Effect Of Nickel And Chromium On Microstructure And Properties Of Silicon Solution Strengthened Ductile Iron

Silicon solution strengthened ductile iron has better comprehensive mechanical property,corrosion resistance and oxidation resistance than ordinary ductile iron,but when the content of Si exceed 4.3%,the silicon solution strengthened ductile iron material is brittle and the application is limited.Therefore,through alloying treatment,improving its corrosion resistance and oxidation resistance,and developing the silicon solution strengthened ductile iron applied for offshore wind power equipment parts,metallurgical sintering machine parts and coking equipment parts,have great engineering significance for expanding its application.In this paper,0.4%to 1.4%Ni and 0.15%to 0.45%Cr were added into silicon solution strengthened ductile iron which contained 3.4%to 3.5%Si.OM,SEM,EDS and XRD were used to research the effects of Ni and Cr on the microstructure,mechanical properties,corrosion resistance and oxidation resistance of silicon solution strengthened ductile iron.The results of the research showed that:The microstructure of specimens with 0.4%?1.4%Ni and 0.15%-0.45%Cr were composed of graphite nodules,pearlite and ferrite.The size of graphite nodules was above grade 7,and the graphite sphere grade was above grade 2 in all of specimens.With the increase of Ni and Cr content,The amount of pearlite,hardness and tensile strength increased,while the elongation decreased.When the Cr content was 0.448%and the Ni content was 1.397%,the amount of pearlite is 40%,hardness was 239.4HB,tensile strength was 697.5MPa and elongation was 4.5%.When the silicon solution strengthened ductile iron specimens were immered in 3.5%NaCl solution for 30 days,the corrosion rate decreased with the increase of Ni and Cr content.When the Cr contents was 0.448%and the Ni contents was 1,397%.the corrosion depth of the specimen matrix was the thinnest,the numbers of falled graphite nodules were the least and the corrosion rate was the lowest,which was 0.04898g·m-2·h-].When the silicon solution strengthened ductile iron specimens were immered in 3.5%NaCl solution for 30 days,Ni and C.r concentrated in the inner side of the rust layer,refining the rust particles.With the increase of Ni and Cr content,the size of rust particle decreased and the density of the rust layer increased.The rust layer of the specimens with different Ni and Cr content were all composed of ?-FeOOH,?-Fe2O3,?-FeOOH and ?-FeOOH.with the increase of Ni and Cr content,the ratio of?-FeOOH and ?-Fe2O3 in the rust layer increased,and the ratio of ?-FeOOH and?-FeOOH decreased.When the content of Ni and Cr is constant,the corrosion current and corrosion potential increased first and then decreased,the impedance spectrum capacitance arc radius in the high frequency region decreased first and then increased,and the impedance spectrum in the low frequency region changed from inductance arc to Weber impedance with the increase of immersion time.The corrosion potential.,corrosion current density and capacitive arc resistance radius of non-immersed specimens increased with the increase of Ni and Cr contents,while the corrosion current density and capacitive arc resistance radius of specimens immersed 20 and 30 days decreased with the increase of Ni and Cr content.Under different dry/wet corrosion conditions,with the increase of drying time proportion in the dry/wet corrosion cycle,the corrosion rate of the specimens with 0.436%Cr and 0.395%Ni increased.The amount of Fe3O4 increased with the increase of drying time within one dry/wet corrosion cycle,which leaded to the corrosion current density increase,and the capacitance arc radius decrease.When the Ni content was constant,the oxidation rate decreased with the increase of Cr content,when the Cr content was constant,the oxidation rate increased with the increase of Ni content.When the Cr contents was 0.448%and the Ni contents was 1.397%.,The oxidation rate of was 0.6630g·m-2·h-1,and the specimen had the best oxidation resistance.Under the condition of 700?,with the increase of oxidation time,the oxide film grew from local to overall,and the thickness of the oxide film increased,The morphology of the oxide film changed from porous to smooth and compact.In the oxidation process,Ni was enriched at the interface between the oxide film and the matrix,which accelerated the oxidation of the matrix,Cr was enriched in the internal oxide film,which increased the density of the oxide film and prevented the oxidation progress.Summarizing the effects of Ni and Cr on the microstructure,mechanical properties,corrosion resistance and oxidation resistance of silicon solution strengthened ductile iron,for silicon solution strengthened ductile iron applied for offshore wind power equipment parts,1.4%Ni and 0.45%Cr are suggested,and for silicon solution strengthened ductile iron applied for metallurgical sintering equipment and coking equipment parts,less than 0.4%Ni and 0.45%Cr are suggested.