Study On Microstructure And Properties Of 2507 Super Duplex Stainless Steel Welded Joints

SAF2507 is one of the best grade duplex stainless steel with excellent mechanical properties and corrosion resistance.It is widely used in marine,bridge pipeline,pressure vessel and other places.SAF2507 has high performance and good welding performance.gas tungsten arc welding,friction stir welding,laser welding and plasma arc welding can be used to weld SAF2507 Super Duplex Stainless Steel.However,the high content of alloying elements in SAF2507 will also make it easy to precipitate intermetallic compounds in welded joints,and the ratio of the two-phase is unbalanced,which will lead to the decrease of welding structural mechanics and corrosion performance,especially in the welding of high energy density energy sources such as laser welding,which to some extent hinders the application of SAF2507 super duplex stainless steel in engineering.In this paper,SAF2507 super duplex stainless steel was welded by gas tungsten arc welding,laser beam welding and laser-MIG hybrid welding.The two-phase unbalance of welded joints was improved by post-weld heat treatment,and the intermetallic compounds in the joints were eliminated.The effects of the microstructural evolution of the three welded joints on mechanical and corrosion properties were compared and analyzed.The results show that laser beam welding has the best weld shape,but due to the rapid cooling rate,the biphasic structure imbalance in the weld area is serious,and the ferrite content is as high as 69.3%.The weld shape of gas tungsten arc welding is the worst,but due to the slower welding speed and larger weld heat dissipation area,the cooling rate is slow,the ferrite transforms into austenite in large quantities,and the biphasic structure is the most balanced,and the ratio is close to 1:1.Laser-MIG hybrid welding joint is divided into upper and lower parts.The upper part is affected by filler wire and laser.The addition of welding wire makes the cooling speed of weld slow.At the same time,the content of austenitic element Ni in the welding wire is higher than that of base metal.More austenitic structure precipitates out.The lower part is only affected by laser,and the cooling speed is faster.The structure is similar to laser beam welding.The intermetallic compound Cr2 N appears in the heat affected zone of the joints by three welding methods,which will affect the comprehensive properties of the joints.The structure of Cr2 N can be eliminated by solution treatment after welding,and the ratio of the two components can be adjusted at the same time.After solution treatment,the biphasic structure of laser beam welded joint is optimized a lot,the ratio of the two phases is close to 1:1,but many sigma phases appear in the filler of laser-MIG hybrid welding and tungsten-argon arc welding joints.The mechanical properties of the joints show that the tensile strength and micro-hardness of the welds are higher than those of the base metal.All the three joints fracture at the base metal location,and the fracture surface presents dimples of different sizes,which indicates ductile fracture.Due to the difference of the ratio of two phases,the impact property of laser beam welded joint is the worst and that of tungsten-argon-arc welding is the best.After solid solution treatment,the impact toughness of tungsten-argon-arc welded joint decreases obviously,from tough fracture to brittle fracture,and the toughness of laser beam welded joint increases.The results of corrosion resistance show that the three kinds of welded joints have good corrosion resistance.The corrosion resistance of gas tungsten arc welding is slightly better than that of laser beam welding and laser-MIG hybrid welding.After solution treatment,the corrosion resistance of gas tungsten arc welding and laser-MIG hybrid welding is reduced due to sigma phase appearing in the welds,while the corrosion resistance of laser beam welding with adjustable two-ratio is improved.The corrosion properties of the three welded joints in the solution containing sulfur ions are much worse,which indicates that the super duplex stainless steel is more sensitive to sulfur ions.The corrosion morphology of welded joints shows that sigma phase is the area where pitting corrosion germinates,and the existence of austenite hinders the further expansion of corrosion pits.Deep corrosion of welded joints results in more ferrite structures being corroded,while most austenite structures are retained,indicating that the corrosion resistance of austenite is better than that of ferrite.