Strengthening of σ phase in a Fe20Cr9Ni cast austenite stainless steel

The strengthening mechanism of σ phase in a Fe20Cr9Ni cast austenite stainless steel used for primary coolant pipes of nuclear power plants has been investigated. The yield and ultimate tensile strengths of aged specimens increased comparing with those of the unaged ones. It was found that the increase of strengths is due to the hard and brittle (σ + γ2) structure which decomposed from α phase in the steel. Fracture surfaces of specimens after in situ tensile test showed that the inhibition of (σ + γ2) structure on the dislocation movements was more significant than ferrite although cracks started predominately at σ/γ2 interfaces. The (σ + γ2) structure behaves like a fiber reinforced composite material.