Research On Niobium Microalloyed Ductile Iron And Its Heat Treatment Process

Ductile iron has excellent mechanical properties and low price,so it has a high output and a wide range of applications.However,with the development of transportation,wind power,nuclear power and other fields,castings such as engine crankshaft,wind turbine hub,nuclear fuel storage and transportation devices put forward higher requirements for the strength and plasticity of ductile iron,but general alloying elements such as Cu and Ni will greatly reduce the plasticity while improving the strength,which can not meet the requirements.As a strong carbide forming element,Nb can combine with C to form carbide,which plays a role of precipitation strengthening to increase the strength with a proper elongation.In this paper,the effect of niobium(Nb)on the microstructure and properties of ductile iron was investigated to obtain a comprehensive properties.The heat treatment process was optimized,and the comprehensive mechanical properties of the developed ductile iron was further improved on the basis of the optimal heat treatment process.The precipitates of ductile iron were analyzed by scanning electron microscope(SEM)and energy dispersive spectroscopy(EDS).The result shows that the precipitates are mainly composed of Nb,Mo and C,the sizes of precipitates are less than 5μm,and most of the precipitates are about 3μm.Through thermodynamic calculation and cooling curve analysis,the phase diagram of ductile iron containing Nb in the composition range of this paper was obtained,and the precipitation behavior of Nb C during solidification was analyzed by phase diagram.The analysis of microstructure shows that with the increase of Nb content,the morphology of graphite deteriorates,the pearlite content increases and the pearlite lamellar spacing decreases.The analysis of mechanical properties shows that with the increase of Nb content,the hardness and strength increase,while the elongation and impact toughness decrease.The optimum mechanical properties occur at the Nb content of 0.14%,with the hardness,tensile strength,yield strength,elongation,and impact toughness are182.3HBW,530.6MPa,405.2MPa,16.6%,and 91.5J/cm~2,respectively.The effects of different austenitizing processes and tempering processes on the microstructure and hardness of normalized/quenched+tempered ductile iron were systematically studied.The analysis of microstructure and hardness shows that the best process of the two kinds of heat treatment is 980℃×1h+560℃×1h.On the basis of the optimum heat treatment process,the effect of Nb content on the microstructure and mechanical properties of heat treated ductile iron was investigated.The analysis of microstructure shows that with the increase of Nb content,the tempered sorbite content increases and the recrystallized ferrite content decrease in normalized+tempered ductile iron,while the content of recrystallized ferrite decreases in quenched+tempered ductile iron.The analysis of mechanical properties shows that with the increase of Nb content,the hardness and strength of normalized/quenched+tempered ductile iron increase,but the elongation and impact toughness decrease.When the Nb content is 0.14%,the comprehensive mechanical properties of heat-treated ductile iron are the best.The hardness,tensile strength,yield strength,elongation and impact toughness of normalized+tempered ductile iron are 36.5HRC,835.1MPa,610.3MPa,4.7%and 31.7J/cm~2,respectively,and those of quenched+tempered ductile iron are33.2HRC,861.3MPa,696.4MPa,3.4%and 23.2J/cm~2,respectively.