Effect Of Mo Content On Microstructure And Mechanical Properties Of Si-Mo Heat-Resistant Ductile Iron

Silicon molybdenum heat-resistant ductile iron is widely used in automobiles,power,machinery,and other fields because of its good mechanical properties and heat resistance.With the development of such industries,the requirements for the mechanical properties and heat resistance temperature of silicon-molybdenum heat-resistant ductile iron are getting higher and higher.Therefore,it is of great significance and value to optimize the additional amount of Mo element in silicon-molybdenum heat-resistant ductile iron to maximize the performance advantages of silicon-molybdenum heat-resistant ductile iron and make it better applied in the corresponding fields.In this paper,the effect of the Mo element on the microstructure,mechanical properties,and high temperature(500°C)mechanical properties of silicon-molybdenum ductile iron was studied by adjusting the alloying element Mo and heat treatment.To obtain heat-resistant ductile iron materials with good comprehensive properties,the main research results are as follows:With the increase of Mo content in the as-cast state,the roundness and spheroidization rate of the spherical graphite of the silicon-molybdenum ductile iron did not change significantly.The graphite grade was 5-6,the spheroidization rate reached more than 80%,and the number of graphite balls decreased slightly.The precipitates of as-cast ductile iron were analyzed by scanning electron microscopy(SEM)and energy dispersive spectroscopy(EDS).The results show that the precipitates are Mo-rich carbides mainly composed of C,Mo and Fe.With the increase of Mo content,the content of pearlite and Mo-rich carbides in the matrix increases gradually,and the pearlite lamellar spacing decreases gradually.The Mo content increases from 0.2%to 0.8%,the pearlite and Mo-rich carbides increase from 2.4%to14.0%and 0.5%to 3%,respectively,and the pearlite lamellar spacing decreases from0.42μm to 0.26μm.With the increase of Mo content,the hardness and tensile strength at room temperature show an upward trend,and the impact toughness and elongation after fracture show a downward trend.When the Mo content increases from 0.2%to0.8%,the hardness and tensile strength increase from 170 HBW and 525 MPa to 205HBW and 578 MPa,respectively.The impact toughness and elongation after fracture decreased from 73 J/cm~2 and 20%to 22 J/cm~2 and 11%,respectively.The increase of Mo improves the high-temperature strength of the alloy.When the Mo content is 0.2%,the high-temperature tensile strength is 284 MPa and the elongation is 26%.With the increase of Mo content,the high-temperature tensile strength increases.When the Mo content is 0.8%,the high-temperature tensile strength increases to 339 MPa and the elongation decreases to 22%.After low-temperature graphitization annealing,the graphite ball diameter of silicon molybdenum ductile iron increases slightly,the graphite grade is grade 5,the flake pearlite in the matrix decomposes into granular,and the morphology of Mo-rich carbides at the grain boundary changes little.With the increase of Mo content,the content of pearlite and Mo-rich carbides increases gradually.With the increase of Mo content,the hardness and strength of annealed Si-Mo nodular cast iron gradually increase,and the impact toughness and elongation after fracture gradually decrease.When the Mo content is 0.4%,the comprehensive mechanical properties of Si-Mo nodular cast iron are the best.The sum of Brinell hardness,tensile strength,elongation,impact toughness,high-temperature tensile strength,and high-temperature elongation after fracture are 175 HBW,543 MPa,22%,81 J/cm~2,327 MPa,and 25%,respectively.