Effect Of Boron On Microstructure And Corrosion Resistance Of S31254 Super Austenitic Stainless Steel

The S31254 super austenitic stainless steel?SASS?has a high content of chromium,nickel,molybdenum,nitrogen and other elements,which providing steel better corrosion resistance than conventional austenitic stainless steel.Hence,it can be instead of nickel-copper alloy and widely used in flue gas,desulfurization,seawater desalination,pulp bleaching,chemical and other strong corrosive environments.However,due to a high alloying element content in S31254 super austenitic stainless steel,the segregation of molybdenum in the solidification process is severe and the second phase is easily precipitated during hot working,which can have a negative influence on its pitting corrosion resistance and intergranular corrosion resistance.Therefore,how to reduce the segregation of Mo at the grain boundary and suppressing the precipitation of the second phase is the key to improving the corrosion resistance.In the boron microalloyed stainless steel,the special segregation behavior of boron at the grain boundary can be utilized to suppress the segregation of Mo at the grain boundary.However,if the boron is excessive,the precipitation of the BN phase is caused,so that the performance of the material is deteriorated.Aiming at these problems,S31254 super austenitic stainless steel containing40ppm boron and boron-free was prepared in the experiment.The effect of boron on the dissolution and precipitation behavior of the second phase was studied by solution treatment and aging treatment.The potentiometric polarization method,electrochemical impedance spectroscopy?EIS?and double-loop electrochemical potential reactivation?DL-EPR?method were used to electrochemically test the heat-treated samples,study the effect of boron on the corrosion resistance of the materials.By comparing and analyzing the experimental results,the following conclusions can be drawn:?1?By analyzing the microstructures of S31254-0B and S31254-40B which were solution treatment at 1220?for 1,2,5 h,the results showed that the second phase of the boron-containing sample was significantly faster.S31254-0B has a residual undissolved second phase at the grain boundary even after solid solution for 5 h,and there is no undissolved second phase in the microstructure of S31254-40B after solution treatment for 2 h.?2?By analyzing the microstructures of S31254-0B and S31254-40B after aged solution at 950?for different times,the result shows that the addition of boron has a significant effect on the precipitation behavior of the precipitated phase.The number and volume of precipitated phases in S31254-0B is large,and the precipitated phase at the grain boundary grows and nucleates with the extension of aging time until they are connected together.In S31254-40B,the phase of the precipitated phase is small,does not aggregate and grow at the grain boundary,but is dispersed throughout the austenite matrix.?3?The potentiostatic polarization,electrochemical impedance spectroscopy and X-ray photoelectron spectroscopy of S31254-0B and S31254-40B after solid solution and aging treatment in 10%NaCl solution,the result shows that addition of boron can make the more content of Cr₂O₃,Mo⁴⁺and Mo⁶⁺in the passivation film of S31254 super austenitic stainless steel,have a more stable passivation film,the corrosion rate is lower in 10%NaCl solution,and has a higher pitting resistance.?4?The double-loop electrochemical potential reactivation method was tested on S31254-0B and S31254-40B after solid solution and aging treatment.The result shows that the boron-containing S31254 super austenitic stainless steel has a lower intergranular corrosion sensitivity.During the solution treatment of S31254-40B,the precipitated phase dissolves rapidly,and the alloying element depleted zone disappears more quickly.The precipitation phase distribution is more dispersed during the aging process,and the Cr and Mo element-poor regions are more easily replenished by the matrix,thus having higher resistance to intergranular corrosion.