Investigation Of Preparation And Application Of Nano-(NbTi)C/Fe Composite Powder

Steel materials play an indispensable role in national infrastructure construction and daily life due to their high strength,good plasticity and toughness.With the demand for different applications,the requirements for the properties of cast steel materials are gradually increasing.In the steel strengthening methods,the grain refinement effect is the most obvious,and it is the only way to simultaneously improve the toughness and the strength of the material.The addition of carbide particles in steel has the characteristics of second phase size and quantity controllability,and simple production process.In this paper,?NbTi?C was selected as the strengthen particles,which was synthesized by laboratory and added to 45 steel and infinite chilled cast iron to achieve the purpose of refining the microstructure of steel materials and improving mechanical properties.The density of NbC?7.82 g/cm³?is larger than that of molten steel?7.2 g/cm³?.In order to reduce the density of NbC and make it more uniformly dispersed in molten steel,three different Ti-doped?NbTi?C solid solutions were designed.The lattice parameters,structural stability,mechanical properties and electronic properties were analyzed by first-principles method.The results show that they are stable in thermodynamics,mechanics and structure.In this paper,the Nb-Ti-graphite system was ball milled for 8 h by mechanical activation+high temperature synthesis and heated at 750?C for 30 min in a vacuum tube furnace to prepare different?NbTi?C nano-powders with different Ti doping amounts,which solved the problem of long ball milling time and high energy consumption in traditional powder preparation.The?NbTi?C/Fe composite powder was prepared by adding an equal amount of Fe powder by ball milling for 3 h and then heated at 750?C to achieve the effect of pre-dispersion of nano-?NbTi?C in Fe powder.The structure in which the?NbTi?C solid solution was embedded in the Fe powder was obtained,and the particle diameters were all about 200 nm.The prepared three kinds of?NbTi?C/Fe composite powders were added to an infinite chilled cast iron melt and cast.The equilibrium phase diagram and the property diagram were calculated by Thermal-Calc software and the microstructure was observed.The results show that during the solidification cooling process L??,a part of?NbTi?C becomes the nucleation core of the primary austenite,which makes the primary austenite grain size smaller;a part of?NbTi?C is repulsed to the grain boundary by the solidification front;in the remaining liquid phase,an L??+?NbTi?C eutectic reaction occurs,and this precipitated?NbTi?C is distributed in the boundary as the form of a eutectic type and connected with?Fe,Cr?₃C.The precipitated?NbTi?C and?NbTi?C repelled to grain boundary together becomes the heterogeneous nucleation core of the cementite,thereby refining the carbides and making the carbide distribution more uniform.During the tempering process of infinite chilled cast iron,the metastable retained austenite structure transforms into martensite,and the supersaturated carbon in the martensite gradually de-dissolves to form carbides.After the addition of?NbTi?C,the austenite grains are refined,the carbide size is reduced,and the amount of retained austenite is increased due to the increased stability of the retained austenite.As the tempering temperature increases,the hardness increases,mainly due to the secondary quenching and martensite desolvation transformation.After the second tempering,some residual austenite remains in the modified steel,resulting in a lower hardness value.The prepared nano-?NbTi?C/Fe was added to 45 steel.The equilibrium solidification phase diagram and property diagram of 45 steel were calculated by Thermal-Calc software,and the thermodynamic and kinetic calculations of?NbTi?C dissolution and precipitation behavior at different temperatures were carried out.The results indicate that inside the dendrites,the diffused?NbTi?C becomes the nucleation core of?-Fe and?-Fe,refining the austenite grains;the?NbTi?C particles distributed in chain-like are formed by the devouring of the solidification front.It has hindered the growth of dendrites.Between the dendrites,the particle-like distribution?NbTi?C is repelled to the intergranular formation during solidification;while the rod-like distribution and granular mixture?NbTi?C is solidified during the formation of L??+?NbTi?C eutectic reaction is formed.There is also a square?NbTi?C precipitation inside the austenite grains.In terms of mechanical properties,the strength and hardness of the steel added with?NbTi?C were higher than those of the unadded steel.The prepared modified 45 steel was subjected to normalizing treatment.At normalized austenitizing temperature,?NbTi?C becomes an austenite nucleation core,and diffused?NbTi?C also hinders the growth of austenite grains.The mechanical properties test results show that the strength and hardness of the steel added with?NbTi?C were higher than those of the unmodified steel,and the elongation was reduced.After quenching and different temperatures tempering,the strength and hardness of the steel added with?NbTi?C were higher than those of the unmodified steel.