Hot tensile properties of 25Cr-8Ni duplex stainless steel containing cellular (σ+γ2) structure after various thermal treatments

The influence of cellular (σ+γ2) structure on the hot tensile properties of duplex stainless steel aged at 750°C was studied. Various solution treatments were employed and subsequent ageing of the specimens at 750°C resulted in (σ+γ2) structure with different morphology and distribution within the austenite matrix. The amounts of σ precipitated at 750°C in different samples did not depend on the prior solution treatment temperature. These samples were tensile tested at 550 and 750°C, respectively. The yield strengths and ultimate tensile strengths increased while the ductility dropped in all aged samples compared with the unaged ones. The increase in strength is due to the replacement of the softer α phase with the harder but brittle (σ+γ2) cellular structure. Fracture surface analysis showed that cracks started predominately at σ/γ2 interfaces. The cracks then merged and propagated in a direction perpendicular to the tensile direction within the cellular structure. With appropriate solution treatment to increase the spacing between the (σ+γ2) structure, the presence of cellular structure in the steel can increase the strength of the material and retains sufficient ductility at high temperatures.