Carbon Diffusion And Graphite Morphology Control During Solidification Of Ni-C Alloy

Ductile iron has good strength and impact performance and can reduce stress concentration effectively when subj ected to a tensile load,so it is used in various fields of engineering manufacturing widely.The mechanical service performance of ductile iron is closely related to its internal graphite morphology,especially in the production of thick and large cross-section cast iron,where often appear "graphite distortion" and"inoculation fading" and other defects,which will affect properties of ductile iron seriously.In order to obtain castings with good comprehensive properties,specific melt treatment techniques are usually used in industrial practice to improve graphite morphologies in cast iron alloys.To improve graphite morphology and quantity,adding elements in melt is a relatively efficient and low-cost process in current production.In the present study,by preparing binary Ni-C alloy and ternary Ni-C-Sb,Ni-C-Bi and Ni-C-Sn alloy,the paper use Micro-CT to observe graphite distribution in Ni-C alloy and Ni-C-Sb alloy creatively,analysing the growth mechanism of graphite deeply,and studying the growth mechanism of Sb,Bi and Sn on graphite.The object of study is to provide basic data and theoretical support to realize engineering application of high performance cast iron.Graphite microstructure in Ni-C alloy was studied by numerous systematic experiment and determining optimal casting parameters.The microstructure of Ni-C alloy were characterized by OM、SEM、EDS and Micro-CT.It was found by SEM that the graphite growth in the-c-direction of Ni-C alloy is mainly through 2D nucleation.In addition,crimp and folding will occasionally occur during the stack of graphite sheet layers in-c-direction,and specific bending angle will appear sometimes."Twin defects"on graphite surface will contribute to the graphite growth sheet in-a-direction.Antimony(Sb)in Ni-C-Sb alloy is mainly distributed in the nickel matrix.It is speculated that Sb have a strong hindrance to the diffusion of carbon atoms,which reducing the average diameter of graphite obviously,but could not promote graphite nucleation effectively.Among them,the optimal inoculation effect can be achieved when 0.04%Sb is added,which the average diameter of graphite decreases from～13μm to～7μm,appearing dispersive distribution and rounded morphology.Sb can cause the mutual transformation between-a-direction and-c-direction of the growth in graphite sheet layer,resulting in "vertical overlap" of graphite sheet layer.Meanwhile,when the Ni-C-0.01%Sb alloy was characterized by Micro-CT,it was concluded that "flowering graphite" in Ni-C-Sb alloy was formed by the interconnection of smaller graphite due to the low limit load of Ni shell containing Sb around graphite.Bismuth(Bi)in Ni-C-Bi alloy is mainly distributed in nickel matrix and the near of graphite core.It was speculated that Bi in the nickel matrix can affect graphite growth by hindering carbon atoms diffusion,and reducing graphite size significantly.When the content of Bi in Ni-C alloy reaches a certain content,it can promote the graphite nucleation effectively.Optimal inoculation function can be achieved when 0.1%Bi content is added,which the average diameter of graphite decreases from～13μm to～4μm,and graphite number is about twice of that in Ni-C alloy.Stannum(Sn)in Ni-C-Sn alloy is distributed in nickel matrix and the surface of the graphite mainly,It is speculated that the graphite growth front and Sn atoms in the Ni matrix would make it difficult for carbon atoms to diffuse to the internal "carbon-rich phase" during the solidification process of Ni-C-Sn melt,which can reduce graphite size obviously,but can’t promote graphite nucleation in Ni-C alloy.During the solidification process of Ni-C-Sn alloy melt,the presence of Sn element will lead to the transformation of the diffusion mode of carbon atoms and the transformation of the growth mechanism of graphite,thus presenting the appearance of rounded graphite and eventually forming the graphite morphology with smaller size,more "perfect" surface and more compact structure,and show a nearly perfect spherical growth pattern.