| Duplex stainless steels(DSS) have been widely used in petrochemical, marine engineering and food engineering industries owning to their high strength and excellent resistance to corrosion. DSS has a two-phase microstructure consisting of ferritic and austenitic.In heat treatment and welding process, the σ phase and the ferrite/austenite ratio have a serious impact on the plasticity, toughness and corrosion resistance. Compared to the normal duplex stainless steels2101,2205etc.,SAF2507super duplex stainless steels have increased levels of Cr、Mo、N(especially the level of N increased),resulting with a higher strength and improve corrosion resistance. Especially in the organic and inorganic acid, the corrosion rate is very low, which is very suitable for the harsh media of chlorine. SAF2507is also a saving of nickel, molybdenum economic duplex stainless steel. With the growing tension of nickel resources, SAF2507greatly reduces the cost of production to increase the advantages of competition. With the increase of SAF2507in industry, welding is highlight problem. Developing new high-efficiency and low-cost welding materials, and promoting the welding technology to ensure the performance of welded joints Has become an urgent subject of SAF2507development. For our country such a lack of nickel resources, research, production and promotion of duplex stainless steel has an important economic and strategic significance.Effect of solution treated temperature at500～1100℃on ferrite/austenite ratio and σ phase precipitation has been studied in this paper, and the tensile properties of solution treatment is compared at950℃and1050℃. The results show that the ferrite/austenite ratio is close to1:1in the range of1050～1100℃; the σ phase forms in the ferrite or ferrite-austenite boundary. The quantity of a phase precipitation increases firstly and then decreases with increase of the solution treatment temperature.The a phase precipitation is a maximum at900℃,and there is almost noσ-phase at above1050℃.Tensile tests show that the tensile fracture is brittle fracture divided and stretched by ductile belt at950℃, while the fracture is ductile fracture at1050℃. In this paper,SAF2507with a thickness of12mm has been welded by shield metal arc welding process with E2510welding rod and0.4,0.6,0.8,1.42kJ/mm heat inputs. The microstructure evolutions, the distribution of σ phase, the impact fractographs and fracture mechanism and in each weld zone are analyzed by optical microscope (OM) and scanning electronic microscope (SEM)The hardness and impact testers are used to evaluate the mechanical properties of the welded joints. The results show that very low heat input (less than0.5kJ/mm) lead to high ferrite content and chromium nitride and secondary austenite precipitation. However, high heat input cause precipitation of brittle σ phase at the y/a interface, including weld metal and heat affected zone (HAZ).The precipitation of σ phase increases the hardness of the region and lead to the impact toughness decrease significantly. Analysis of impact fracture scanning shows that the fracture is brittle fracture along the grain boundary. Heat input range in the range of0.6-0.8kJ/mm is the best process specifications, which can inhibit the precipitation of σ phase and prompt the ferrite and austenite ratio to close to1:1. |
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