Grain refinement of Cr25Ni5Mo1.5 duplex stainless steel by heat treatment

A technology employing σ transformation (α→γ+σ) and its reverse transformation (γ+σ→α) to refine grain of duplex stainless steels has been devised. The experimental results indicated that the σ phases nucleate and grow both at the α/γ phase interfaces and in the interior of the α phases when the temperature was higher, whereas the σ phases nucleate at the α/γ phase interfaces primarily when a lower temperature was used. The coarse α grain decomposed into fine γ and σ grains at 750 °C for 38 h (the σ treated), and fine α grains were obtained after a revise transformation (γ+σ→α) being performed, leading to an significant improved toughness. The highest impact toughness value was achieved by the process that σ transformation was carried out at 750 °C for 38 h followed by a reverse transformation, heated at 1200 °C for 1.5 min, at a heating rate of 13 °C/s.