tests, softening phenomena are observed.