| As is a kind of high strength cast iron material,the mechanical properties of nodular cast iron are close to those of steel.Currently,our country has entered a well-off society in an all-round way.As the demand of people for vehicles has greatly increased,every type of vehicle needs to be manufactured by stamping.Nodular cast iron is often used as the material for stamping dies of automobile panels.After high-strength and high-frequency impact,the die of this material will deform and crack due to insufficient strength,which will result in the decrease of die dimensional accuracy,and seriously affect the service life of automobile panel die,thus bringing about the increase of production cost and slow down of production efficiency.Therefore,it is very important for enterprises to develop an automobile stamping die with high strength and toughness.In order to improve the properties of nodular cast iron,this study intends to use multi-factor and multi-level visual analysis method to optimize the composition of alloying elements based on the composition ratio of nodular cast iron in production practice,and explore the influence of different normalizing temperatures on the microstructure and properties of nodular cast iron in combination with the composition ratio of nodular cast iron in production practice.On the basis of this study,the waste heat normalizing process was used for practical application research combined with the actual production conditions,and then high strength and toughness nodular cast iron castings were obtained,which provides technical support for enterprises to produce high strength automobile panel dies and improves their core competitiveness.The main research contents and results are as follows:(1)Multi-factor and multi-level visual design method was used to design 9 groups of experiments.After smelting,spheroidizing,inoculation and pouring,nodular cast iron samples were obtained.At the same time,the tensile strength and percentage elongation after fracture were measured after processing.Through the analysis of the optimized data,the influence of different alloy element content on the tensile strength and percentage elongation after fracture of nodular cast iron was clarified,and verified by corresponding experiments.It was found that the tensile strength of nodular cast iron increased with the increase of Cu content,but this trend was no longer obvious when Cu content was higher than 1%,because the distribution coefficient of Cu in iron was only 1.09.However,with the increase of manganese content,its tensile strength increased,but its elongation decreased obviously,which resulted from the fact that manganese could enlarge austenite phase region.Moreover,Mn atoms increased the phase transformation resistance of nodular cast iron castings,hindered the self-diffusion of iron atoms,slowed down the decomposition of austenite,increased the critical undercooling of eutectoid transformation,and improved its tensile strength.However,Mn atoms could easily form a network cementite with carbon in the matrix,which reduced the toughness of castings and resulted in the decrease of elongation of nodular cast iron.The experimental results showed that the nodular cast iron with tensile strength of 800.3 MPa and elongation of3.7% could be prepared with manganese content of 0.53%,copper content of 1.18% and nickel content of 0.32%.(2)The influence of normalizing temperature on the mechanical properties of nodular cast iron was studied by using partial austenitizing normalizing process and holding temperature of 750 ℃,775 ℃,800 ℃,825 ℃ and 850 ℃ respectively.It was found that the spheroidization rate of graphite spheres in nodular cast iron decreased with the increase of normalizing temperature,and flocculent graphite and non-rounded graphite morphology appeared,which resulted from the fact that the saturation of carbon in austenite decreased during cooling,which could lead to the enrichment of carbon atoms in austenite to the surface of graphite spheres.However,due to the short cooling time of normalizing,carbon atoms could not be completely enriched to the surface of graphite spheres,resulting in the appearance of flocculent graphite.The pearlite content in nodular cast iron matrix increased with the increase of normalizing temperature.It is possible that the austenitizing degree in the microstructure increases with the increase of normalizing temperature during heating,and austenite undergoes eutectoid transformation and decomposes into pearlite during cooling,thus increasing the pearlite content in the microstructure.In addition,the pearlite lamellar spacing in the matrix was calculated by statistical method,and it was found that the pearlite lamellar spacing was nearly doubled when the normalizing temperature increased from 750℃ to 800℃.The nodular cast iron with tensile strength of 850 MPa and elongation of 8.7% could be prepared at the normalizing temperature of 800 ℃.The nodular cast iron with tensile strength of more than 800 MPa and elongation of more than 5% could meet the requirement of high strength and nodular cast iron with tensile strength of more than 800 MPa and elongation of more than 5%.At the same time,it was explained that pearlite lamellae could prevent crack propagation and increase the tensile strength of nodular cast iron castings.(3)On the basis of composition optimization and normalizing treatment,the influence of residual heat normalizing on the microstructure and mechanical properties of nodular cast iron was studied in combination with production practice and residual heat normalizing process.Nodular cast iron with tensile strength of 744 MPa and elongation of 7.4% was prepared by waste heat treatment.By comparison,it was found that normalizing with waste heat could effectively increase the number of graphite spheres in matrix,improve the roundness and spheroidization rate of graphite spheres,promote the formation of pearlite and significantly refine the lamellar spacing of pearlite,and improve the tensile strength of nodular cast iron.However,the elongation decreased to some extent,which resulted from the fact that nickel copper manganese and other elements in the matrix could reduce the eutectoid transition temperature,increase the pearlite content,reduce the pearlite lamellar distance and improve the tensile strength of the matrix.Compared with as-casting samples,the tensile strength of samples normalized by waste heat could be improved.However,due to the lack of heat preservation stage in normalizing treatment,there is still a certain gap in mechanical properties compared with normalizing treatment. |
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