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 degreesC 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 a phase even at the initial stage of aging. Two kinds of Cr-, Mo-enriched intermetallic phases, sigma and chi, were found to precipitate preferentially at delta/gamma interface boundary and within delta-ferrite grain after 5 min of aging at the temperature range of 875-900 degreesC. The volume fraction of a phase was continuously increased with the time of aging and sigma phase developed into a coarse particle due to the high diffusibility of solute atoms at high temperatures. The precipitation of Mo-enriched chi phase at the initial stage of aging is presumably enhanced due to the low interfacial energy of highly coherent chi/delta interface with a characteristic cubic-to-cubic orientation relationship. However, this pre-formed chi phase was re-dissolved eventually into the sigma phase when the specimen aged above 750 degreesC. Accompanied with the growth of secondary phases, the gamma phase nearby the interface of sigma or chi phase was induced to grow into the delta-ferrite region which was depleted in Cr and Mo. The delta-ferrite in original duplex structure would be completely decomposed into the a phase and secondary gamma after long-term aging. (C) 2002 Elsevier Science B.V. All rights reserved.