Nanoindentation behavior and creep-induced cracking of long-term crept austenitic steel at 650 °C

The grain boundary (GB) damage of long-term crept HR3C (25Cr–20Ni–Nb–N) austenitic steel with solid solution state was investigated by nanoindentation test accompanied with in-situ electron back-scattered diffraction. The corresponding microstructure was characterized by scanning electron microscopy and transmission electron microscopy. Results show that the increase in nanoindentation hardness at the GBs and triple grain junctions may be related to the dislocation accumulation and carbide growth during the creep. Coarsened M 23 C 6 and dislocations piling-up at the GB accelerate the nucleation and coalescence of creep cavity along the GB. The nanoindentation hardness in grains varies with orientation of the stress axis. The orientation difference of neighbor grains may induce local high geometrically necessary dislocation densities and strain gradients near the GB, consequently causing stress concentration and subsequent crack growth at specific GBs.