Microstructure And Mechanical Properties Of Low Temperature High Toughness Ductile Iron

Low temperature high toughness ductile iron(low temperature ductile iron)has excellent comprehensive mechanical properties,which is widely used in high-speed railway and wind power.The impact toughness decreasing is nature characteristic of the ductile iron material with the decrease of temperature,and it's impact toughness is influenced by the graphite morphology,the ferrite quantity and impurity.Therefore,taking effective measures to optimize the chemical composition and microstructure of the ductile iron to improve the low temperature impact toughness maximally,which have a positive promoting role in the development of China's high-speed rail and wind power as well as other industries.The purpose of this paper is to improve the microstructure and properties of low temperature ductile iron by adding alloy elements Co and Ni to ferrite ductile iron.Under the experimental conditions,four kinds of high toughness ductile iron alloy at low temperature were prepared,cobalt and nickel content were:WCo=1.5%,WNi=0.4%;WCo=l.8%,WNi=0.4%;WCo=2.2%,WNi=0.4%;WCo=2.5%,WNi=0.4%.The effects of cobalt on the microstructure and mechanical properties of the alloys in-50 ? were investigated by means of phase constitution,quantitative metallographic analysis,micro-fluorescence spectroscopy,fracture scanning and mechanical properties.The results show that:.The microstructure of as-cast ductile iron is composed of nodular graphite,ferrite and a small amount of pearlite,has no carbide and phosphorous eutectic.After heat treatment,the pearlite disappeared,the matrix organization is all ferrite.Witli an increase of cobalt content,the graphite nodule count increases,the sphere diameter becomes smaller,the chunky graphite decreases and the graphite precipitate increases.And the pearlite content decreases with the cobalt content in the as-cast ductile iron.In the as-cast and heat-treated ferrite ductile iron,silicon content has "U" shaped segregation model.Compared to the as-cast ductile iron,the "U" shaped segregation model degree of heat-treated ductile iron reduces.In different ferrite grain of graphite nodule periphery,the silicon content has certain deviation with same diameter circle.Even within the same ferrite grain,silicon content also has certain volatility along the outer graphite nodule diameter round distribution,but,compared with the as-cast,the uniformity of silicon distribution is improved in heat treatment of ductile iron.Under the condition of as-cast and heat-treated,the degree of solid solution strengthening increases with the increase of cobalt content,so as to increase the tensile strength of the ductile iron,and the elongation increases firstly and then decreases,the impact toughness of the as-cast ductile iron increases firstly and then decreases,and the impact toughness decreases greatly at low temperature,the low temperature impact toughness after heat treatment is higher than that of as-cast.The hardness of the as-cast ductile iron was fluctuated within the range of 149HBW?159HBW,while the hardness of the heat-treated ductile iron was stable,and its value was 145HBW.As-cast,when the content of cobalt and nickel is WCo=1.8%and WNi=0.4%,the comprehensive mechanical properties of ductile iron are the best:the tensile strength of 405MPa,the elongation of 19.7%,the impact energy of the average value achieve 12.7J in-50 ?,single value is greater than 9J,the ductile iron QT400-18L could meet the requirements in the high cold region.