The effect of aging temperature on the morphology of $\sigma$-phase in a ferrite-austenite ($\delta-\gamma$) duplex stainless steel was investigated with Fe-26Cr-6Ni-3Mo alloy. An attempt was made to express the rate of precipitation of $\sigma$-phase with conventional kinetic equations.
The TTT diagram for the $\sigma$-phase precipitation after solution treated at 1050°C for 1hr was constructed. The TTT diagram was C-type with a nose at about 850°C and the time required for 1% $\sigma$-phase precipitation was within 10 minutes.
The $\sigma$-phase nucleated predominently on the $\delta/\gamma$ interface and subsequently grew into the adjacent $\delta$ grains as a eutectoid decomposition of $\delta$-phase into $\sigma$ - and $\gamma$-phase ($\delta\to\sigma+\gamma$), resulting in a $\sigma/\gamma$ duplex structure.
On aging at 700°C, 750°C, and 800°C, the growth of $\sigma$-phase was analogous to the growth of $\sigma/\gamma$ lamellar structure and the lamellar structure was finer at lower aging temperature. However, at 900°C above nose, the growth of $\sigma$-phase was the expansion of the domain of $\sigma$-phase instead of the $\sigma/\gamma$ lamellae.
The kinetics of $\sigma$-phase precipitation during aging treatment followed approximately the Johnson-Mehl equation.