Three-dimensional characteristic and evolution of creep cavity and microcrack of HR3C austenitic heat resistant steel after long-term creep at 650 °C

Three-dimensional characteristics of creep cavities and microcracks in HR3C austenitic steel after creep at 650 °C were investigated using high resolution X-ray computed tomography (CT) and three-dimensional atomic probe (3DAP). The variation of cavity volume fraction is related to the increasing fraction of cavities undergoing growth and coalescence. Irregular lamellar or “W” shaped microcrack along the grain boundary were observed by the CT and the number large sized microcracks decreased significantly with decreasing stress levels. 3DAP results show that the concentration of P elements was high at the M₂₃C₆/γ interface at the early stage of creep. With the increase of creep time, M₂₃C₆/γ interface have been found to not only contain high concentration of P element, but also be enriched with Si. The coarsening of the carbides at the grain boundary and the enrichment of P and Si elements at the interface creates the conditions for the nucleation of creep cavities and microcracks under stress. Thus, stress has an tremendous influence on the distribution of creep cavities and the propagation path of microcracks. Additionally, with the decrease of stress, the creep damage mode changes from intergranular/transgranular mixed cracks to creep cavity.