| Cylinder liner is one of the important core components in the engine,which has a significant impact on the performance and service life of the engine.Engine cylinder liner works in high temperature,high pressure and other environments,which is easily affected by high temperature and high pressure and easily damaged.Repeated friction between cylinder liner and piston also easily leads to its failure.With the development of super-large engines,higher requirements are put forward for components,which require high strength and excellent wear resistance of high-power engine cylinder liners.However,there are a series of bottlenecks in the production of super-large high-performance cylinder liners at present.In this paper,aiming at the low strength and hardness of commercial gray cast iron cylinder liners,alloying and cooling rate control are adopted to improve the microstructure and mechanical properties.Firstly,gray cast iron was alloyed with Nb and mixed rare earth(La,Ce),and the effects of different contents of Nb and rare earth on the microstructure and mechanical properties of cast iron were studied.Secondly,four different casting materials were used for casting,and the characteristics of microstructure and properties of gray cast iron under different cooling rates were investigated.Experiments show that Nb can obviously refine flake graphite in gray cast iron.In addition,Nb can obviously refine the lamellar spacing of pearlite in the matrix structure,and the lamellar spacing is refined from 530 nm to 210 nm.With the increase of Nb content,the tensile strength,compressive strength and hardness of cast iron also increase.When Nb content reaches 0.35 wt.%,the tensile strength,compressive strength and Brinell hardness of the samples reach the maximum values of 537 MPa,1650MPa and 387 HB respectively.In gray cast iron,a small amount of Nb is dissolved in the matrix,and most of Nb is distributed in the matrix structure in the form of rich Nb phase.Nb has the functions of second phase strengthening,solution strengthening and fine grain strengthening in cast iron.The addition of mixed rare earths(La,Ce)can refine the microstructure and improve the mechanical properties.With the addition of rare earth elements,graphite gradually changes from coarse flake to fine and evenly distributed flake.When the content of rare earth is 0.6wt.%,it can promote whitening and the amount of cementite increases obviously.The addition of rare earth elements in cast iron is beneficial to the reduction of pearlite lamellar spacing.When RE content reaches 0.4 wt.% and 0.6 wt.%,the tensile strength and compressive strength of the samples reach the maximum value of 501 MPa and 2209 MPa,respectively.Trace rare earth elements can promote graphitization,at the same time,increase supercooling degree in pearlite transformation process,reduce lamellar spacing,and improve performance significantly.However,if the content is too high,the formation of Fe3 C will be promoted,and the tensile strength will decrease and the hardness will increase.Experiments show that the increase of cooling rate will improve the tensile strength,compressive strength and hardness of the samples.With the increase of cooling rate,the tensile strength increased from 346 MPa to 421 MPa,while the compressive strength and Brinell hardness increased from 1115 MPa and 255 HB at lower cooling rate to 2165 MPa and 409 HB at higher cooling rate.Increasing cooling rate will make graphite fine and uniform,at the same time,eutectoid transformation supercooling degree will increase,and pearlite lamellar spacing will decrease.This is the main reason for the change of mechanical properties of cast iron.The experimental results obtained in this paper can provide theoretical and practical guidance for the selection of materials and processes for super-large cylinder liners. |
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