Microstructure evolution during isothermal annealing of a standard duplex stainless steel type 1.4462

Small alterations in chemical composition, even within the boundaries of th e international standards, can drastically alter the formation kinetics of intermetallic phases in a stainless steel. Therefore, by means of isotherma l annealing experiments, the time-temperature-precipitation (TTP) diagram w as constructed for an industrially cold rolled and annealed standard duplex stainless steer of type 1.4462 (X2CrNiMoN22-5-3), having a distinct compos ition. Temperature was varied from 600 to 1050 degreesC, with annealing tim es from 10 to 3.10(5) s Two intermetallic phases were observed with scannin g electron microscopy (SEM): sigma phase and chi phase. sigma precipitation occurred in a slightly higher temperature range than chi precipitation, in addition, at high temperatures a was the first phase to appear, while at l ower temperatures chi was the first. This could be explained by the driving force for transformation, which is larger for sigma at high temperatures a nd larger for chi at low temperatures. The microstructural changes during the heat treatment were studied in detai l in order to provide a complete overview of all the phenomena that occur d uring annealing. At temperatures between 750 and 900 degreesC precipitation was fastest and all the alpha was replaced by gamma and sigma after prolon ged times. The presence of neighbouring ferrite seems to be a necessary con dition for the chi phase to be stable. The appearance of large volume fract ions of sigma above 700 degreesC was accompanied by a strong growth of the austenitic phase resulting in a more isotropic microstructure. Beneath 700 degreesC, the precipitated volume fractions of sigma were relatively small and consequently the original banded structure remained clearly visible. At these lower temperatures the mobility of alloying elements is limited and a Widmannstatten like austenite was observed to grow into the ferrite in a needle-like manner.