Effect of aging on secondary phases and properties of an S304H austenitic stainless steel

Effect of aging up to 3000 h at 923 K on the microstructure, the secondary phases, the mechanical properties, and the intergranular corrosion was examined in an S304H-type austenitic stainless steel. The Cu-rich and NbC-type particles with sizes of 1.5 nm and 50 nm, respectively, were uniformly distributed throughout whileM23C6-type carbides with a size of 40 nm precipitated mainly on the grain boundaries after aging for 100 h. Upon further aging a relatively fast coarsening of the boundary M23C6 carbides in accordance to a power law relationship with a particle growth exponent of 4 led to the formation of semi-continuous chains of these carbides along the grain boundaries. These chains diminished the fracture toughness; moreover, the steel became susceptible to intergranular corrosion. On the other hand, the uniform dispersions of NbC carbides and Cu-rich particles were less susceptible to coarsening under aging. The growth behavior of these particles could be expressed with a particle growth exponent of 3. A depletion of solid solution due to precipitate coarsening led to the ferrite appearance primarily along the austenite grain boundaries. The dispersion strengthening by carbide/nitride particles gradually decreased while that from Cu particles increased during annealing, resulting in the tension strength slightly increased with aging duration.