| Duplex stainless steels were known by its excellent corrosion resistance and comprehensive mechanical properties because of the organization with ferrite and austenite, therefore,they played a decisive role in the oil transportation,marine exploration and many other field, which were related to the lifeline of the national economy. In practical application, welding joint between duplex stainless steel components with other types of metals were performed on a lot of equipments and large-scale industrial products,so that it can not only get excellent industrial structure and also can improve the utilization rate of materials. However, the significant differences in the chemical composition and properties of metals among the different types materials made the welding process difficult. For example, during the dissimilar welding between duplex stainless steel and carbon steel,the problems like welding residual stresses and solidification transition layer in the welded joint would appears inevitably,which makes welding more difficult between different types of metals compared with the same type of metal. Due to the differences in chemical composition, thermal expansion rate and other physical and chemical properties of dissimilar metal, thus generated brittle microstructure, precipitated phase and other welding defects in welding joint,which severely decreased the service life of welded joints. Based on this problem,this article focused on the investigation of microstructure and performance of dissimilar welding joint between SAF2507 duplex stainless steel and Q235 A steel metal and first principles calculation was used to simulate the diffusion behavior of carbon element in the fusion zone of Q235 A steel surface, which expounded the mechanism of the formation of carbon migration from the atomic perspective.Comparing the welding characteristic of SAF2507 duplex stainless steel and Q235 A steel, this paper finally decided to use the measure of manual metal arc welding. ER2510 electrodes were choosed as filler metal because theirs chemical composition have high Cr and Ni content. Metallographic microscopy showed that the microstructure of weld joints were made up of ferrite and austenite phase.which is similar to the microstructure of SAF2507 duplex stainless steel. Microhardness test indicated that the microhardness between weld metal and SAF2507 duplex stainless steel have a little difference. The joint met the standard demanded by welding structure in practical application.Optical microscope,scanning electron microscope,energy dispersive spectrum analyzer and other instrument were used to investigate the phase structure and mechanical properties of Q235 A steel fusion zone,HAZ and welded joint. It confirmed that,the difference of the carbon element content resulted in the carbon element diffusion in the interface of Q235 A steel fusion zone.Near the weld fusion line position there are carbon depleted region and carbon enriched regions. In this area,we discovered that the Cr, Ni element performed as gradient distribution. According to the microhardness test, the hardness of decarburized region appeared lower than that of Q235 A steel base material,and the hardness of carburized layer region is higher than the two regions discussed above.First principle was used to simulate carbon atom migration process in Q235 A steel fusion zone surface,which can provide the theoretical basis for the carbon element diffusion in the surface of fusion zone. The calculation results showed that the carbon atoms dissolved in the α-Fe(110),α-Fe-Cr(110) surface structure system,and formed stable structure. With the increase of Cr content in α-Fe-Cr-C(110) system, the segregation of carbon content slowed down, indicated that carbon atom was easier to segregate in the α-Fe(110) interface,which made carbon atom segregation in the carbon steel side of interface between Q235 A steel and austenitic steel. α-Fe-Cr(110) surface of the structural stability was strengthened gradually and forming ability increased. |
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