| The development of high alloy super austenitic stainless steels is one of the most important directions of high quality stainless steels. It is well known that super austenitic stainless steels (SASS) provide the highest corrosion resistance obtainable with stainless steels due to its high alloy (Cr, Mo, Ni and N) content, which makes them becoming alternative materials of nickel-based alloys and titanium alloys. However, super austenitic stainless steels are very susceptible to secondary phase formation because of their high Cr and Mo content, and these secondary phases usually influence the manufacture and application of SASS and reduce the service life of steels. Therefore, studying the aging precipitation behavior and its influence on the corrosion resistance of the steels is able to provide guidance for the heat treatment and hot working processes and the application of super austenitic stainless steels.In this article, a super austenitic stainless steel of Fe-23.4Cr-21.8Ni-7.5Mo-0.42N is melted by a 500 kg vacuum induction furnace. Aging precipitation behavior of the super austenitic stainless steel with high Mo (7.5%) and N (0.42%) (HHSASS) is well studied by Thermo-Calc software, optical microscope (OM), scanning electron microscope (SEM), X-Ray Diffraction (XRD) and transmission electron microscope (TEM), compined with phase transformation theories. The anodic polarization test is conducted to investigate the pitting corrosion resistance of solution annealed HHSASS in different solution systems. The influence of aging treatment on the intergranular corrosion resistance of the steel is studied by double loop electrochemical potentiokinetic reactivation (DL-EPR) test and laser scanning confocal microscopy.The results made by Thermo-Calc software shows that HHSASS forms four precipitates (σ phase, Laves phase, nitride (hcp) and M23C6) when being exposed at high temperature,σ phase is the main precipitate and the content of M23C6 is very low. The upper limit of precipitation temperature of σ phase, Laves phase and nitride is about 1170℃,800℃ and 1200℃. C and N dely the precipitation of σ phase and Cr and Mo promote the formation of σ phase and Laves phase. The volumn fraction of precipitates is decreasing with the temperature increasing.The solution treated condition of HHSASS is identified by OM, SEM and XRD as 1170 ℃ for 60 min, followed by water quenching to make the secondary phases completely dissolve into austenitic matrix and prevent the grains coarsening too much. Initial time-temperature-precipitation (TTP) curve of aged HHSASS which starts with 4% and 7% precipitation volume fraction is investigated by quantitative metallography method. The TTP curve is C-shaped and the "nose" temperature of precipitation is found to be 1000℃.At 1000℃, the volumn fraction of precipitates of HHSASS increases and the rate of precipitation decreases with the aging time increasing. The precipitates form on the grain boundary at the beginning of aging, then with the aging time increasing, the precipitates connect along the grain boundary and start forming as needle-shaped within austenite grain. Then the number of precipitates continues to increase and the nucleated precipitates are growing. The field is filled with precipitates when HHSASS is aged for 5 h. The precipitates on the grain boundry are ellipsoidal and the precipitates in the grain are needle-shaped. The precipitates are rich in Cr and Mo. Three phases (σ phase, Laves phase and M23C6) in aged HHSASS are found by TEM. Nucleation of precipitates on the grain boundry of HHSASS is heterogeneous nucleation, and the nucleation rate decreases to zero rapidly. The growth rate of precipitates on the grain boundry is inversely proportional to t1/2. At the temperature ranging from 600℃ to 1000℃, the volumn fraction of precipitates is increasing. However, it decreases at 1050℃, which shows the higher dissolution rate of the precipitates.The pitting corrosion resistance of HHSASS and 316L is investigated by anodic polarization test. The HHSASS has longer passivation zone and higher pitting potential in 3.5%NaCl,1 mol/L NaCl+1 mol/L H2SO4 and 11.4% H2SO4+1.2% HCl+1% CuCl2+1% FeCl3 than 316L, which shows its higher corrosion resistance. The PREN of HHSASS is also much higher than that of 316L.With aging time increasing, the Ir/Ia and Qr/Qa of HHSASS investigated by EPR method increases, which shows the increasing intergranular corrosion sensibility. The reason is that three phases (σ phase, Laves phase and M23C6) are found in HHSASS when aged at 1000℃ and σ phase is the main precipitate. These phases are rich in Mo and Cr, and form on the grain boundry at the beginning of aging, which makes the diffusion of Mo and Cr to the grain boundry from the matrix. This makes the deletion of Cr and Mo near the grain boundry, which results in the intergranular corrosion of HHSASS. Therefore, with the aging time increasing, the content of precipitates in HHSASS increases, making the intergranular corrosion sensibility increases. |
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