Probing strain rate effect on the creep-fatigue fracture mechanism of 9%Cr steel-welded joint via nanoindentation characterization

The strain rate-related creep-fatigue (CF) interactions on the long-term service damage of 9%Cr steel-welded joint were investigated at low-strain amplitude (0.2%). Initially, the average CF life gradually increased from 1,103 to 2,185 cycles as strain rate increased from 0.0001 to 0.002 s(-1), and then it slowly decreased from 2,185 to 1,503 cycles as continuously increase the strain rate from 0.002 to 0.005 s(-1). By increasing strain rates, the microscale observation indicates the CF fracture location could be shifted from the fine-grained heat-affected zone (FGHAZ) to the base metal (BM) region. Meanwhile, the local microscale mechanical properties including hardness, elastic modulus, creep resistance, and strain rate sensitivity on the fractured specimens with a series of strain rates were detected by instrumental nanoindentation. With increasing CF strain rates, the nanoindentation hardness and creep resistance evidently dropped. Based on the characteristics of microstructural evolution and the variation in local mechanical properties, strain rate effects of CF interaction on fracture mechanism of the welds were systematically discussed.

Automated summary

The study investigated the strain rate-related creep-fatigue interactions on the long-term service damage of 9%Cr steel-welded joint, revealing that CF life and fracture locations are influenced by strain rates. The local mechanical properties such as nanoindentation hardness and creep resistance were found to decrease with increasing CF strain rates.