| Abstract Scope |
Duplex stainless steel (DSS) is composed of 50% austenite (γ) and 50% ferrite (α), which has a combination of good mechanical properties and corrosion resistance. Currently, large-scale-complex-shaped DSS components (e.g. impeller blades in nuclear industry) are manufactured through casting, which is highly sophisticated. Only a few companies worldwide possess the necessary processing knowledge and long production times have to be accepted by customers. Wire Arc Additive Manufacturing (WAAM) is attracting attention of industry due to its abilities to create large metal components with high deposition rate, low equipment cost, and consequent environmental friendliness. It’s a feasible method to manufacture large-scale-complex shaped DSS components. However, recent investigations on the WAAM DSS using commercial welding wires as feedstock material showed that the austenite content in the as-deposited component is significantly higher than the ideal γ/α ratio. Additionally, secondary austenite(γ2), which deteriorates the corrosion resistance is found at the layer bands of as-deposited DSS components. In this investigation, we managed to adjust the austenite content and removed the γ2 precipitated at layer bands through post-manufacturing heat-treatment. It was noticed that the pitting corrosion resistance of the sample heat-treated at 1300℃ was comparable to the hot-rolled 2205 DSS.
The set-up of WAAM process was based on FCAW. The direction along which the wall was built layer by layer was defined as the direction of the Z axis, the direction along which the welding torch moved back and forth was defined as the direction of the Y axis. The direction perpendicular to the Y-Z plane was defined as the direction of the X axis. Commercial E2209T0-4/1 DSS wire was used as feedstock material. The equilibrium phase transformation map of the deposited metal was calculated using Jmat-Pro software. It was found that the temperature at which the γ/α ratio reached 1:1 was Teq=1218℃ and the melting point was Tm=1383℃. In order to achieve a balanced γ/α ratio after 1h heat-treatment, the heat-treatment temperature should be set between 1218℃ and 1383℃. Thus, the post-AM heat -treatments were performed at 1250℃, 1300℃,1350℃ for 1h followed by water quench.In the industry application, the Y-Z surface has a direct interface with the corrosive media. Therefore, all of the test surfaces of this investigation were taken from the machined Y-Z surface. Potentiodynamic polarization tests were performed to evaluate the pitting corrosion resistance. The test solution was 3.5wt% NaCl at 25℃±0.1℃. A saturated calomel electrode (SCE) worked as the reference electrode, a platinum grid worked as the counter electrode, and the specimen embedded in epoxy resin acted as the working electrode. The volume fraction of austenite was calculated using Image Pro Plus software. To observe the precipitation of chromium nitride, the samples were electrolytically etched in 10% oxalic acid solution at 10V for 30s. The crystallographic information of each sample was investigated using EBSD technology. The specimens were electrolytically polished in 5% HClO+95% alcohol solution at 30V for 20s.
The average austenite ratio in the as-deposited WAAM DSS was 66% but the average austenite content in the samples heat-treated at 1250℃, 1300℃, 1350℃ were 48%, 45% and 28%. As specified in NORSOK M-630, the austenite content in DSS should be in the range of 35%-65%. Therefore, a balanced phase ratio was achieved at the heat-treatment temperature of 1250℃ and 1300℃. γ2 was found in the layer bands of the as-deposited component but dissolved in the heat-treated samples.The potentiodynamic polarization curves of the as-deposited and heat-treated WAAM DSS suggested that the 1300℃ heat-treated sample showed the best pitting corrosion resistance that is comparable to the hot-rolled 2205 DSS. In order to explain the pitting corrosion resistance performance, the chromium nitride precipitation behavior and EBSD characterization was conducted on the samples.It was found that chromium nitride precipitated in the as-deposited sample and the sample heat-treated at 1350℃. Comparatively, no chromium nitride precipitated in the sample heat-treated at 1250℃ and 1300℃ which was beneficial to the pitting corrosion resistance. The EBSD characterization suggested that in the 1300℃ heat-treated sample, the austenite showed <111>//X texture and it has lower crystalline defect density and lower grain orientation spread comparing with other samples. We believe that the striking corrosion resistance of the 1300℃ heat-treated sample was the result of the balanced γ/α ratio, the non-existence of chromium nitride, the <111>//X texture of austenite, the decrease of crystalline defect density and the low GOS of austenite.
In this investigation, WAAM DSS was heat-treated at 1250℃, 1300℃, 1350℃. γ2 were removed in the heat-treated samples and a balanced phase ratio was achieved in the 1250℃ and 1300℃ heat-treated samples. The 1300℃ heat-treated sample showed the best pitting corrosion resistance that is comparable to the hot-rolled 2205 DSS plate. The striking pitting corrosion resistance was the integrated result of the balanced phase ratio, the absence of γ2, the non-existence of chromium nitride, the <111>//X texture of austenite, the low crystalline defect density and the recrystallization of austenite. |