Effects Of Heat Treatment Process And Alloying On Microstructure And Mechanical Properties Of Fe-based Powder Metallurgy Materials

Fe-based powder metallurgy materials are widely used in the manufacturing of key autoparts due to their advantages of near net forming,mass production and high material utilization.In recent years,with the rapid development of the automobile industry,there are higher requirements for the comprehensive mechanical properties of Fe-based powder metallurgy parts.Alloying and heat treatment are two effective ways to improve the strength,toughness and wear resistance of materials.We first studied the microstructure and mechanical properties of three Fe-Ni-Mo-Cu-C series sintered steel of powder metallurgy with different Ni contents,and then studied the heat treatment process of tempering at low temperature after the complete quenching(QT),heated up to the intercritical quenching(HIQ),cooled down to the intercritical quenching(CIQ)and W-Mo double glow plasma surface alloying technology on microstructure and mechanical properties of sintered materials by optical microscopy(OM),scanning electron microscopy(SEM),energy dispersive spectrometer(EDS),X-ray diffraction(XRD)and numerous mechanical properties tests.The main conclusions were summmarized as follows:Compared with sample without Ni,the addition of Ni resulted in the formation of Ni-rich austenite,martensite/bainite and ferrite as well as the pearlite structure without Ni in the local area of sintered steel during the cooling process with the decrease of Ni content.The Ni-rich microstructure significantly increased the density,hardness,tensile strength and elongation.The FN-0400 sample was7.247 g/cm³,97 HRB,869 MPa and 4.32%,respectively.The tensile fractures of sintered samples with different Ni contents all presented a mixed fracture mode of cleavage brittle fracture and microporous aggregate ductile fracture.The Ni-rich austenite and martensite structure in Ni-containing samples made the crack development more tortuous and consume energy,delay the crack growth,and significantly improved the plastic toughness of the material.The study of heat treatment process showed that the mechanical properties of samples without Ni under the intercritical quenching process were obviously inferior to that of the complete quenching process,while the ferrite,martensite and austenitic complex structure obtained under the intercritical quenching process of Ni-containing samples guaranteed the strength and significantly improved the ductility and toughness.In addition,compared with HIQ process,CIQ process obtained more austenite wrapped by martensite.In particular,when the intercritical quenching was done at 780?under CIQ process,the density,hardness and tensile strength of FN-0400 sample reached QT process level,while the elongation was much higher than that of QT process,reached 7.261 g/cm³,45 HRC,288 MPa and 4.05%,respectively.Under the QT process,the fracture of all samples showed microporous aggregation ductile fracture.Under HIQ and CIQ processes,the fracture surfaces of samples without Ni presented cleavage brittle fracture,and the number of cleavage plane increased with the increase of intercritical quenching temperature.The cracks in Ni-containing samples were mainly generated at the ferritic/martensitic phase interface and pores,and expanded along the phase interface and inside the austenite.Because of the presence of austenite,the number of cleavage surfaces of Ni-containing samples decreased significantly with the increase of the intercritical quenching temperature,Especially under the CIQ process,even at the lower quenching temperature,the fracture of FN-0400 sample presented a microporous aggregate fracture with good plasticity and toughness.The homogeneous and dense W-Mo alloyed layer was prepared on the surface of Fe-based powder metallurgy gear materials by double glow plasma surface technology.The nano-hardness and elastic modulus of the alloyed layer were 7.7 GPa and 221 GPa,respectively,which were 2.02 times and 1.43 times greater than that of substrate.The friction and wear experiments showed that,under the high load of 670 g,the specific wear rate of the alloy layer was 2.62×10^-5 mm³·N^-1·m^-1,which was only 18.6%of that of the substrate.Compared with the substrate,the wear mark surface of the alloy layer only presented narrow and shallow grooves and slight spalling,showed good wear resistance.