Effect Of Cobalt And Nickel On Microstructure And Properties Of High Toughness Ductile Iron At Low Temperature

High toughness ferrite ductile iron at low temperature has excellent mechanical properties at room temperature and higher fracture toughness at low temperature,it's mainly used in high speed rail,wind power,oil pipeline transportation and other fields.The impact toughness decreasing is nature characteristic of the ductile iron material with the decrease of temperature,and the solidification strengthening degrce,the graphite shape and size,the ferrite quantity,the impurity element and other factors together affect the impact toughness of the material.Therefore,taking effective measures to optimizing the graphite shape and size of the alloy,reducing the degree of solid solution strengthening,increasing the amount of ferrite and effective controlling the impurity content,and thus ensuring the strength of the ductile iron to improve the impact toughness,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 high toughness ductile iron at low temperature by adding alloy element Co and Ni.Under the experimental conditions,four kinds of high toughness ductile iron alloy at low temperature were prepared,cobalt and nickel content were:WCo=3.0%,WNi=0.6%;WCo=3.0%,WNi=0.4%;WCo=2.0%,WNi=0.4%;WCo=1.0%,WNi=0.4%.Thc effects of cobalt and nickel on the microstructure and mechanical properties of the alloys were investigated by means of phase constitution,quantitative metallographic analysis,micro-fluorescence spectroscopy,fracture scanning and mechanical properties.The results show that:The structure of high toughness ductile iron at low temperature is mainly composed of ferrite and spherical graphite.With the increase of cobalt content,the graphite nodule count increases,the sphere diameter becomes smaller,the chunky graphite decreases and the quantity of ferrite increases.While with the increase of nickel content,the quantity of pearlite increases,and the shape and size of graphite spheres have no obvious change.Cobalt and nickel have little effect on intergranular carbides and oxides.With the increase of cobalt content,the amount of ferrite increases,and the degree of solid solution strengthening for ferrite matrix increases,so that the strength and hardness of ductile iron increase,the elongation increases firstly and then decreases,and the fracture toughness also shows the same variation tendency.When the cobalt content is increased to 2%,the elongation and impact toughness of ductile iron reach the maximum value,and the fracture morphology of the impact fracture at-40? from the toughness fracture into cleavage fracture.With the increase of nickel content,the quantity of pearlite increase,and the tensile strength and hardness of the alloy increase,while the elongation and impact toughness of the alloy decrease.When the content of cobalt and nickel of alloying elements is WCo=2.0%and WNi=0.4%,the comprehensive mechanical properties of ductile iron are the best:the tensile strength>420MPa,the yield strength>280MPa,the elongation>18%,the impact of the average value achieve 13J>12J in-50?,the ductile iron QT400-18L could meet the requirements in the high cold region.