Effect Of Cu-rich Nanophase On Microstructure,Mechanics And Corrosion Property Of Steel

Designing materials and then greatly improving the toughness of structural steel by means of nanophase precipitation strengthening is the trend of technology development.Two representative Cu-rich nano-phase strengthened steels,ie.ship steel and mold steel for plastic were selected as research objects,metallographic microscope(OM),scanning electron microscope(SEM),transmission electron microscope(TEM)and Energy Dispersive Spectroscopy(EDS)were used to observe the microstructure,precipitates and composition of the tested steels subjected to various heat treatments.The microstrainαvalue,the phase constitution and its content in the samples heat treated by various processes were estimated by X-ray diffraction profile analysis.Electrochemical corrosion experiments such as polarization curve and AC impedance spectroscopy were carried out to evaluate the corrosion resistance of different materials heat treated by various processes.Some regular changes of the microstructure,mechanics and corrosion properties of two kinds of Cu-rich nano-phase strengthened steels with precipitation of Cu-rich nano-phase were obtained.(1)The results of strength and corrosion resistance of Cu-rich nano-phase strengthened ferritic ship steels with different Y contents show that under-aging leads to the formation of micro-cells with nano-precipitates,which reduces corrosion resistance.At the peak of aging,the nanoparticles are largely precipitated to the size limit of the coherency,and the nano-precipitates are connected to the substrate in a sheet with good corrosion resistance.When it is over-aged,due to the growth of the nanoparticles,the nano-precipitates are gradually separated from the substrate,and the corrosion resistance is decreased.The optimum heat treatment process using 0.01wt.%Y steel is solid solution at 900℃for 0.5 h and aging at 550℃for 1 h.(2)Cu-rich nano-precipitates and Y-containing compounds in the range of50-100 nm are formed in the steel.The average size of the nano-precipitates in Y-containing steel is larger,but the number density is lower,and the ferrite grains are refined.The tensile strength and extensibility of Y-containing steel after air aging are improved.The drag effect of Y thins the oxide layer and enable itto closely contact the substrate.Grain refinement,oxidation resistance and purification from Y addition in Y-containing steels ameliorate the brittleness of air aging of nano-phase precipitation-strengthened ferritic ship steels.(3)The results of micro-strain,hardness and corrosion resistance of aged Cu-rich nano-strengthened mould steel for plastic show that aging at 300-450℃,theαincreases with aging time up to 8 h,then decreases with longer aging time due to precipitates losing coherency;the variation of hardness exhibits a similar trend as theα.Aging at 600℃,theαpeaks at 2 h,which is attributed to the two opposite influences:coherent microstrain and reverted austenite;the reverted austenite makes the hardness exhibit a slight variation.The changing laws with aging time for the R_f and theαexhibit a good polynomial correlation of inverse relationship in the coherent regime,i.e.the R_f decreases with increasing the α.