Investigations On Oxidation,Tensile And High-Cycle Fatigue Properties Of Compacted Graphite Iron

Compacted graphite iron(CGI)has become the preferred material for the preparation of diesel engine cylinder heads due to its good comprehensive properties.With the increase of diesel engine power density and increasingly stringent environmental requirements,the cylinder heads will face more serious service environment,which will seriously damage their mechanical properties.For the cylinder heads,high-temperature oxidation and high-cycle fatigue(HCF)are common engineering problems during service.In this thesis,the static high-temperature oxidation behavior and related mechanism of CGI were studied.The matrix microstructures of CGI were adjusted by heat treatment,and the effects of pearlite content on tensile properties,fatigue properties and related damage mechanisms were investigated.The main research contents and conclusions are as follows:(1)The CGI was placed in the atmospheric environment of 450℃and 500℃to carry out static discontinuous oxidation experiments.It is found that the oxidation rate of CGI follows the parabolic rate law.The matrix and graphite exposed on the surface of the specimens are oxidized first,and the thickness of the oxide layer on the surface shows a parabolic growth rule with the extension of oxidation time.The severely oxidized compacted graphite on the surface forms“oxidation channels”that allow oxygen to enter the interior of the matrix.Oxidized compacted graphite tends to diffuse along the pearlite connected with it,resulting in the decrease of pearlite content in the matrix.(2)The pearlite content in CGI was adjusted by annealing and normalizing,and tensile and HCF tests of CGI with different pearlite contents were carried out at room temperature.The results show that both the tensile strength and fatigue strength increase with the increase of pearlite content,and the tensile strength increases more obviously.Fatigue microcracks tend to initiate at the graphite/ferrite interface,and these microcracks form the main crack through bridging,leading to the final fatigue fracture.The fatigue ratio shows a linear decreasing relationship with the increase of pearlite content.Therefore,a method to predict the fatigue strength of CGI by pearlite content and tensile strength is proposed:σ_w=(m C_P+n)·σ_b.(3)High-temperature tensile and HCF tests of CGI with different pearlite contents were carried out at 500℃.It is found that increasing the pearlite content can improve the high-temperature tensile strength and fatigue strength of CGI.Compared with room temperature,the strengths of CGI decrease significantly at high temperature.At 500°C,due to the weak oxidation resistance of pearlite,fatigue cracks can not only initiate at the graphite/ferrite interface,but also initiate at the graphite/pearlite interface.A shorter oxidation time has a greater effect on fatigue crack propagation,while a longer oxidation time has effects on the initiation and propagation of fatigue cracks.It is proved that this prediction methodσ_w=(m C_P+n)·σ_b can accurately predict the high-temperature fatigue strength of CGI.