Study On Thermal Analysis Testing Of Ductile Iron Solidification And Shrinkage Tendency

Due to its high strength and toughness,ductile iron is widely used in various fields of industry and is one of the very important cast iron materials.For metallurgical and technological factors,the stability of ductile iron solidification characteristics and spheroidization and inoculation effect cannot be guaranteed,and the solidification shrinkage type casting defects often appear in production practice.Most of the control of ductile iron solidification shrinkage defects in foundry production plants is off-line detection afterwards,which is not conducive to online adjustment of the quality state of the molten iron.Since the thermal analysis test means are limited by the structure and size of the specimen,it is not suitable for the analysis and application of the solidification characteristics and expansion and contraction process of ductile iron.Therefore,this project realized the joint detection of multi-parameters of relative thermal conductivity temperature curve,normal temperature cooling curve and displacement expansion and contraction curve along the diameter of the specimen using the thermal analysis sample cup with a large melt specimen made of resin sand to analyze the relevant characteristic parameters and correlation of solidification characteristics and expansion and contraction process of ductile iron.On this basis,the effect of metallurgical related factors on the solidification and expansion-shrinkage tendency of ductile iron with thermal analysis using dry clay sand cylindrical sample cups of small size specimen,and the results of the study showed that:The use of a special shaped relative thermal conductivity thermal analysis of large specimen structure to achieve two temperature curves and a linear displacement curve multi-parameters detection,which is suitable for ductile iron solidification process and expansion and contraction structure formation characteristics research and analysis.The cooling timeτ₁₀₀in the 1160-1060℃section of the relative thermal conductivity curve has a good correlation with T₃,and the nodular ratio N_A%of the lower end of the shaped sample has a good correlation with T₃,and R²is up to 0.95.The cooling curve of the large diameter hot center at the upper end of the shaped specimen is significantly influenced by the tendency of solidification shrinkage characteristics,and the ductile iron solidification has the minimum shrinkage tendency under experimental conditions with characteristic parameters K（eutectic expansion coefficient）<0.35;α_S（included Angle between the end point of solidification）<45°.Thermal analysis of small specimens of dry clay sand sample cups showed that the carbon and silicon content change the solidification mode and transformation characteristics by significantly affecting the final carbon equivalent and eutectic degree of the ductile iron.The eutectic solidification state has the smallest initial crystal shrinkage coefficient S and the largest eutectic expansion coefficient K.As the carbon potential energy of hypereutectic melt decreases,S increases instead,and K decreases,which is detrimental to the expansion of eutectic graphitization and self-replenishment shrinkage of melt solidification.All three graphite characteristic parameters:graphite sphericity N_A,graphite area rate S_Aand graphite sphere number A_S,reach their maximum values in the eutectic solidification mode state,indicating that the factors favoring stable eutectic graphitization and spherification are beneficial to the self-replenishing shrinkage of solidified graphite expansion.The thermal analysis with dry clay sand sample cup shows that the spheroidization treatment has less effect on the T_Lof the initial crystallization temperature of under-eutectic ductile iron melt solution.The 1.40%and 1.50%spheroidization treatment,the eutectic minimum temperature T_EUsignificantly reduces by 2～3°C,and the eutectic maximum temperature T_ERchange little,resulting in an increase in the initial eutectic solidification temperature region and an increase in the eutectic reflection temperatureΔT_R.The coefficient of shrinkage S of initial crystallization of melt solidification increases,and the coefficient of expansion K of eutectic solidification does not change significantly.The three graphite characteristic parameters of typical graphite sphericity N_A,graphite area rate S_Aand graphite sphere number A_Sin the center of the thermally analyzed specimens are all maximum at 1.40%spherification treatment condition.Under spheroidization or over spheroidization is detrimental to the graphitization and spheroidization properties of melt solidification,which is not conducive to eutectic graphite expansion and self-replenishing shrinkage of the solidification process.1.0%～0.45%secondary incubation was simulated in the cup of dry clay sand samples,and the inoculant addition significantly affects the initial crystal temperature T_Land eutectic nucleation temperature T_EUof the melt.The molten iron inoculated in 0.40%inoculation cup is in eutectic solidification state,the primary shrinkage coefficient S appears the minimum value,and the eutectic expansion coefficient K seems to be inclined to the minimum value.The volume budget is the most stable in the eutectic solidification mode of molten iron inoculated in the sample cup.With the increase of inoculation amount in the sample cup,the number of typical graphite spheres（A_S）in the center of the thermal analysis sample increases,and the graphite area ratio（S_A）and spheroidization ratio（N_A）both have the maximum peak values in the eutectic mode.It can be seen that eutectic graphitization and spheroidization are more stable in the state of inoculation eutectic solidification mode,which is conducive to the self-regulation between solidification shrinkage and expansion and shrinkage of molten iron.