What effect does heat treatment have on the properties of duplex stainless steel plates?

The main reason for the decrease in corrosion resistance of duplex stainless steel after heat treatment at different temperatures is the change in phase ratio and the precipitates during the heat treatment process. The influence of the two-phase ratio on pitting corrosion performance is mainly related to the change of alloy elements in the phase. The pitting corrosion potential is closely related to the austenite content. As the austenite content increases, the pitting corrosion potential of the steel decreases. Piting corrosion occurs on the austenite phase within the ferrite crystal. The chromium content in the ferrite phase increases with the increase in the austenite content in the steel, while the chromium content in the austenite phase within the grain boundaries increases. It is relatively reduced, but the content of the austenite phase in the northern grade is higher. The distribution of nickel content in the two phases is exactly opposite. The nickel content in the austenite phase grain boundaries is lower than that in the grains. The molybdenum content does not change much in the two phases. It can be seen that since the chromium and molybdenum content in the austenite phase is lower than that in the ferrite phase, and the chromium content of the austenite phase within the grain is lower than that at the grain boundaries, the austenite phase within the grain has priority. Corrosion pitting occurs. In the two-phase ratio, the higher the austenite content ratio, the greater the chance of pitting corrosion. The test results are consistent with the two-phase ratio analysis and theory.

Other precipitated phases during heat treatment are also a possible reason for the decrease in pitting corrosion resistance of duplex stainless steel. Duplex stainless steel is an ultra-low carbon stainless steel. The amount of carbide precipitation at the phase boundary is very small and cannot even be distributed to all phase boundaries. Therefore, there is no chromium deficiency caused by the precipitation of carbide and thus intergranular corrosion. The results of the nitric acid corrosion test and the ferric sulfate sulfate corrosion test show that the duplex stainless steel after heat treatment does have obvious intergranular corrosion tendency.

This needs to be analyzed from the influence of the precipitation of other phases during the heat treatment process on the corrosion resistance of duplex stainless steel. Phase is the most harmful precipitation phase in duplex stainless steel. The existence of molybdenum expands the temperature range of σ phase precipitation and shortens its formation time, so that it can still exist at high temperatures and can precipitate within a few minutes. The σ phase is a brittle phase that precipitates along the grain boundaries, seriously affecting the intergranular corrosion resistance of stainless steel. Duplex stainless steel is a duplex stainless steel containing molybdenum. During the slow cooling to room temperature, the σ phase will inevitably precipitate. However, after the duplex stainless steel is dissolved, the precipitated σ phase can be basically eliminated, ensuring that it has good corrosion resistance. . The corrosion resistance of duplex stainless steel after solution treatment at 1050°C is significantly better than that after other mild treatments.