Understanding Limitations and the Power of Knowledge

Abstract

Casting is a complex process that often produces parts that suffer from a variety of porosity voids and inclusions when not understood and controlled. The discontinuities in castings affect both the monotonic and cyclic mechanical properties of castings. The various discontinuities that are often unique to castings or are absent in wrought and forged parts have caused a bias against the use of castings. The size and shape of the porosity voids, inclusions and second phase particles have a major influence on the properties of castings. While they have a limited effect on yield strength, discontinuities have a major effect on tensile elongation and tensile strength. The way in which these discontinuities influence tensile properties is discussed. Various methods have been proposed to reduce these discontinuities and data are presented to demonstrate the effectiveness of some techniques used to increase the strength of castings. Discontinuities are even more damaging to fatigue strength than to tensile properties. The role of discontinuities in the fatigue process is described. While porosity and other discontinuities reduce the monotonic and cyclic strength of castings, the tremendous freedom of design with castings can make cast parts competitive with wrought parts and welded assemblies. Investigators have shown that the design strength can be substantially improved by redesigning a part, thus making castings quite competitive with wrought parts. A good understanding of the power of design makes a casting stronger and more tolerant of the imperfections that can occur in castings.