Effect Of Ni And Annealing Process On Microstructure And Properties Of Low-Temperature High-Toughness Ductile Cast Iron

The mechanical properties of low-temperature high-toughness ductile cast iron are excellent,and the impact toughness can be maintained even at low-temperature environment.It is widely applied to the fields of rail transit,large-scale ships,nuclear power,wind power generation and the like.The service environment of the castings used in this field is very bad,and even long-term operation in low temperature environment.Therefore,the quality and performance of castings are strictly required,especially the low temperature impact toughness.At present,the main methods to improve the low temperature impact toughness of ductile cast iron are alloying and heat treatment.In this paper,on the basis of QT400-18 ductile cast iron,the low temperature impact toughness of ductile cast iron was improved by Ni alloying and annealing.In this paper,the influence of Ni content on the as-cast microstructure and mechanical properties of ductile cast iron was studied by casting Y samples.The results show that the content of pearlite increases and the matrix structure is refined after adding Ni element.When the content of Ni increases from 0.0%to0.6%,the number of graphite spheres increases,and they are more fine and round,and the ferrite grains are refined.However,when the content of Ni continues to increase,the spheroidization rate and uniformity of graphite slightly decrease,and the refining effect on ferrite grains is also weakened.With the increase of Ni content,the tensile strength and hardness of as-cast ductile cast iron increased,while the elongation and low temperature impact toughness decreased.Secondly,the heat treatment process of Y samples without Ni and with 0.6%Ni was optimized by comparative analysis.The single-stage and two-stage graphitization annealing processes of low temperature(760?),medium temperature(850?),high temperature(920?)were used.The results show that the comprehensive mechanical properties of ductile cast iron are the best after two-stage annealing at low temperature(760?×4h+730?×4h).Then,the effect of Ni content on the microstructure and mechanical properties of heat treated ductile cast iron was discussed under the best heat treatment process.The results show that after heat treatment,the content of ferrite in ductile cast iron is over98%,and the comprehensive mechanical properties of ductile cast iron with 0.6%Ni are the best.The tensile strength,elongation,low temperature impact toughness and Brinell hardness of the sample with 0.6%Ni reached 410MPa,26.3%,18.8J/cm~2,155HB.The optimized Ni content and annealing process are applied to the simulation test block of 300m×300mm×300mm heavy section ductile cast iron.The results show that the number of graphite balls in the heavy section ductile cast iron with 0.6%Ni increases slightly,the morphology of graphite balls is improved,it becomes finer and rounder,the matrix structure is also refined,and the ferrite grain size is reduced.After annealing at low temperature and two stages,the content of ferrite in the structure can reach more than 95%.The mechanical properties of the two test blocks are the highest at the edge position.With the prolongation of solidification time,deformed graphite appears in the samples,which leads to the sharp decrease of mechanical properties.Compared with that without Ni,the heavy section ductile cast iron with 0.6%Ni has higher mechanical properties.At the edge position,the tensile strength,elongation,low temperature impact toughness and Brinell hardness of the test block with 0.6%Ni increased by 6.8%,25%,14.8%,4.0%.At the center position,the tensile strength,elongation,low temperature impact toughness and Brinell hardness of the test block with 0.6%Ni also increased by 1.6%,11.1%,3.8%,5.6%.