Demystifying Shrinkage Porosity in Ductile Iron Castings
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Copyright 2020 American Foundry Society https://doi.org/10.1007/s40962-020-00440-3
Abstract Shrinkage porosity or volume deficiency in ductile iron castings manifests itself in many different physical shapes and sizes. The terminology used to describe the various forms of shrinkage confuses operating personnel when they try to solve the problems. There is also confusion as to the use of thermal analysis events in estimating the carbon equivalent. Irons with wide range of chemistry may solidify as eutectic. To maximize the potential of ductile iron and to produce castings with consistent high quality, it is essential to understand the fundamental nature of ductile iron
solidification and use appropriate process controls. In this paper, an attempt is made to interpret various kinds of shrinkage porosity occurrences and make sense of process variables that may be contributing to them.
Introduction
still be cost competitive to other materials and processes. Success depends on whether one is ‘‘Cooperating with Mother Nature’’ or ‘‘Attempting to Fool Mother Nature’’ as posed by Rundman.1
Ever since the start of production of ductile iron, foundries are confounded with shrinkage porosity. Seemingly same irons processed similarly can have quite different shrinkage characteristics. Castings are produced every day, some without any separate risers and others with very big risers, with mold yield varying from less than 40% to higher than 80%. It is not uncommon to encounter many different names to describe the shrinkage porosity, and it may lead to steps that may not actually address the problems. Do these names suggest that there are different types of shrinkage rather than physical manifestations? This leads to confusion and complexity in determining the root cause and solving the problems. Sometimes combination of variables, even when each of them is within control limits, can cause defects. It is the intent of this paper to clarify as much as possible, the nature of shrinkage defects and process variables that can affect them. If we understand these factors fundamentally, then we will be able to control and produce ductile iron castings which meet or exceed the demands of design engineers and This paper is an invited submission to IJMC selected from presentations at the 6th Keith Millis Symposium on Ductile Iron held October 23–26, 2018 at the Sonesta Resort, Hilton Head Island, SC. It is published in the IJMC bypermission of the DIS (Ductile Iron Society).
International Journal of Metalcasting
Keywords: eutectic, carbon equivalent liquidus (CEL), local super-saturation, exudation, austenite grain size, segregation, riser piping
There are many terms used to describe the porosity found in castings, such as open shrinkage, macro-shrinkage, micro-shrinkage, suck-in, surface depression, gate shrink, surface sunk, dispersed porosity, dendritic porosity and so on. Shrinkage porosity results from lack of compensation of contraction during liquid cooling and contraction during solidification. In cast irons, contraction
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