Assessment of ductility dip cracking susceptibility on Ni based alloy by FEM simulation

Abstract

Alloy 690 is a Ni–Cr–Fe type alloy with a widespread application in nuclear power plants. However, the companion filler metal 52M (FM-52M) is susceptible to ductility dip cracking (DDC), which seriously affects safety and life extension in nuclear application. To provide a better understanding of DDC formation in the varestraint test, local strain is adopted to assess materials resistance of DDC instead of average bending strain. In this work, spot varestraint tests of FM-52M were performed with an average strain (0·25–7%). Then, a thermomechanical simulation of the spot varestraint tests was modelled based on the experiment. The simulation obtained preferential propagation orientation of DDC and accumulated strain increment during a susceptible temperature range. Simulated local strain caused at an elevated temperature is much larger than the average strain. Besides, prediction of DDC distribution agrees well with experimental results. The finite element model presents a clear local strain distribution and provides an extra understanding of DDC formation.