| In the present work, two kind of duplex stainless steels, 2205 and 2507, were modified by using High Current Pulsed Electron beam(HCPEB) with the aim of improving the overall surface properties. The surface and cross section morphologies and the phase components before and after the HCPEB treatment was investigated by scanning microscope(SEM), optical microscope(OM) and the X-ray diffraction(XRD). The changes in microhardness on the surface and cross section after HCPEB treatment and corrosion resistance of the surface before and after irradiation in 3.5wt%Na Cl water solution were carefully measured. The surface microstructure and properties of two kinds of duplex stainless steel were investigated after different pulsed electron beam treatments, and the reasons for these changes were discussed.After surface modification treatments by HCPEB, the microstructure and morphology of treated surfaces were investigated. The results showed that after the HCPEB treatment, the grain in surface were refined. The element distribution was more homogeneous after the repeated HCPEB treatment, which caused the formation of more ferrite phase in the surface layer. Due to the fast heating and cooling induced by HCPEB treatments, craters formed on the treated sample surfaces as a result of local eruption. With the increase of irradiation pulses, the surface roughness is increased, while the number of crater is decreased but its size becomes bigger and the depth is decreased. However, it should be noted that no matter the increase of irradiation pulses, the craters still exist. The density and size of craters on the 2507 duplex stainless steel is bigger than that of 2205 steel. After the surface treatment by HCPEB, the melted layer depth is about 8-9μm for the 2 stainless steel. It is also observed that the depth of melted layer has no relation with the number of irradiation. At the same time, on the surface of duplex stainless steel of 2507, there are some slip bands induced by plastic deformation caused by the strong thermal stress.After measurement of the microhardness from cross sections of HCPEB treated specimens, it shows that the microhardness of all specimens is increased after the HCPEB treatment. A heat affect zone(HAZ) underneath the melted layer is formed as well. With the increase of depth, the microhardness is changing with the characteristic of fluctuate at a certain range, which is the result of thermal stress wave propagates in the depth. The research results show that, the irradiation pulse number has little effect on the surface microhardness of the specimen, but when the irradiation pulse is increased, the depth of hardened layer is becoming higher. The reason for the improvement of surface microhardness is that the surface grain was refined and more ferrite phase forms after the HCPEB treatments. Alloying elements dissolved into the substrate, which was beneficial to form the supersaturated solid solution. Meanwhile, the generation of the thermal stress and the large number of defects and slip bands are effective factors to improve the surface microhardness of the specimen to a certain extent.The electrochemical corrosion behaviors of the untreated and HCPEB treated specimens in 3.5wt% Na Cl water solution were tested. The results show that the corrosion potential of the two materials becomes nobler with the increase of pulse number. This illustrates that the irradiation of HCPEB makes the microstructure and composition in surface of the material more homogeneous, and the grain is refined as well. And with the increase of the number of pulses, such effect is more obvious. After the HCPEB treatment, the corrosion current density of 2205 duplex stainless steel is reduced, while the polarization phase impedance and capacitive arc radii in the low frequency range are increased. It shows that the corrosion resistance of this material is significantly improved. The corrosion resistance of the 2205 steel sample after 15 pulses was better than that of the 5 pulsed sample.The reason why the corrosion resistance of the 2205 is improved lies in the fact that the HCPEB treatment makes surface grain finer and has the surface purification effect. Although the crater on the surface of the specimen will reduce the corrosion resistance, but for the 2205, the number of craters on the surface is limited, and the size is small. With the subsequent pulse processing, most of the crater were removed and the corrosion resistance can be improved finally.For the duplex stainless steel of 2507, its corrosion current density increased after the HCPEB treatments, while the polarization impedance and capacitive arc radii in the low frequency range are decreased. The results illustrate that corrosion resistance property is deteriorated. The reason for such degradation is that more craters formed on 2507 steel after the HCPEB treatment, and depth of crater is deeper, which are the possible pitting sites during corrosion in Cl- containing environment and the corrosion properties are becoming worse. |
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