| dc.description.abstract | Correlation of the structure of cast iron with its mechanical properties through the assessment of micro stresses has been addressed in this thesis. The large variation in the shape-spherical to flake-size, and distribution of the dispersed low strength graphite in a matrix similar to that of steel produces a wide range of mechanical properties. Spheroidal graphite irons have high strength and ductility in contrast to flake graphite irons.
Previous work on structure-property correlations in materials, considering the distribution of second phase on the matrix and employing analytical closed form solutions, numerical schemes (FEM, BEM), or experiments, has been carried out with steels, non ferrous alloys, and composites including porous materials and plastics. Studies on the distribution of stresses in the microstructure of cast irons appear to be limited.
It was proposed to investigate this problem by linear elastic and elastoplastic finite element analysis of representative chosen microstructures of spheroidal, vermicular, and grey cast irons. Two dimensional models, containing graphite treated as voids, were subjected to uniformly distributed uniaxial tensile load in ferrite, pearlite, or bainite matrix. Stress distribution in the structure in the elastic and plastic regions was plotted. Peak stress concentration factor (SCF) was computed. Inhomogeneity of stress distribution, with peak stresses at specific locations in the matrix close to graphite, is the result of the influence of stress concentration across the loading direction and stress shadow effect along the loading direction, as influenced by the shape and distribution of the graphite.
Load-SCF curves for the three irons, along with knowledge of yield band development in the plastic region, have enabled a clear understanding of the role of microstructure in determining the properties of cast irons. High strength and ductility in spheroidal graphite iron, particularly with low hardening coefficient ferritic matrix, is due to multiple, branched, and wide yield bands developed with a relatively low peak stress concentration factor of 3. Low strength and poor ductility in flake graphite irons can be explained on the basis of a higher stress concentration factor of 8-9 and a single narrow yield band traversing the structure with no branching. Several issues, such as the influence of the size and number of spheroids or flakes on the properties of irons, have been resolved. This investigation has established that FEM is a powerful tool in effectively delineating the structure-property relationships in the family of cast irons.
Photoelastic analysis of 2D plate models of a range of typical microstructures of the irons has proved to be a reliable, sensitive, and rapid method of assessing the influence of a number of parameters-viz., shape, size, and distribution of graphite-on micro stresses. It has also aided in the validation of results of FEM analysis. | |
