| Crankshaft,a key component of the engine,not only has high strength and toughness,but also has good balance and wear resistance.The requirements for material selection and heat treatment process system are often high,and the production process is complicated.Therefore,how to directly obtain the crankshaft that meets the performance requirements in the as-cast condition has become the main research direction for shortening the crankshaft production process and improving the competitiveness of the product,which greatly conforms to the development trend of"casting forging" and "taking iron for steel".In this study,the iron-type sand-casting casting process was used to develop a new preparation process for the as-cast crankshaft by optimizing the alloy composition,perfecting the spheroidizing technology and improving the pouring conditions.The optical microstructure(OM),scanning electron microscope(SEM),tensile test,impact test,hardness test and other methods were used to study the residual heat normalization process,the unpacking cooling conditions and the microalloying properties of the as-cast crankshaft.The influence and its mechanism of action,and the key control technology of high-performance as-cast crankshaft in the production process.The main findings are as follows:(1)After the orthonormal crankshaft is normalized,the spheroidal rate is significantly increased and the roundness is also increased.With the advancement of the unpacking time,the cooling rate of the casting during the pearlite transformation increases,and the degree of subcooling increases,which promotes the nucleation and growth of the pearlite,and the number thereof increases significantly,and the sheet spacing is significantly reduced.When the unpacking time is 6 min,the micro structure and mechanical properties of the as-cast crankshaft can be comparable to that of the normalized crankshaft.When the unpacking time is 10min,the obtained as-cast crankshaft has the best comprehensive mechanical properties:tensile strength 930MPa,elongation5%,hardness 263N/mm2,impact toughness 50J/cm2.(2)After the air-cooling,air-cooling and mist-cooling treatment of the as-cast crankshaft obtained by unpacking in 15 minutes,the cooling rate is further increased,and the pearlite volume is increased while the pearlite sheet spacing is refined.The mist cooling is compared with the air cooling.The content is increased by 13%.The as-cast crankshaft obtained by fog cooling has a tensile strength of 880MPa,an elongation5%,a hardness of 262N/mm2,and an impact toughness of 44 J/cm2.(3)After the crankshaft is strengthened by Cu alloy,the spheroidization rate increases and the average graphite length decreases.Cu can reduce the eutectoid transformation temperature to provide greater subcooling for the formation of pearlite and enrichment in the graphite-matrix.The diffusion of C atoms promotes the conversion of the candle body.As the content of Cu increases,the amount of pearlite increases and the spacing of the denser pearlite sheets decreases.The as-cast crankshaft obtained by adding 1.1%Cu has the best comprehensive mechanical properties,tensile strength 1000MPa,elongation 4.5%,hardness 284N/mm2,impact toughness 45J/cm2.(4)After the as-cast crankshaft is strengthened by Cu-Sn alloy,Sn can increase the starting temperature of pearlite transformation to provide greater subcooling for its transformation and the enrichment layer of Sn hinders the diffusion of C atoms to promote pearlite shape.With the increase of Sn content,the matrix tends to 100%pearlite structure,the pearlite volume change does not significantly maintain the 4%growth trend;the as-cast crankshaft obtained by adding 1.0%Cu and 0.021%Sn has the best comprehensive mechanical properties.The tensile strength at this time was 1011MPa,the elongation was 4.5%,the hardness was 302N/mm2,and the impact toughness was 34J/cm2. |
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