The Control Technology Of Black Speck On Heavy Section Ferrite Nodular Cast Iron

In the past40years, the black speck in the centre part of heavy section ferritenodular cast iron has been bothering researchers around the world. The black speck,the most typical defect of heavy section ductile iron, is due to the graphite distortedinto chunky graphite and gathered together, and that is a kind of macro performance,which lowers the mechanical properties of the castings greatly. In order to eliminatechunky graphite, improve mechanical properties, manufacturers have used heattreatment, or adding alloying elements (such as Cu and Sb). These measures madegratifying achievements, but also rose the manufacturing cost. In order to improve themarket competitive power, the as-cast heavy section ductile iron with excellentperformance is the priority selection.The main content of this paper is about the effect of several processes, such asadding different content of trace element Bi and residual rare earth, on graphitemorphology and mechanical properties of heavy section ferrite nodular cast iron. Theeffect of mixed RE spheroidizer is also discussed. With the multi-functionaltemperature collection device and the analytical method of eutectic solidificationcooling curve, we analysed the eutectic solidification cooling curve of heavy sectionferrite nodular cast iron, then studied the solidification process of nodular iron.Observed the graphite morphology by optical microscopy, analysed the tensile fracturemorphology and the element composed of the heterogeneous nucleation core and grainboundary segregation by SEM and EDX. Reaearch conclusions are as follows:(1)For180mm×180mm×200mm specimen, adding0.012wt%Bi could obviouslyeliminate the chunky graphite. When the size of test block is250mm×250mm×300mm, adding trace Bi could only inhibit chunky graphite in a certain extent. With theincreased amount of Bi, the inhibition of chunky graphite enhanced at first and thenweakened, the best value of Bi between0.010wt%and0.012wt%. Meanwhile, thevalue of tensile strength, elongation and-40℃low temperature impact toughness was337MPa,10.4%and10.5J, respectively. Reduced the content of RE and made RE/Biadd=1.3~1.5, could further reduce the chunky graphite and improve the comprehensivemechanical properties. The process of adding trace Bi is an effective controltechnology of black speck. In addition, adding Bi can restrain the pearlite. The tensilefracture of the sample with lots of equiaxial toughening nest is belonged to ductile fracture. Heterogeneous nucleation core of graphite is composed of the oxygen orsulfur compounds of Magnesium and the high melting point intermetallic, such asCeBi and LaBi.(2)The process of spheroidizer mixed heavy and light Rare Earth is an effectobvious control technology of black speck. Compared the different kind of heavy REspheroidizer, The inhibition of chunky graphite of the Tian-He D-2spheroidizer ismuch better than that of Long-Yi DY-2spheroidizer. Compared the differentcomponent mixture ratio, the microstructure and properties of the mixture ratio of4:6is obviously better than that of the mixture ratio of3:7. It means that the inhibition ofchunky graphite would enhance with the increased mixture ratio of the heavy rare earthspheroidizer. Grain boundary segregation may be composed by steadite, compounds ofMagnesium and high melting point compounds of Rare Earth.(3)With the increased wall thickness of ductile iron from70mm to250mm, thecrystallization temperature interval would enlarge from32.1℃to59.8℃, the coolingspeed would decreased from2.74℃/min to0.59℃/min, and solidification time wouldextend from11.7min to101.1min. Then the spheroidizing level would be decreasedfrom2grade to3-4grade, the size of spherical graphite would enlarge from6grade to5grade and eutectic group grain would be bigger and less, and graphite morphologyand mechanical properties would deteriorate gradually. Chunky graphite obviouslylowers comprehensive mechanical properties. When the wall thickness is below180mm and solidification time is less than48.2min, spherical graphite would separate outand grow up, and mechanical performance is good. Then, if the wall thickness continueto increase, graphite distortion would be more likely. In addition, there is an obviousrecalescence phenomenon on the solidification cooling curve of nodular iron with70mm wall thickness.