BStrength-Toughness Properties And Thermal Conductivity Research Of High-performance Compacted Graphite Iron

Cast Iron with higher strength and ductility,better thermal properties,casting properties and damping properties has been the ideal material for the cylinder head of diesel engine. With the continuous improvement of advanced diesel engine power,the higher requirements of better toughness and themal conductivity for engine cylinder head material are needed. It is not only requires further high obdurability,heat resistance and fatigue resistance,but also superior thermal performance in order to reduce thermal stress which caused by local overheating. There are lots of studies on the obdurability of different alloyed cast irons by the researchers,but little attention to the obdurability and thermal conductivity comprehensively.Four kinds of the multi-component micro-alloying compacted graphite cast iron (CGI) are designed and prepared, which are normal CGI, Cu-Mo alloyed CGI,Cu-Mo-Sn alloyed CGI and Cu-Mo-Sn-Cr alloyedCGI.Microstructure,mechanical properties,damaging micro-mechanisms and thermal conductivity are studied and analyzed comprehensively by use optical microscopy,scanning electron microscopy,mechanical properties testing, in-situ tensile testing and thermal conductivity test in the paper.The results show that the obvious effects of multi-component micro-alloying elements on vermicularizing was not observed,but the graphite are thinning.Improving formation of pearlite accounts,the lamellar spacing of pearlite obviously narrows and distribution more uniform when elements Cu,Mo,Sn and Cr are added. Meanwhile,the strength of the multi-component micro-alloying CGI at room temperature are increased.Compared to the normal CGI,the tensile strength of Cu-Mo-Sn-Cr alloyed CGI increase from358.9MPa to508.5MPa,improves about42%,the yield strength improves about55%,hardness improves about82%,the compression strength improves about125%,the σ0.2/σb increased from0.8to0.88,even the elongation over1.5%.The strength and retention rate of strength at elevated temperaturesare improve greatly.Compared to the normal CGI, at500℃, the tensile strength of Cu-Mo-Sn-Cr alloyed CGI improves from202.9MPa to376.1MPa,improved about87%,the yield strength improves about71%,the retention rate of strength increased from0.55over to0.64.The micro cracks can be appear on graphite-graphite interface and graphite-matrix interface of CGI.Extend along the direction of graphite curved or favorable and stress direction.The formation of the main crack is in the sample boundary of thick graphite.The more and more micro cracks initiation under load.The micro cracks and crack are selectively connected. Crack along ferrite exend easily in the matrix. With higher pearlite content the more fracture stress are needed and the lamellar cementite is cut.While the load is higher than the fracture need energy, the main crack quickly connected and extended to the whole sample. Compared to GCI and CGI,it found that the cracks, micro cracks between vermicular graphite base can have multiple. Bending degree larger cracks and tearing phenomenon of graphite are obviously appeared.The effect of multi-component micro-alloying and vermicularizing on thermal conductivity of CGI are comparatively analysis. Show that under200℃, with the increase of the content of pearlite, the thermal conductivity reduce. Compared to the pearlite content about40%Cu-Mo alloyed CGI the thermal conductivity is38.8W·m-1K-1at100℃.With the increase of temperature, the thermal conductivity decreased to varying degress. Especially,Cu-Mo-Sn-Cr alloyed CGI decreased most slowly and its thermal performance is the best over400℃.With the increase of vermicularizing the thermal conductivity also increase.With the increase of temperature from100℃,80%and50%vermicularizing CGI the thermal conductivity increase slowly.Every100℃increase about0.4W·m-1K-1～0.6W· m-1K-1,and the maximum value is between250～350℃and then decreased, the thermal conductivity of20%vermicularizing CGI decreased with the temperature decreased slowly under500℃.Between100℃and400℃only reduc1W·m-1K-1.Thermal conductivity changed show different characteristics with the temperature increase. Both the graphite and matrix affect it.Thermal conductivity of CGI matrix is mainly free electron conductivity. While the graphite is mainly by the phonon thermal conductivity.With the increase of temperature,the thermal conductivity of some CGI increased,which caused by the thermal conductivity of graphite.