Failure competition behavior of 9Cr/617 dissimilar welded joint during LCF test at elevated temperature

Failure competition behavior was found after low cycle fatigue (LCF) tests for the dissimilar welded joint between modified 9Cr steel and nickel-based alloy 617 at 600 degrees C. The failure location shifted from base metal (BM) and fine grain heat affected zone (FGHAZ) on the 9Cr steel side to steel/Ni interface with the increase of the strain amplitude. At higher strain amplitude, strain concentration at the steel-Ni interface resulted in steel-Ni interface failure due to the differences in deformability. With the decreasing of the strain amplitude, the lower degree of transformation from lath to cell structure in coarse grain heat affected zone (CGHAZ) caused the competition between interface and FGHAZ failure. FGHAZ failure was governed by higher fraction of dynamic recrystallization (DRX) and formation of cell structure in FGHAZ. DRX was found promoting the formation of cell structure in FGHAZ, thus contributing to its lowest microhardness after LCF tests. As the strain amplitude further decreased, the reduction of DRX fraction made the formation of cell structure dominate the failure. The competing behavior between FGHAZ and BM led to the presence of case for BM failure.