Development of high-modulus high-strength low-carbon spheroidal graphite cast irons.

摘要

While conventional spheroidal graphite cast irons possess manufacturing and mechanical and physical property characteristics that have resulted in their use in a wide range of automotive and transportation applications, the relatively low modulus of elasticity of these alloys (compared to that of steel) has been of concern to design engineers attempting to reduce component weight or to increase the stresses imposed upon components. As a result, design engineers have looked at alternative materials which, in many cases, exhibit less satisfactory manufacturability and design flexibility.; It is proposed to investigate the feasibility of producing a low carbon spheroidal graphite cast alloy with high strength and high elastic modulus. Three types of alloys were examined: low carbon equivalent ductile irons, high carbon graphitic steels, and magnesium treated malleable irons.; Experiments on heat treatment responsiveness were performed, correlating the pearlite-ferrite ratio in the matrix with various foundry variables (i.e., section size, chemical composition, nodule count, and SSG cooling rate).; Microstructural study indicated that minimum heat treating cycle can be achieved by eliminating as cast carbides and obtaining a high nodule count. Interdendritic graphite segregation and foundry characteristics (castability, shrinkage, etc.) were also rationalized.; Mechanical properties were evaluated by comparing with the ductile iron quality baseline. Castings with a nearly fully pearlitic matrix exhibited better tensile properties than that of conventional ductile irons. The yield strength is affected primarily by the presence of solid solution strengthening elements in the ferrite.; For these cast alloys, the "round house" nature of the stress-strain curve in the elastic region is a result of localized plastic deformation taking place in the matrix. Accordingly, measured modulus values depend not only upon graphite morphology and matrix structure, but also on the techniques used to determine the modulus.; A modified measuring method was investigated. The proposed hyperbolic plots of strain versus strain/stress clearly shows the different deformation regimes occurring during "elastic deformation" and can be used to reproducibly measure the true elastic modulus as well as characterize the non-linearities that occur below the yield stress.