Research On The Composition Optimization And Property Improvement Of Nodular Cast Iron Used For Piston Ring

Nodular cast iron has been widely used for engine piston ring because of high strength and high toughness.In accordance with the problems encountered in the production of Shuanghuan company,the influence of composition(especially Si and Mn)and heat treatment on microstructure and mechanical properties was investigated in this paper.The main results are as follows:The micro structure and properties of Shuanghuan and model company were compared and analyzed.Results showed that there was great difference on composition between two company and high C content,low Si content and low Mn content were found in Shuanghuan.Bad graphite nodule was found in Shuanghuan because of lower nodularity.The micro structure of these two samples was both tempered sorbite that remained shape of queching martensite,but shape of Shuanghuan was much thicker and bigger.Besides,mechanical properties of Shuanghuan were worse due to lower hardness,lower bending strength and lower fatigue strength.The influence of Si and Mn content on the micro structure and mechanical properties was investgated.Results showed that with the increase of Si content,bending strength and hardness of nodular cast iron gradually increased because Si could improve graphite shape,promote the precipitation of carbide and strengthen the matrix.However,when Si content was too high(≥3.0%),graphite shape became worse and carbide began to gather and grow,which reduced strength.Mn produced less impact on graphite shape.With the increase of Mn content,bending strength and hardness of nodular cast iron gradually increased because Mn could improve hardenability of nodular cast iron,make carbide more dispersed distribution and strengthen the matrix.However,when Mn content was too high(≥0.40%),martensite transition temperature dropped and retained austenite content increased,which made strength decrease.Besides,Si or Mn could strengthen the matrix and reduce plasticity,which decreased deflection of nodular cast iron.The influence of heat treatment on the micro structure and mechanical properties was analyzed.Results showed that when tempering temperature remained unchanged,with the increase of quenching temperature,bending strength and hardness of nodular cast iron gradually increased because C content of austenite raised that enhanced dispersion strengthening of carbide after quenching and tempering treatment.However,when quenching temperature exceeded 890℃ or 910℃,needlelike martensite became much thicker and retained austenite content increased,which made shape and distribution of carbide worse and reduced strength.Besides,with the increase of quenching temperature,deflection of nodular cast iron gradually decreased because needlelike martensite became much thicker and retained austenite content increased.When quenching temperature remained unchanged,with the increase of tempering temperature,carbide became more uniform and much thicker,finally reducing bending strength and increasing deflection.Through the experiment,optimum composition is obtained and that is 3.7%C,3.0%Si and 0.15%Mn.In addition,optimum heat treatment is found and quenching treatment is 890℃×60min and tempering treatment is 550℃～560℃×90min.Under these circumstances,mechanical properties of nodular cast iron are optimal and bending strength is 1906MPa,deflection is 3.731mm and hardness is 35.7HRC.Former properties of Shuanghuan are as follows:bending strength is 1741 MPa,deflection is 2.382mm and hardness is 34.8HRC.Compared with former properties,optimal properties of nodular cast iron remarkably improved.