Effect of Middle Temperature Aging on Precipitated Phase and Intergranular Corrosion Properties of 2205 Duplex Stainless Steel
Various precipitation phases appear in duplex stainless steel during heating. This paper mainly studies the effect of isothermal aging on the microstructure and intergranular corrosion properties of DSS2205 duplex stainless steel. Studies have shown that for solid solution samples, there is no precipitated phase; after aging at 650°C for 0.5h, there is a small amount of precipitated phases, and as the aging time increases, the precipitated phases increase. The double-loop electrochemical dynamic potential test showed that the solid solution sample has the greatest resistance to intergranular corrosion. As the aging time increases, the degree of intergranular corrosion of the material gradually increases.
Duplex stainless steel has been widely used in chemical, petroleum and other fields because of its good mechanical properties and corrosion properties in harsh environments. Due to the high content of alloying elements and their segregation in the two phases, the duplex stainless steel shows complicated phase transitions and precipitation phenomena during improper heat treatment or welding at 300 to 1000 ℃, and a large number of secondary harmful phases will be precipitated. For example: carbides, nitrides, metal mesophases (χ,σ), amplitude modulation decomposition brittle phases (Cr-rich α’) [1-2]. The precipitation of these secondary phases leads to the appearance of chromium-depleted areas around them, causing the corrosion performance and mechanical properties of the material to decrease [3-4]. In order to develop high-performance stainless steel products and design a reasonable processing technology, it is necessary to deeply study the effect of heat treatment on the structure and corrosion properties of duplex stainless steel. This article mainly studies the microstructure and intergranular corrosion properties of 2205 duplex stainless steel after aging at 650℃ for different times, and discusses the effect of heat treatment process on the local corrosion properties of 2205 duplex stainless steel.
Experimental materials and methods
Sample heat treatment
The composition of the 2205 duplex stainless steel used in the experiment is shown in Table 1. After cutting the 2205 wire to a 10mm×10mm×5mm block shape, it was solution treated at 1050°C for 0.5h, and then water quenched. The samples after solution treatment were aged (sensitized) at 650°C for 0.5, 8, and 100 hours, respectively.
Tab.1 Chemical composition of DSS2205 steel experimental samples(wt%)
The samples were sequentially polished with 150#, 240#, 600#, 1200# SiC sandpaper, polished with diamond paste, acetone ultrasonic cleaning, and hot air drying. First use 10% oxalic acid for electrochemical corrosion at 2V voltage for 10s, and then use 30% KOH for electrochemical corrosion at 2V voltage for 10s . The morphology of the microstructure was analyzed with a scanning electron microscope-energy spectrum system (SEMPhillipsXL30FEG).
Intergranular corrosion performance test
The double-ring potential dynamic reactivation method (DL-EPR) is used to characterize the intergranular corrosion performance. The test is divided into three steps:
- ① Cathodic polarization at -0.9V potential for 120s to remove the oxide on the surface to be tested.
- ② In the case of an open circuit, measure the potential at both ends and record the open circuit potential E0 after the potential is stable.
- ③ Scanning from E0 to the anode direction at a certain voltage scan rate and an activation peak appears. When the scan potential enters a certain position in the passivation zone (this article is set to 0.3V), it immediately scans back at the same scan rate until the potential reaches the original Stop the test after the open circuit potential.
Experimental results and discussion
Organizational structure characterization
Figure 1 shows the XRD diffraction pattern of DSS2205 duplex stainless steel aged at 650℃ for different times. It can be seen from the figure that after the sample is aged for 8 hours, other diffraction peaks other than ferrite and austenite are still not observed on the XRD diffraction pattern, and the weak characteristic peak of σ phase is not observed until 100 hours of aging. This is mainly due to the small number of precipitated phases at this temperature and the insufficient detection sensitivity of the instrument. Correspondingly, as the aging time increases, the peak value of the characteristic peak of the austenite phase increases, while the peak value of the characteristic peak of the ferrite phase decreases. This shows that after the sensitization treatment of DSS2205 duplex stainless steel, the ferrite phase has undergone an obvious eutectoid transformation (δ→σ+γ2 and δ→σ+Cr2N) [5-6].
Figure 2 shows the microscopic morphology of a series of samples at different aging times at 650℃. It can be seen from the figure that there is no precipitation phase in the sample after solution treatment, and the basic structure is still composed of austenite and ferrite (Figure 1(a)). However, after 0.5h of aging treatment, a small amount of σ phase precipitated at the grain boundary between austenite and ferrite (Figure 1(b)). With the extension of time, a large amount of σ phase precipitated in the austenite and ferrite grain boundaries and in the original ferrite in the samples after the aging treatment for 8,100h (Figure 2(c), (d)).
Fig.1 XRD diffraction patterns of DSS2205 samples after aging at 650℃ for different time
Fig.2 SEM microstructure of 2205 duplex stainless steel after aging at 650℃ for different time
Intergranular corrosion performance
Figure 3 is a typical bicyclic potential reactivation method DL-EPR curve measured after DSS2205 steel is aged at 650℃ for different time. The maximum current of forward scanning is defined as activation current (Ia), and the maximum current of reverse scanning is defined as reactivation current (Ir). It can be seen from the figure that all the samples have obvious passivation areas. The samples without sensitization treatment have no flyback current (Ir) peaks after the DL-EPR test, while the DL- Tiny Ir peaks can be found in the EPR test curve. This shows that under the test conditions, the samples without sensitization almost have no intergranular corrosion, while the samples after sensitization for 1h have lighter intergranular corrosion. When the sample is sensitized for 8 hours, two obvious Ir peaks appear on the DL-EPR curve. WuTF et al.  researched that the peak with more positive potential corresponds to the intergranular corrosion current, while the peak with more negative potential Pitting current from the substrate. When the aging time is 100h, the peak reactivation current is the largest. Due to the aging treatment, various precipitation phases are produced, and these precipitation phases are all chromium-rich phases. Therefore, a chromium-poor zone appears around, and the passivation film in the chromium-poor zone is relatively weak. During the potential flyback, the surface passivation film at the chromium-poor zone is broken, resulting in a reactivation current peak. From the previous analysis, it can be seen that the aging time is prolonged, the precipitation phase increases, and the chromium-depleted area caused by the precipitation phase also increases, so the peak value of the reactivation current also increases. A chromium-depleted zone appears around the precipitated phase, and intergranular corrosion is mainly caused by the chromium-depleted zone.
Figure 4 shows the microstructure after the intergranular corrosion test. It can be seen from the figure that after the intergranular corrosion test, the structure of the sample after aging for 0.5h is not significantly corroded, while the sample after aging for 8h not only corrodes the ferrite/austenite grain boundary, but also the ferrite/ferrite grain boundary It is also corroded, which is consistent with the results of DL-EPR. The aging time is prolonged and the intergranular corrosion of steel is aggravated.
Fig.3 Double-loop electrochemical potentiokinetic reactivation (DL-EPR) test curves
Fig.4 Intergranular corrosion microstructure of 2205 duplex stainless steel after aging at 650℃ for different time
- (1) During the aging treatment of DSS2205 steel, the α→γ2+σ and α→Cr2N+σ reactions mainly occur, and a chromium-depleted zone appears around the precipitated phase. The occurrence of intergranular corrosion is mainly caused by the depleted chromium area around the precipitated phase.
- (2) For solid solution samples, there is no precipitated phase; after aging at 650°C for 0.5 h, there is a small amount of precipitated phase. As the aging time increases, the precipitated phase increases.
- (3) When DSS2205 steel is aged at 650℃, the degree of intergranular corrosion of the material gradually increases with the extension of time.
Source: China 2205 Flanges Manufacturer – Yaang Pipe Industry Co., Limited (www.steeljrv.com)
(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)
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