Qualitative and quantitative analysis of σ and χ phases in 2205 duplex stainless steel

The ferritic matrix in duplex stainless steels undergoes a variety of decomposition processes when aged in the temperature range 650-900 °C. These processes involve the precipitation of austenite, sigma (σ) and chi (χ) Frank-Kasper phases. The intermetallic σ and χ phases are found at both the grain boundaries and inside the ferritic grains. The effect of isothermal treatment at 750 °C on the microstructure of 2205 duplex stainless has been investigated. The purpose of the research was to work out the qualitative and quantitative analysis of precipitations nucleated after aging 2205 duplex stainless steel. Two kinds of Cr- and Mo-enriched intermetallic phases, σ and χ, were found to precipitate preferentially at ferrite-austenite interface and within the ferrite phase. The volume fraction of σ phase increased with the time of aging and developed into coarse particles. A methodology of duplex stainless steel microstructural description was developed, including: preparation, selective etching methods, image acquisition and analysis, selection of the quantitative description parameters and surface distributions of stereological parameters.