Effect Of Oxidation On Microstructure And Properties Of Vermicular Graphite Cast Iron

With the continuous progress of the national material science,the requirements for power and emission diesel engines are becoming higher and higher.The vermicular graphite cast iron has become more and more popular in diesel engine cylinder head and cylinder block because it has better comprehensive performance than that of gray cast iron and nodular iron,and gradually becomes the best choice for diesel engine cylinder head and cylinder block material.The diesel engine cylinder head which is subjected to heat load and mechanical load is a typical and complex thin-walled part.At the same time,it is corroded by the oxygen and sulfur gas at high temperature resulting in producing the oxide layer and oxygen diffusion layer on its near surface,which significantly reduces the mechanical and physical properties of the cylinder head Micro-cracks are generated and continuously expand into the matrix under severe conditions,which would reduce the service performance and reliability of the cylinder head and affect the service life.The tissue deterioration and degradation of performance of high power density diesel cylinder head during high temperature working condition were studied.The effects of holding time at the temperature of 400? and 500? respectively on micro-structure,mechanical properties and thermal conductivity of vermicular graphite cast iron that under atmospheric and vacuum conditions were researched by the analysis and measure of OM,SEM and tensile properties.And the evolution mechanism of the structure and properties of Vermicular graphite cast iron was analyzed by means of SEM and EDS.The effect of holding time on thermal fatigue resistance of vermicular graphite cast iron under atmospheric and vacuum conditions was studied by means of thermal fatigue testing machine and scanning electron microscope at the temperature of 500?.And the mechanism of fatigue resistance evolution of vermicular graphite cast iron in atmosphere and vacuum was analyzed.At the same time,the oxidation behavior of vermicular graphite cast iron at high temperature was revealed by means of OM,SEM,oxidation weight gain,oxidation kinetics test and so on.The results show that the surface oxide layer consists of oxide film layer and oxygen diffusion layer when the vermicular graphite cast iron is kept warm at the temperature of 400? and 500? in the atmosphere.With the increasing of the holding time,the thickness of oxide film layer and oxygen diffusion layer is growing rapidly first and then slowly,until to balance.The tensile strength of vermicular graphite cast iron at room temperature and high temperature(500?)increases firstly and then decreases with the extension of the holding time at the temperature of 400?,but its tensile strength increasingly decreases with the extension of the holding time at the temperature of 500?.Under the same test conditions,the tensile strength of vacuum keep warming is 10-30%higher than that of atmospheric environment holding temperature.The thermal conductivity of vermicular graphite cast iron in atmospheric and vacuum environment decreases firstly and then increases with the extension of the holding time at the temperature of 500?,but the thermal conductivity of vacuum keep warming is 8-13%higher than that of atmospheric environment holding temperature.With the extending of holding time,the curve of oxidation weight gain of vermicular graphite cast iron is parabola.The oxidation weight fitting curve at the temperature of 400? is as follows:?W1=0.0045t-2.11x10-5t2+2.56x10-8t3,The oxidation weight fitting curve at the temperature of 500? is as follows:?W2=0.014t-3.63x10-5,t2+3.54x10-8t3.The oxide producing from vermicular graphite cast iron in the atmospheric environment of 400? and 500? is mainly Fe3O4.Graphite is the main channel for oxygen diffusion.The depth of the oxygen diffusion layer is mainly affected by the graphite shape and content of the surface.Under the same oxidation conditions,the worm graphite oxygen diffusion layer depth is 4-5 times larger than that of ductile iron.At the same time,the larger the graphite cluster is,The deeper the oxygen diffusion layer is.The thermal fatigue crack length of vermicular cast iron in the environment of atmosphere and vacuum firstly expands rapidly and then goes down with the extension of holding time.The crack initiation is mainly affected by the holding environment and the thickness of oxide film,and the crack propagation is mainly affected by the thermal fatigue upper limit temperature and the depth of oxygen diffusion layer.After the atmosphere holding temperature,the interaction of oxidation and thermal fatigue accelerates the initiation and expansion of fatigue cracks.The fitting curve of crack expansion is as follows:?2=393.72N-66.13N2+3.95N3.After the vacuum environment holding temperature,the interaction between micro-oxidation and heat stress produced by thermal fatigue makes the fatigue crack initiation and propagation speed become smaller compared with the atmosphere holding temperature.The fitting curve of the thermal fatigue crack expansion is as follows:?1=675.09N-103.82N2+5.13N3.The thermal fatigue resistance performance of vermicular graphite cast iron in vacuum environment is better than that of the atmospheric environment.