| The 904 L super stainless steel has excellent mechanical properties and high corrosion resistance,as it contains high amounts of chromium,nickel,molybdenum,and nitrogen.And is widely used in flue gas desulfurization equipment parts.However,the high alloy content will lead to the segregation of elements during the solidification process to precipitate the second phase,which seriously affects the corrosion resistance of the material.The cooling rate during solidification is one of the important factors affecting the precipitation of brittle phases between stainless steels.Therefore,studying the effect of cooling rate on the phase composition of 904 L super austenitic stainless steel during solidification and the corrosion resistance in a certain simulated flue gas desulfurization environment has important theoretical,and it also has guiding significance for the production of practical materials.In this paper,the solidification mode of 904 L stainless steel under equilibrium state was predicted by equivalent empirical formula.With the help of JMat Pro simulation software,the solidification process under equilibrium state,the temperature transformation diagram of precipitates and the continuous cooling transformation diagram were simulated,and the solidification phase diagram,the TTT curve and CCT curve of precipitates were plotted.The solidification structures of 904 L stainless steel at different cooling rates(6,50,100,500 and 1000 ℃/min)were obtained by controlling the cooling rate with high temperature laser confocal scanning microscope(HT-CLSM)equipment.The influence of cooling rate on pitting corrosion resistance of904 L stainless steel in simulated flue gas desulfurization(FGD)environment was studied by electrochemical workstation,XPS analysis and corrosion morphology observation.The new insights and research findings of this paper include:(1)The Creq/Nieq of 904 L stainless steel is calculated to be 0.95 by classical equivalent formula,which conforms to A-type solidification mode.The calculation results of JMat Pro software show that the matrix structure of 904 L stainless steel is austenite,and the precipitated phases include carbides,M23C6,Sigma phase and Laves phase.The precipitation temperature of the austenite phase is 1396 ℃,while the precipitation temperature of the main precipitation phase,Sigma,is 950 ℃.The precipitation amount is the largest at 517 ℃,and the mass fraction is close to 12%.The main alloy components of austenite are Fe,Cr and Ni,and the main alloy components of Sigma phase are Cr,Fe and Mo.The TTT curve of Sigma phase presents a typical C shape.The temperature of ’nose tip’ was 910 ℃,the incubation time was 16 min,and the initial transition temperature was 950 ℃.The CCT curve reflects the absence of precipitation of the Sigma phase at cooling rates greater than100 ℃/min when the steel is cooled in a continuous cooling mode.(2)The solidification evolution law of 904 L stainless steel under different cooling rates was observed in situ by HT-CLSM,and the results showed that the cooling rate had no obvious effect on its solidification mode.As the cooling rate increases during the solidification process,the initial solidification temperature and secondary dendrite spacing decrease.The EPMA surface scanning revealed that the interdendritic content of Cr,Mo,Mn,Si,and Cu was greater than the dendrite stem.Secondary precipitation in steel is primarily the distribution of interdendrites formed by Sigma phase,and with increasing cooling rates,the ratio of Sigma phase to interdendrites increases and then decreases.In contrast,the content of Cr and Mo in the Sigma phase decreased first and increased with increasing cooling rate,before reaching their minimal values at 100 ℃/min(22.72 wt.% and 13.98 wt.%).(3)The electrochemical corrosion test of 904 L stainless steel was carried out using an electrochemical workstation.It was determined that the pitting corrosion resistance of the sample increased with an increase in cooling rate followed by a decrease,and the optimum temperature was 100 ℃/min.Analyzing the passive film formed on the surface of 904 L stainless steel by X-ray photoelectron spectroscopy(XPS),and it was found that the passive film was mainly composed of Cr2O3,Cr(OH)3,Fe2O3,Fe(OH)3,Mo O3,Ni O and Ni OOH.The protective Cr2O3 content is less at higher or lower cooling rates,resulting in a decrease in the protection of the passivation film.Observing the morphology after corrosion,it is found that the pitting corrosion is very sensitive to the increase of Cr and Mo contents in the Sigma phase,that is,the higher the Cr and Mo contents in the Sigma phase,the worse the corrosion resistance. |
PDF Full Text Request