Development Of Broken Ferrite Ductile Iron Crankshaft With High Strength And High Toughness

Crankshaft plays an extremely important role in the automobile, and it will directly determine the quality of the life of a vehicle. Because of this, engine is called "the heart of car," and crankshaft is the "power source of the automobile heart". In recent years, as people continuing to improve vehicle performance requirements, the power of vehicle growing, the crankshaft in the engine is bound to bear greater alternating load, and thus the strength and toughness and reliability of the crankshaft material is also put forward higher requirementsIn the light of the unmatched practicality and economy of ductile iron crankshaft comparing with forged steel crankshaft, research and development of high strength and high toughness ductile iron crankshaft will certainly have broad market and development prospects. Ferrite in as-cast ductile iron looks like bull’s eye surrounding the spheroidal graphite. Its performance is characterized by high strength and hardness, and low ductility and toughness, not suitable as the crankshaft material directly. In this paper. Incomplete Austenitizing Normalizing (IAN) treatment has been carried out on the as-cast ductile iron, and pearlite and dispersed broken ferrite appeared in the matrix of ductile iron. By analyzing the effect of temperature and holding time on the microstructure and mechanical properties of ductile iron, it is found that in the three-phase coexistence region of austenite, ferrite and graphite, and within a certain holding time, with the increase of temperature and holding time, the content of pearlite increases, while that of ferrite decreases after normalizing. At this moment, the strength and hardness of the sample rise while the toughness and plasticity decline to some extent.Ductile iron was treated by Complete Austenitizing Normalizing (CAN) and Two-stage Normalizing (TSN) and then compared with the sample treated by Incomplete Austenitizing Normalizing. It is found that the percentage of ferrite in sample by CAN is fractional, and because of the coarse pearlite groups, properties of the sample has not been improved. Ferrite in TSN sample is along the grain boundaries, while the one in IAN sample distributed radially. At the same holding temperature, the percentage of sample by TSN is slightly lower than the one by IAN, thus the strength of the former is higher than the latter, while the elongation opposite. After a comprehensive comparison of the three normalizing processes, the IAN treatment is the best choice because of its simple and efficient process and being suitable for the actual production. Crankshaft which was treated in830℃-840℃and held in this temperature for1h has the best comprehensive mechanical properties. Under this process, the tensile strength, brinell hardness, elongation, and impact toughness is in the range of820MPa,266HBW, over7%and51-55J/cm2, respectively.On the basis of experiments in laboratory to obtain excellent results, the IAN treatment process was put into field experiments. After the insulation in the field800℃-830℃, single parking time in6.5min-7.5min in continuous normalizing furnace, crankshaft has been complete austenitizd. It has a high strength and toughness but the plasticity and toughness still need to be improved. In the next stage of the experiments, the normalizing temperature and holding time of IAN need to further reduce in order to obtain broken ferrite ductile iron crankshaft product which has better overall performance to meet the actual production.