A Systematic Investigation of Precipitates in Matrix and at Grain Boundaries in an Alumina-Forming Austenitic Steel During Creep Testing at 700 °C

A systematic investigation of the microstructure in an alumina-forming austenitic (AFA) stainless steel at different time (0 to 2821 hours) reveals the diverse evolution of precipitates both at grain boundaries (GBs) and in matrix. In the initial stage at 700 °C, the second-phase precipitates in the AFA steel occurred in the sequence of M₂₃C₆ carbide at GB (0.5 hour), B2-NiAl phase at GB (1 hour), MC carbide in matrix (5 hours), and L1₂-Ni₃Al in matrix (10 hours). During creep test, the coarsening of L1₂-precipitates in the matrix followed Oswald ripening law, i.e., the size was proportional to t^1/3 (t as the creep time). However, the width of precipitation free zone near GB was proportional to t^1/2 and B2 phase at GB increased linearly with time. Meanwhile, the M₂₃C₆ carbides, which mainly precipitated at GB, kept at nanoscale within 500 hours and grew ultimately to micrometer size. Consequently, the strength of GBs weakened faster than the grain interior and became similar to the strength of the grain interior after the equicohesive time t_E. The creep rupture mode correspondingly changed from transgranular to intergranular after long-term creep rupture. Hence, both strengthening and weakening by precipitation ought to be considered for creep life prediction.