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

Abstract

Small alterations in chemical composition, even within the boundaries of the international standards, can drastically alter the formation kinetics of intermetallic phases in a stainless steel. Therefore, by means of isothermal annealing experiments, the time-temperature-precipitation (TTP) diagram was constructed for an industrially cold rolled and annealed standard duplex stainless steel of type 1.4462 (X2CrNiMoN22-5-3), having a distinct composition. Temperature was varied from 600 to 1050 °C, with annealing times from 10 to 3-10 5 s Two intermetallic phases were observed with scanning electron microscopy (SEM): a phase and X phase, a precipitation occurred in a slightly higher temperature range than X precipitation. In addition, at high temperatures a was the first phase to appear, while at lower temperatures X was the first. This could be explained by the driving force for transformation, which is larger for a at high temperatures and larger for X at low temperatures. The microstructural changes during the heat treatment were studied in detail in order to provide a complete overview of all the phenomena that occur during annealing. At temperatures between 750 and 900 °C precipitation was fastest and all the a was replaced by y and a after prolonged times. The presence of neighbouring ferrite seems to be a necessary condition for the X phase to be stable. The appearance of large volume fractions of a above 700 °C was accompanied by a strong growth of the austenitic phase resulting in a more isotropic microstructure. Beneath 700 °C, the precipitated volume fractions of a 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.