Properties and characteristics of duplex stainless steel
The relative amount of ferrite and austenite in the rolled or processed product of duplex stainless steel depends on its chemical composition and hot working history. As the phase diagram shows, small changes in composition can have a large effect on the relative volume fraction of the two phases. A single alloying element promotes the formation of ferrite or austenite. The phase equilibrium of ferrite/austenite in metallographical structures can be predicted by multivariate linear regression as follows:
Creq = %Cr + 1.73% Si + 0.88% Mo
Nieq = %Ni + 24.55%C + 21.75%N + 0.4%Cu
% ferrite = -20.93 + 4.01 Creq - 5.6 Nieq + 0.016 T
T (Celsius) is the annealing temperature, 1050-1150°C, and the element content is percent by weight (wt%).
By adjusting the content of chromium, molybdenum, nickel and nitrogen, and controlling the heating operation, the desired phase balance can be obtained, that is, the proportion of ferrite phase is 45-50%, and the rest is austenite phase.
For the rolling production of duplex stainless steel, the best results can be obtained by solid solution annealing at an appropriate annealing temperature followed by water quenching. It is important that the time between the material leaving the heating furnace and water quenching is as short as possible, thus minimizing heat loss, which can lead to the precipitation of harmful phases before the water is quenched to room temperature.
For the welding of duplex stainless steel, the optimal heat input must be given for each grade and welding configuration so that the cooling rate is fast enough to avoid the precipitation of harmful phases, but not so fast that excessive ferrite is retained near the fusion line. In practice, this may occur when welding parts with large differences in section size or when welding thick sections with low heat input. In this case, the thin weld on the thick section is quenched too quickly and does not allow sufficient time for a sufficient amount of ferrite to be converted to austenite, resulting in excessive ferrite in the welding heat affected zone.
Since nitrogen increases the temperature at which austenite begins to form from ferrite, as shown in Figure 3, it also speeds up the conversion of ferrite to austenite. Therefore, if the stainless steel contains nitrogen, even at a relatively rapid cooling rate, the amount of austenite can almost reach the level of equilibrium. For second-generation duplex stainless steels, this effect reduces the possibility of excess ferrite in the heat-affected zone of the weld.
2 Precipitated phase
The harmful phase can be formed in a few minutes at the critical temperature, as shown in the isothermal precipitation diagram of 2205 and 2507 duplex stainless steel in Figure 4. The precipitation phase significantly reduces the corrosion resistance and toughness of stainless steel, so the cumulative residence time in the precipitation temperature range, such as the forming operation, especially the welding and cooling time after annealing, must be reduced as much as possible. Modern duplex stainless steel grades that maximize corrosion resistance and delay harmful phase precipitation have been developed to enable smooth manufacturing. However, once the harmful phase is formed, it can only be removed by complete solution annealing followed by water quenching
