Preparation Of 23 Cr Lean Duplex Stainless Steel And Investigations On Its Microstructure And Properties

Because of the special two-phase structure,duplex stainless steel has excellent mechanical properties and corrosion resistance,so it has been widely used.Due to the shortage of raw materials and the rising price of Ni,lean duplex stainless steel has been developed rapidly in recent years.Replacement of expensive Ni by inexpensive Mn and N has become a major way to reduce the cost of duplex stainless steel.In this paper,referring to the alloy composition of 2304 duplex stainless steel as the standard,using Ni to replace Mn and N reduces the cost while ensure its performance.The purpose is to develop the 23Cr lean duplex stainless steel.The main contens of this paper are as follows:(1)The equilibrium phase diagrams of the five alloy compositions are calculated by Thermo-Calc.There are only ferrite and austenite phase during 950?1200?.Cr2N precipitates appear during 500?950?.? phase appears in experimental steels of 23Cr-1,23Cr-2 and 23Cr-5.(2)Solution treatments at different temperature are carried out on experimental steels.Microstructure changes and the impact on the mechanical properties and corrosion resistance are studied.The results show that the optimum solution treatment temperature is 1000?.The two-phase distribution is uniform,and the two-phase ratio is close to 1:1 between 1000?1050?.Mechanical properties and corrosion resistance are also the best;The ferrite phase content is as high as 60%at 1150?,that leads a serious imbalance between the two-phase ratio.Ferrite grains are too large at the same time so that the mechanical properties and corrosion resistance are the worst.(3)Isothermal aging treatment at different temperature for different time are carried out on experimental steels.The precipitation behavior of nitrides under different isothermal aging conditions,and the impact on the mechanical properties and corrosion resistance are analyzed.The results show that the precipitates of Cr2N mainly appear on the phase boundary and the ferrite grain boundaries.Mechanical properties and pitting corrosion resistance are the worst at 700?.Comprehensive mechanical properties and pitting corrosion resistance are lower than the solid solution state when isothermal aging for 30min at 700?.The comprehensive mechanical property is superior to the solid solution state when isothermal aging for 240min at 700?.But the pitting corrosion resistance is still far below the solid solution state.(4)Cr and Mo are mainly enriched in ferrite phase,while Ni,N,Mn and Cu are mainly enriched in the austenite phase.As a result,the PREN of ferrite is lower than phase ferrite phase.So pitting preferentially occurs in the ferrite phase,while austenite phase has a retarding effect on the expansion of the pitting pits.The addition of 0.5%Cu can effectively improves the pitting corrosion resistance of the experimental steel.