The effect of high-temperature exposure on the microstructural stability and toughness property in a 2205 duplex stainless steel

A series of aging treatments at the temperature range of 650–975 °C for different time intervals in a solution-treated 2205 duplex stainless steel were carried out. The corresponding microstructure of aged specimens was observed and the impact toughness was measured. More attention was paid to the secondary phase precipitation and the transition of ductile-to-brittle fracture. The results indicated that the impact toughness of duplex stainless steel was sensitive to the precipitation of σ phase even at the initial stage of aging. Two kinds of Cr-, Mo-enriched intermetallic phases, σ and χ, were found to precipitate preferentially at δ/γ interface boundary and within δ-ferrite grain after 5 min of aging at the temperature range of 875–900 °C. The volume fraction of σ phase was continuously increased with the time of aging and σ phase developed into a coarse particle due to the high diffusibility of solute atoms at high temperatures. The precipitation of Mo-enriched χ phase at the initial stage of aging is presumably enhanced due to the low interfacial energy of highly coherent χ/δ interface with a characteristic cubic-to-cubic orientation relationship. However, this pre-formed χ phase was re-dissolved eventually into the σ phase when the specimen aged above 750 °C. Accompanied with the growth of secondary phases, the γ phase nearby the interface of σ or χ phase was induced to grow into the δ-ferrite region which was depleted in Cr and Mo. The δ-ferrite in original duplex structure would be completely decomposed into the σ phase and secondary γ after long-term aging.