A Study on the Cyclic Heating Induced Intergranular Fracture of Ferritic Spheroidal Graphite Cast Irons

• Main Authors: Hung-Mao Lin, 林宏茂
• Other Authors: Truan-Sheng Lui
• Format: Others
• Language: zh-TW
• Published: 2004
• Online Access: http://ndltd.ncl.edu.tw/handle/91006054052637992749

博士 === 國立成功大學 === 材料科學及工程學系碩博士班 === 93 === 　Ferritic spheroidal graphite (SG) cast iron is often used as a candidate alloy for structural components that operate at high temperatures and undergo repeated heating/cooling conditions, even the external load after cyclic heating. Under these circumstances, the components periodically operate at a high temperature ranging around AC1 of approximately 850℃. The thermal stress arising from a steep temperature gradient may largely which lead to cyclic heating induced cracking that causes ductility deterioration, which is called cyclic heating induced intergranular fracture in this dissertation, in these components after cyclic heating. The ductility deterioration thus caused reduces the application reliability of this material for cyclic heating. Therefore, it is vital to investigate and prevent the cyclic heating induced intergranular fracture of ferritic SG cast iron. 　The effect of the highest temperature on the cyclic heating induced intergranular fracture of ferritic SG cast iron was investigated during thermal cycling with a temperature range of 25℃ to AC1 (where the temperatures selected were respectively 650℃, 700℃, 750℃, 800℃ and 850℃). The susceptibility to cyclic heating induced intergranular fracture was most severe at the heating temperature of around 750℃. Further increasing or decreasing the heating temperature could prevent the cyclic heating induced ductility deterioration. The better resistance to cyclic heating induced intergranular fracture at the lower heating temperature of 650℃ was related to the smaller temperature gradient within the specimen during thermal cycling. On the other hand, recrystallization of ferrite grains could be observed after heating at the maximum temperature of 800℃. The occurrence of recrystallization could improve the resistance to cyclic heating induced intergranular fracture. In addition, when the temperature of thermal cycling was 850℃, there was a significant deterioration of elongation that resulted from the partial martensite phase transformation of the ferrite matrix. 　Eutectic cell wall regions where inclusion particles are clustered are present in the central region among graphite nodules. The inclusion particles are mainly MgO which are formed since the spheroidizer used in the casting process of SG cast iron contains magnesium, i.e., the Fe-45wt%Si-8wt%Mg alloy. The degree of inclusion particles clustered in the eutectic cell wall regions can be successfully revealed by electrochemical etching. The electrochemical evidences of the specimens before and after cyclic heating and tensile test results, show that the clustered inclusion can promote the nucleation of cyclic heating induced cracks and the cumulative concentration of thermally induced deformations around the eutectic cell wall regions, where completely intergranular brittle fracture occurs. Furthermore, the magnesium at the final solidificational region and the internal oxidation of grain boundaries can promote the propagation of cyclic heating induced cracks, therefore the occurrence of the cyclic heating induced intergranular fracture. 　The degree of inclusion clustering is affected by the soliification history of SG cast irons. The degree of MgO particles concentrationrises as the silicon content increases. Materials with a high silicon content have both higher degree of inclusion clustering and flow stress than those with a low silicon content. Hence, in the case of the former, thermally induced cracks nucleate easily and therefore cyclic heating induced intergranular fracture occurs. Furthermore, in the case of high-silicon SG cast iron that has a better resistance to oxidation, the count of inclusion clustering and the degree of metallic magnesium residues increase as the residual magnesium content rises. This promotes the cyclic heating induced intergranular fracture of high-silicon SG cast iron. In addition, refining microstructures by increasing solidification rate leads to a larger number of eutectic cell wall regions and hence a smaller degree of inclusion particle clustering. A high solidification rate also decreases the degree of the metallic magnesium residues on the grain boundaries of the material. Consequently, thermally induced cracks are hard to nucleate and the cyclic heating induced intergranular fracture is impeded. Since molybdenum is added in the ferritic SG cast iron, the strengthening effect of molybdenum on cell walls together with the formation of Mo2C in the eutectic cell wall regions can impede the formation and propagation of cyclic heating induced cracks and prevent the occurrence of the cyclic heating induced intergranular brittle fracture. 　On the other hand, according to the tensile testing results of high-silicon SG cast iron at 400℃, the intermediate-temperature intergranular embrittlement occurs and the ductility drops drastically. Furthermore, the ductility of high-silicon SG cast iron at 400℃ decrease as the residual magnesium content rises. The fractography indicates an intergranular fracture. Some MgO particles aggregate in the eutectic cell wall regions and metallic magnesium segregation can be detected. Refining of microstructures by iron-mold casting, which has the effect of reducing clustering of MgO particles, can eliminate the intermediate temperature intergranular embrittlement at 400℃. The abovementioned experimental results confirm that there is a close relation between the cyclic heating induced intergranular fracture and the intermediate temperature intergranular embrittlement in this material. Through this study that investigates the cyclic heating induced intergranular fracture of ferritic SG cast iron, the factors affecting the embrittlement behavior are clarified. It is also possible to prevent the occurrence of the embrittlement and improve the application reliability of the material for cyclic heating.