Effects Of Vanadium And Heat Treatment Process On Microstructure And Properties Of Nodular Cast Iron

As a high-strength cast iron material,nodular cast iron has good comprehensive mechanical properties and is widely used in wind power generation,transportation,and other fields.With the development of various industrial fields in the direction of portability,low energy consumption,and high efficiency,it is difficult for ordinary ductile iron to continue to meet the market requirements.Higher requirements are put forward for the comprehensive properties of nodular cast iron.Therefore,this paper takes ductile iron as the research object,with vanadium as the alloying element,by adjusting the vanadium content and optimizing the heat treatment process,the influence of vanadium and the heat treatment process on the microstructure and mechanical properties of ductile iron is studied and analyzed.In this paper,the effects of vanadium content on the as-cast microstructure and mechanical properties of ductile iron were studied by casting standard Y-shaped test blocks with a wall thickness of 25 mm and vanadium content of 0,0.05%,0.1%,0.15%,and 0.2%respectively.The results show that with the increase of vanadium content,the graphite spheroidization rate and spheroidization grade in the sample structure has little effect.The spheroidization rate is maintained at 80%-90%,the spheroidization grade is grade 3,and the ball diameter grade is grade 6.The matrix structure is pearlite,ferrite,and a small amount of vanadium and molybdenum composite carbides.With the increase of vanadium content,the pearlite content increases first and then decreases,the ferrite content decreases first and then increases,and the number of carbides increases gradually.When the vanadium content is 0.1%,the pearlite content reaches the highest of 74.8%,and the ferrite content is at least 22.3%.The tensile strength and hardness of as-cast ductile iron increased first and then decreased,while the elongation and impact toughness decreased.When the vanadium content is 0.1%,the tensile strength and Brinell hardness reach the highest,which are 564 MPa and 210 HBW,respectively.The elongation and impact toughness is 12.1%and 6.3 J/cm~2,respectively.Secondly,the heat treatment process of samples with different vanadium content was optimized,and the optimum vanadium content and heat treatment process were selected.Normalizing temperature is set to 840°C,890°C,940°C,990°C,and1040°C,holding 2 hours,observe the different normalizing temperatures and different vanadium content on the microstructure and mechanical properties of the matrix to determine the best-normalizing temperature.The results show that the matrix consists of pearlite,bainite,and ferrite after normalizing.With the increase of normalizing temperature,the content of bainite decreases,the content of pearlite increases,and the content of ferrite changes little.When the normalizing temperature is 940°C,the comprehensive performance of the sample is the best.To eliminate the bainite and residual stress formed in the matrix structure after normalizing,the samples after the best normalizing temperature were subjected to high-temperature tempering treatment.The tempering temperature was set to 530°C,560°C,590°C,and 620°C,and the holding time was 1 hour.The results show that after high-temperature tempering,bainite decomposes to form tempered sorbite.When the tempering temperature is 560°C,the number of free cementite in the tempered sorbite structure is the least,the precipitated carbides are evenly distributed,and the Brinell hardness is significantly lower than that after normalizing.Therefore,the heat treatment process of normalizing temperature at 940°C,holding for 2 hours,tempering temperature at 560°C,and holding for 1 hour,the comprehensive performance of ductile iron is the best.Then the effect of vanadium content on the microstructure and properties of heat-treated ductile iron was discussed under the heat treatment process.The results show that after heat treatment,the content of pearlite in ductile iron reaches more than 95%,and there is also a small amount of tempered sorbite.When the vanadium content is 0.15%,the comprehensive performance of ductile iron is the best.At this time,the tensile strength,elongation,impact toughness,and Brinell hardness of ductile iron reach 1020 MPa,3.9%,2.8J/cm~2,and 294 HBW,respectively.