The Effects Of Fe₂O₃ Content And Annealing Process On Microstructure Of Micro/Nano Structure 2507 Super Duplex Stainless Steel Prepared By Aluminothermic Reaction

Super duplex stainless steel, which comprise similar quantities of ferrite(δ) and austenite(γ) in solid solution organization, the less content of either phase must be more than 30%. With its combination of good toughness, high strength of ferritic stain-less steel and excellent corrosion resistance, mechanical properties, good casting and welding properties of austenitic stainless steel, it has replaced 304,316 L, even 904 L austenitic stainless steel, can be used in a variety of applications including marine engineering, petrochemical industry, shipbuilding and other fields. Aluminothermic reaction casting is a novel method to prepare nanostructural alloys,high nucleation rate,alarge degree ofsupercooling and cooling copper substrate to provide a high cooling rate(about150k/s) so that grains remain in nanometer size after solidation. In this paper, we use thermite reaction melting process, by adding different content of Fe2O3 pre-pared large dimension bulk 2507 super duplex stainless steel with nano/micro structur e, the effect of different residual Al content and annealing process on microsteucture of micro/nano structure 2507 steel are summarized as follow:1.When the Al and Fe2O3 meet the ratio of reactants 1:2 in theory,due to high content of alloying elements(25% Cr, 7% Ni, 4% Mo) in 2507 steel reduces the reaction heat, moreover, Al can freely soluble in liquid steel, which result in incompleteness of thermite reaction, at last there are about 2% to 3% residual aluminum dissolve into the steel. Although the chemical compositions of the steel meet the requirements at this time, Al promotes transformation of ferrite lead to the steel comprise single ferrite(δ) phase. The average grain size and volume fraction of nanocrystalline/microcrystaline in cast steel are 23 nm and 180 nm, 85% and 15%, respectively.2. By the addition of an excess of Fe2O3 made Al in residual steel transfor m into Al2O3 thus eliminated the effect of residual Al on the microstructure of steel. When excess of 10% Fe2O3, the volume fraction of ferrite and austenite were 68% and 32%, respectively. Although dual phase scale accorded with dupl ex steel, fewer austenite phase because of residual 1.2% Al. When excess of 15% Fe2O3, volume fraction of ferrite and austenite were 63% and 37%, respectiv ely. There was 0.9% residual Al distributing in steel. When excess of 20% Fe2O3, volume fraction of ferrite and austenite were 57% and 43%, respectively. T his moment the micro-nanocrystalline duplex steel had better dual phase scale, a nd about 0.2% residual Al in steel which had no significant effect on dual phase scale.3. In order to regulate the dual phase scale of nanocrystalline/microcrystalline a nd improve the cast steel ductility, excess of different weight fraction steel anne aled 8h at 800℃,then the average grain size of nanocrystalline/microcrystalline were 25 nm and 227 nm, the volume fraction of microcrystalline was 20%. Comp ared with cast 2507 steel, the grain size and volume fraction of nanocrystalline had little change, and grain size of microcrystalline increased, indicating that du ring anneal process nanocrystalline had no obvious grow up behavior, but micro crystalline grew up significantly, however the volume fraction of m, but microcr ystalline grow up significantly, however the volume fraction of microcrystalline change little. After annealed 0.5h at 1000℃,the average grain size of nanocrys talline/microcrystalline were 31 nm and 306 nm, respectively. The volume fraction of microcrystalline was 30%. After annealed 0.5h at 1000℃, Both nanocrystalli ne and microcrystalline grow up obviously, the volume fraction of microcrystalli ne increased evidently, indicating that annealed at 1000℃ was suit for regulatin g the grain size and volume fraction of nanocrystalline/microcrystalline, as well as the dual phase distribution in steel.4. The micro-nanocrystalline duplex steel had better dual phase scale when exce ss of 20% Fe2O3, but had significant Si C inclusions and σ phase. Annealed at 1000℃ eliminated metalloid inclusions、detrimental precipitates and regulated n anocrystalline/microcrystalline dual phase scale. After annealed 1h, spot and stri p precipitates almost completely disappeared, and δ transformed into γ made γ i ncrease, moreover, microcrystalline grew up and nanocrystalline had no obvious change. After annealed 1.5h, dual phase presented the pattern of “massive distri bution” and evenly, spot non-σ phase decreased significantly, nanocrystalline clu stered and grew up obviously, microcrystalline was widespread. After annealed 2h, α transformed into γ, nanocrystalline and microcrystalline grew up evidently, and σ phase fully dissolved in steel.