Effect Of Nickle On Microstructure And Mechanical Properties Of Low-Temperature High-Ductility Ductile Iron

As an excellent type of engineering materials, low-temperature high-ductility ductile iron was widely used to produce wind power equipment, such as wheel hub and Gearbox. Because of the harsh working conditions, the low-temperature impact toughness of the equipment material was strict required. In this paper,0-1.2wt.% nickel was added into low-temperature high-ductility ductile iron, and then'Y'model was poured. Moreover, heat treatment process was optimized. Microstructure observation, Scanning electron microscopy, Spectrum analysis, Tensile strength test, Brinell hardness test and Low-temperature impact toughness test were done to research the effect of nickel on the microstructure and mechanical properties of low-temperature high-ductility ductile iron.The as-cast organization of low-temperature high-ductility ductile iron was constituted of spheroidal graphite, pearlite, ferrite and little cementite. Graphite spheroidization was not influenced by nickel. The graphite size class of each sample was six degree or over, and the spheroidization class was two degree or over. Ferrite occurred in matrix decreases in value with the increase of nickel, while pearlite was just opposite. The content of cementite which had no relationship with nickel was between 3% and 5%.The heat treatment of low-temperature high-ductility ductile iron had been studied. And the best process scheme was 920℃×4h+740℃×4h two-stage graphitization annealing. Morphology of the graphite had no changed with annealing. The content of ferrite was significantly increased, which reached 98% or over. And cementite disappeared. Little non-decomposed pearlite appeared when the content of nickel was above 0.88%.Tensile strength and brinell hardness of the material were enhanced gradually with nickel, and respectively increased from 403Mpa to 421Mpa and from 127HBW to 146HBW. But the regulation of elongation was different. With the increasing of nickel, elongation improved in the beginning. Afterward, when the content of nickel reached 0.71%, elongation was up to 27.4%. And then it dropped. The tensile fracture morphology of each sample which was made up with a large amount of dimple presented dimple mode. Nickel was found uniform distribution in the annealing tissue by Scanning electron microscope Spectrum analysis. Ferrite was strengthened, and the microstructure was also refined which lead to the rise of tensile strength, extensibility and brinell hardness. With the increased of solid solubility, degree of pattern distortion was growing, which induced the extensibility falling down.The results of the low-temperature impact toughness experiment showed that effect of nickel on low-temperature impact toughness of the materials were obscure. The macrography and microscopic view of the sample fracture had been analysised. It showed that fracture patterns and mechanism were nearly the same among the different nickel content sample under the similar temperature. The microscopic view of the fractures which were under-20℃condition test, constituted a large quantity of dimple, sharply represented ductile rupture. While, the sample which were under-40℃condition test showed brittle fracture. Vast scale of river patterns and some tongue-shaped patterns were turned up in the frature.A few abnormal phases which accumulated on the grain boundary were observed, and their component were also analysed. The results indicated that these abnormal phases which embodied residual ecmentite(Fe3C),occluded foreign substance which contained titanium(Ti) and phosphorus(P), oxysulfide sulfoxide which contained residual magnesium(Mg), calcium(Ca) and some rare-earth(RE), cerium(Ce) for example, would hindrance dislocation-slipping and be the chief reason that reduce the low-temperature impact toughness of the materials.The effect regularity of Nickel on microstructure and mechanical properties of low-temperature high-ductility ductile iron was generalized by analysing the distribution and shape of nickel in ductile iron. The consequence indicated that nickel couldn't obviously influence the ductile-brittle transition(Tc).Adding suitable dosage of nickel could intensify tensile strength and brinell hardness of the material on the promise of satisfing the low-temperature impact toughness, thus compensated for the silicon declined.