Research On The Influence Of Vermicular Rate On Crstallization Process And Shrinkage Tendency Of Compacted Graphite Cast Iron

As a kind of new excellent engineering materials,compacted graphite cast iron（CGI）has received more and more attention from the world.However,the formation mechanism of compacted graphite is complicated specially,and the crystallization process of CGI is still a controversial issue.Compared to gray iron,the casting shrinkage defect of CGI has become one of the most critical factors that seriously affects the quality.However,The research on the prediction of shrinkage tendency is still less and insufficient.So,exploring the crystallization process and accurate defect prediction is of great significance for both the CGI actual production and academic research.In this paper,the influences of vermicular rate on the primary crystallization process were studied by using the instantaneous liquid quenching method at the characteristic points of the thermal analysis curve.At the same time,the thermal analysis technique was used to study the connection of the characteristic curves、vermicular rate and shrinkage tendency that cased by the changes of carbon content、silicon content、EMg and instantaneous inoculation addition.By studying the associated mechanism,we explore the influence of vermicular rate on the shrinkage tendency of CGI,put forward a useful shrinkage forecast model successfully.The experimental results show that:（1）The vermicular graphite is mainly formed by the transformation of spherical graphite,no matter the vermicular rate is high or low.However,the transformation is more earlier and rapid for the CGI with high vermicular rate.（2）The eutectic solidification starts from bottom to top in the thermal analysis cup.According to the eutectic degree,it is generally divided into three parts:pitting zone,transition zone,light zone.With the increase of vermicular rate,the eutectic reaction rate of CGI becoming more faster,and so the eutectic solidification process does.（3）Due to the influence of heat convection,the thermal analysis characters does not reflect the metallographic change at thermocouple point,the change of thermocouple temperature is a reflection of the integral heat dissipation among solidification process.（4）In the case of hypoeutectic composition,with the increase of vermicular rate,the reasonable graphitizing expansion enhances its self-complement effectively and adequately,and the tendency of shrinkage tends to decrease obviously.In the case of hypereutectic composition,the graphitizing expansion force is too large in the earlier stage,which resulting in squeezing molten iron to the riser and model expansion violently;the graphitizing expansion caused by the eutectic transformation of residual carbon is not enough to compensate for shrinkage,which leads to the shrinkage tendency increases drastically.（5）In this experiment,the shrinkage tendency is minimal at the treated condition of C3.8%,Si2.4%,Mn0.5%,0.36wt.%_V,0.3wt.%_N.（6）The tendency of shrinkage is related to the characteristic values:TAL、TSEF、TEU、TER and so on,and the signatures of time have the same effect on it.During the thermal analysis cooling curve period,the period of TAL to TSEF is liable to form shrinkage cavity;it’s easy to form expansion during TEU to TER;the longer the period of TER to TED,the easier it reduces shrink tendency;and it’s easy to form slight shrinkage during TED to TES.（7）The shrinkage forecast model is:S=-3.926·T_TAL-TEU+80.17·T_TER-TEU-2.426·T²_TER-TEU-15.2·T_TER-TES+0.12·T²_TER-TES+5.034·t_TAL-TEU-3.93·t_TER-TEU+5.407·t_TER-TED+243.13·Ln(t_TED-TES)-68.44·TEM’+1.9·TEM’²-169.6·1/TED〞-15.29·TES’-1525.4...