Research On The Microstructure And Properties Of Heavy Section As-cast Pearlite Nodular Iron

In recent years, the rapid development of major equipment manufacturing industry promotes the research and application of heavy section nodular iron. The requirement of the market for high strength heavy section nodular iron is more and more urgent. Although our country has made a lot of achievements on the field of heavy section nodular iron, the heavy section nodular iron production technologies of most domestic manufacturers are not good enough to meet the needs of the market. Dependence on imports for key large castings has limited the development of the equipment manufacturing industry. Therefore, the study on the microstructures and properties of heavy section pearlite nodular iron is of great significance to promote the foundry industry and the equipment manufacturing industry.In the present paper, the heavy section as-cast pearlite nodular iron was prepared by using resin-coated sand casting. The effects of Cu, Sb, Si content and specimen thickness on the structures and properties of nodular iron were investigated. The effects and mechanisms of Sb element on the graphite balls were researched by using SEM and EDX. The pearlite morphology of the heavy section as-cast pearlite nodular iron was observed. The effects of Cu element and thickness on the solidification process of the nodular iron were studied by analyzing the cooling curve. The microstructures and properties of the different location in the heavy section pearlite nodular iron were compared.The paper has got the following conclusions:(1) As for the specimen whose size is250mm×250mm×300mm, the chunky graphite could be restrained which would reduce the mechanical properties by adding a small amount of Sb. The graphite spheroidization would be improved, and the pearlite content would increase in this specimen too. So the mechanical properties of this specimen would be improved by adding defined amount of Sb. If the Sb addition amount was excessive, the graphite spheroidization of this specimen would deteriorate. When the Sb addition amount was0.015wt.%, the specimen with good performance and microstructure could be obtained. The tensile strength, yield strength, elongation and hardness in the central part were up to525MPa、317MPa、4.1%and200HBW,which were23%,10%,33%and8%higher than those of the specimen without minor elements. The intermetallic with high melting point which is composed by Sb, Ce, and La could act as the heterogeneous nuclei of the graphite.(2) The spheroidization rate and the pearlite content in the central part of the specimen whose size is250mm×250mm×300mm could be increased by adding proper content of Cu. So the strength and hardness in the central part would be improved, but the ductility would be reduced. When the Cu addition amount was0.50wt.%, the mechnical properties of the specimen were the best. The liquidus temperature(TL), the eutectic temperature starting(TE) and ending temperature(TS), the austenitic crystallization temperature range(TL-7emim) and eutectic coagulation time were1170.2℃,1110.3℃,9.6℃and97.3min.1166.2℃. But these were1164.9℃,1097.4℃,9.8℃and95.2min when the specimen was not added Cu. Cu element could promote nodular iron graphitization, reduce its chilling tendency, make austenite dendrite precipitate ahead of time, and reduce the branching of austenite dendrite.(3) When Si content was2.1wt.%, the specimen whose size is250mm×250mm×300mm could get good spherical graphite and high pearlite content. So the mechanical properties of this specimen were high.(4) As for70mm×70mm×180mm size specimen,120mm×120mm×180mm size specimen,180mm×180mm×200mm size specimen, and250mm×250mm×300mm size specimen added with0.50wt.%Cu and0.50wt.%Sb, the performance in the central part first increased, then decreased as the size increased. The mechanical properties of120mm×120mm×180mm size specimen were the best. The tensile strength, yield strength, elongation and hardness in the central part were up to640MPa,371MPa,5.2%and225HBW. Because the70mm×70mm×180mm size specimen is small and cooling fast, for this size specimen Cu and Sb addition amount was too high, which caused the mechanical performance significantly reduced.(5) When the sample size was250mm×250mm×300mm, the pearlite layer spacing in the central part was reduced to0.3～0.6μm by adding0.50wt.%Cu and0.015wt.%Sb. The pearlite morphology was irregular. There were a number of fractured pearlite and globular pearlite.(6) The microstructures and properties in position40mm away from the center and in the center part of250mm×250mm×300mm size specimen added with0.50wt.%Cu and0.015wt.%Sb were better than those in position72mm and104mm away from the center. The microstructure and properties in position40mm away from the center was the best. The tensile strength, yield strength, elongation and hardness were up to529MPa,319MPa,4.3%and202HBW.The characteristics of heavy section nodular iron make Mg and rare earth elements segregate in position72mm and104mm away from the center which caused the mechanical performance significantly reduced in this area.