| Gray cast iron is a kind of traditional structural material, it has good wear resistance, shock absorption, thermal conductivity, casting formability and low cost, at present, the material selection of Engine cylinder block is still given priority to gray cast iron. However, the mechanical properties of gray cast iron are relatively poor, including high brittleness and low strength for example. Therefore, in the past, the study of gray cast iron emphasised on its mechanical properties improvement, the most common method are inoculation treatment and adding alloying elements. But in recent years, with the increase of the mechanical properties of gray cast iron used in engine cylinder block, it was found that the machining performance is obviously reduced, the engine cylinder block usually be cutting on automatic lathe and assembly line under the condition of high speed, if cutting tool changed so frequently that lower down the productive efficiency severely, the production problem is attracting comprehensive attention extensively.The addition of Nb as a trace element to steel for improving the strength and the wearing resistance usually. What effects on niobium addition on micro structures and properties of gray cast iron under the equivalent RECaBa inoculant supply were studied in this paper. Current correlative researches are not so many, this paper study on this field. In the experiment, measuring tensile strength, hardness, section sensitivity, micro hardness of gray cast iron samples by using universal material testing machine, brinell hardness tester, micro hardness tester respectively. And the machining performance was tested by a dynamic strain amplifier and a octagonal ring. Morphology of primary austenite dendrite, graphite, eutectic cell, pearlite and hard phase were studied by using optical microscope, SEM and XRD, and the relations of microstructure and property were researched.The results show that with the increase of Nb content, the number of primary austenite dendrite in gray cast iron is increased, dendrite arm spacing decreases, the number of eutectic cell increase in per unit area, graphite morphology translate from rough type A to tiny uniform type A and then to diffuse distribution of type D, pearlite amount do not change significantly but the spacing decreases. The tensile strength of gray cast iron increased gradually with the increase of Nb content. Compared with the sample without adding Nb, tensile strength of samples adding0.15%Nb and0.25%Nb increased28%and41%respectively. With the increase of Nb content, The hardness increase and section sensitivity improve gradually, the standard deviation which represent homogenous of microstructure decreases first and then increases, the cast iron with0.15%Nb showed the best microstructure uniformity.The cutting force of gray cast iron increased gradually with the increase of Nb content. But the machinability of samples changes appear difference in different stages of Nb content, when the content of Nb is0%-0.15%in the sample, the cutting force increased slowly, the standard deviation which represent machining stability is reduced, the improvement of machining stability is closely related to the improvement of the microstructure uniformity. when the content of Nb is0.15%-0.25%in the sample, the cutting force increased rapidly and the standard deviation is also increased significantly, which is closely related to the precipitation of hard phase.Finally the article make a conclusion that the fitting level of Nb for gray cast iron is0.15%under the RECaBa inoculant. The graphite morphology of this gray cast iron is fine and uniform type A graphite, the number of graphite is big, tensile strength arrive to315MPa, the gray cast iron has good section sensitivity and microstructure uniformity, there is no hard phase containing niobium and Nb in solid solution is existed in the gray cast iron, this gray cast iron has an uniform matrix and good cutting stability, showing a good machinability... |
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