Date

December 1985

Document Type

Thesis

Degree Name

M.S.

Department

Dept. of Materials Science and Engineering

Institution

Oregon Graduate Center

Abstract

Antimony's influence on segregation during solidification, mechanical properties, fracture mode, and microstructure were investigated in this study. A single heat of a modified 4720 alloy was innoculated with varying concentrations of Sb. In the as-cast condition, metallographic analysis was performed to show microstructural variations as a function of Sb content. Microstructural differences were related to changes in microhardness, and the microsegregation of Sb, C, Cr, Mo, Ni, and Si. Equipment used to characterize microsegregation in this study include: energy dispersive X-ray analysis, wavelength dispersive X-ray analysis, and electron microprobe analysis. Several heat treating cycles were given to selected samples in order to show the effects of Sb additions on resulting CVN transition temperature values, diffusion characteristics, microstructure and fractography. Changes in CVN transition temperature were related to fracture/microstructure variations. The results indicate that Sb additions retard the peritectic solidification reaction. This leads to an increased segregation ratio of alloying elements, causing specific regions of the as-cast microstructure to change in alloy element concentration. Standard normalizing, quenching and tempering heat treatments were ineffective in reducing the degree of microsegregation obtained during solidification. Carbide size varied significantly depending upon its location in the dendrite structure. Fracture behavior of CVN samples was a function of carbide size. Carbide size increased in the general heat treated microstructure with increasing Sb content, changing the fracture mode from ductile dimple to a cleavage fracture mode. Intergranular fracture resulted after heat treatment in those locations which previously contained proeutechtoid ferrite in the as-cast condition. Intergranular fracture was found to be a function of both carbide size and Sb segregation. The carbide size decreased in grains exhibiting intergranular fracture. Increased concentrations of alloy and impurity elements in these regions increased the segregation ratio and hence the interactive co-segregation of Sb to grain boundaries leading to embrittlement and intergranular fracture. As a result, the addition of Sb tended to increase the CVN transition temperature after heat treatment by increasing the propensity for both cleavage and intergranular fracture. Hence Sb content should be restricted to 0.005 wt % in this alloy system.

Identifier

doi:10.6083/M4GQ6VP0

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