| Fe-Ni-Mo-Cu-C low alloy powder metallurgy(PM)steels are widely used in the automotive industry due to its excellent integrated mechanical properties.Howerver,for occasions requiring high mechanical properties,the strength and toughness of the sintered steel cannot meet the performance characteristics due to the existence of inherent porosity in the material.Use of high-performance lubricants to improve green density,addition of alloying elements,and heat treatment process are effective methods to improve the toughness and wear resistance of PM low alloy steel.In this paper,the effects of composite lubricant(a mixture of polyoxyethylene and erucic amide),microalloying element vanadium and quenching-low temperature tempering heat treatment on the process,microstructure and mechanical properties of Fe-Ni-Mo-Cu-C sintered low alloy steels were investigated by means of powder flowmeter,optical microscopy analysis,scanning electron microscopy analysis and mechanical properties testing.The results indicate that the addition of composite lubricant significantly reduced the flowability and the apparent density of the mixed powder.The mixed powders could not flow out of Hall flow meter when the composite lubricant contained 40%of erucamide.A higher green strength and density was achieved by using the composite lubricant compared to the green specimens containing zinc stearate,and the mixed powder containing composite lubricant could be pressed to green specimens with a density of 7.126 g/cm~3,and 0.101g/cm~3 more than that of green containing zinc stearate at a pressure of 600MPa.Compared to specimens containing zinc stearate,higher mechanical properties and little dimensional change rate of sintered specimens were achieved by composite lubricant.The addition of vanadium refined the as-sintered microstructure of medium-carbon Fe-1.75Ni-0.5Mo-2Cu-0.45C and high-carbon Fe-1.75Ni-0.5Mo-2Cu-0.8C PM low alloy steels and reduced the grain size.The addition of vanadium increased the content of polygonal ferrite and granular bainite in the medium-carbon sintered PM steel.The distribution of carbon in the sintered high-carbon PM steel alloyed with vanadium was more uniform and the number of carbon-rich zone and grain boundary were reduced.V(C,N)particles were formed in specimens during sintering in a atmosphere containing nitrogen.Ferrovanadium did not completely dissolve into the steel matrix during sintering process,and V(C,N)particles precipitated during cooling process.The addition of 0.1-0.3%vanadium increased the strength and hardness of the sintered PM medium-carbon and high-carbon steel.Addition of 0.2%V improved the transverse rupture strength of medium-carbon steel from 772.6MPa to830MPa,and improved the hardness increased from 116.8HV to 123.3H;transverse rupture strength of high-carbon steel increased from 894.3MPa to 963.5MPa and the hardness increased from 128.5HV to 137.2HV.Further increase in V content to 0.3%led to a negative effect on fracture toughness of the steel.V(C,N)limited the growth of austentite size during austenization.Compared with the vanadium-free samples,the sintered samples containing vanadium after quenching obtained more fine lath martensite.The addition of 0.1and 0.2%vanadium increased the transverse rupture strength of the heat-treated specimens,however,the strength of the heat-treated steel decreased significantly when0.3%vanadium was added.Tribological tests indicated that the addition of V improved the hardness and wear resistance of heat-treated high-carbon PM low alloy steel,but reduced the hardness and wear resistance of the heat-treated PM medium-carbon low alloy steel. |
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