Effect Of RE On The Microstructure And Properties Of Super Ferritic Stainless Steel

Owning to its outstanding mechanical properties and corrosion resistance,super ferritic stainless steel has become established as the preferred material for sea water cooled heat exchanger tubing in coastal power plants.However,due to the requirements of high-alloy and high purification,only a few international steel enterprises master the core technology.Based on the full investigation of on-site process and product quality and taking super ferritic stainless steel with different rare earth content as an object,this research was involved in evaluating the impact of the rare earth content on the microstructure,mechanical properties and corrosion resistance of super ferritic stainless steel and further optimized the alloy composition,through the observation of microstructure,charpy impact test,tensile test at ambient temperature,electrochemical workstation and immersion test.Steel samples were super ferritic stainless steels containing 0,0.081%,0.106%and 1.98%rare earth smelted by 10 kg vacuum induction furnace.The sulfur content in steels decreased with the rare earth content increasing.On the thermodynamic analysis of La-Ce-O-S quaternary system,it was found that the deoxidation capacity of La was stronger,while the desulfurization capacity of Ce was stronger.When 0.73<[%S]/[%O]<1.2,La2O3 was mainly generated;when[%S]/[%O]<0.73,the products were La2O3 and Ce20O3 By the prediction of the thermodynamic model,the products of desulfurization and deoxidation were mainly RE2O2S and RE2O3.After adding rare earth,the increased number and finely distribution of inclusions in the super ferritic stainless steels were seen.The average diameter of inclusions in the super ferritic stainless steel containing 0.106%rare earth got the smallest.The affect of rare earth not only referred to modifying inclusion,but also promoting the transition from irregular rectangle to spherality.On the condition of ultra-low sulfur and oxygen,rare earth reacted with phosphorus,which inhibited the segregation of phosphorus.After the addition of rare earth,the super ferritic stainless steel grains had been refined,and a feasible amount of rare earth increased the grain size to 8.6 grades.In the annealing process,the segregation of rare earth at grain boundary inhibited grain growing.RE improved the C,N solubility,thereby reducing the secondary phase precipitation.In order to dissolve inter-metallic phase and carbo-nitride precipitate phase in the super ferritic stainless steel completely,the annealing temperature needed to reach above 1030?.Mechanical performance test showed that after adding rare earth,the tensile strength and yield strength of the super ferritic stainless steel was improved,and the elongation and reduction of area increased,which was considered the results of the inclusion distribution,grain refinement and solid solution strengthening of rare earth.On the other hand,the impact toughness has been markedly improved,and grew slowly with the rare earth content increasing.The impact energy of the super ferritic stainless steel containing 0.106%rare earth was 45.25 J,rising by 217%compared with the steel without rare earth.Meanwhile,rare earth promoted the impact fracture mechanism transiting from brittle fracture to ductile.The Brinell hardness was improved since the addition of rare earth.Due to the cooperation of dynamic recrystallization and grain boundary precipitation of secondary phases,the Brinell hardness decreased firstly,then increased,and eventually became flat at different annealing temperatur-re.Corrosion test showed that the addition of rare earth reduced sensitive part number to pitting.With increasing rare earth content,the corrosion current density in 3.5%NaCl solution at 30? dropped successively with the pitting potential increasing.Owning to the promotion of the passive film stability by rare earth,the corrosion current density did not varied with the environmental temperature.After the addition of rare earth,the intergranular corrosion rate of the super ferritic stainless steel decreased,and the steel sample with 0.106%rare earth behaved the most outstanding resistance.Since rare earth could suppressed the ions diffusion in the passivation film,the polarization resistance of the super ferritic stainless steel increased with the rare earth content rising in the electrochemical corrosion process.