by clicking the arrows at the side of the page, or by using the toolbar.
by clicking anywhere on the page.
by dragging the page around when zoomed in.
by clicking anywhere on the page when zoomed in.
web sites or send emails by clicking on hyperlinks.
Email this page to a friend
Search this issue
Index - jump to page or section
Archive - view past issues
button in toolbar for more information.
Metal Casting Technologies : March 2009
Back to B A S I C S Using chills to improve casting soundness J. F. Meredith, Casting Solutions Pty Ltd Figure 1a. Model of casting with isolated hot-spot C hills can be defined as materials of higher thermal conductivity than the mould material which are positioned in the mould or core so as to accelerate solidification or modify structure at a certain point of the casting. Chills are usually made from metallic blocks but other materials such as silicon carbide or graphite are often preferred if a less severe chilling action is required. Chill aggregates such as zircon or chromite are very useful because of their ability to be moulded to the contours of the casting; their effectiveness as a chill is, however, rather limited and falls off dramatically with increasing casting section. Many foundries tend to use chills as a last resort, when all else fails, usually because of bad past experiences which are generally the result of misapplication. Properly applied, chills can be a valuable tool in improving casting soundness and yield. External chills External chills are used to: 1. Eliminate porosity be neutralising hot-spots, causing them to solidify at least as quickly as the adjoining thinner sections. 2. Reduce hot-tearing tendency by accelerating skin- formation and thus increasing the energy required to initiate tearing. 3. Increase sound feeding distance in uniform casting sections by increasing thermal gradients. 4. Improve yield by making it possible to reduce the size and/or number of feeders. 5. Eliminate adverse metallurgical structures by reducing solidification time. An example of the use of chills for neutralising hot-spots is shown in figures 1 and 2. Common problems encountered with external chills: 1. Chills welded to casting, are usually the result of using 50 www.metals.rala.com.au Figure 1c. Plot of shrinkage through centre of casting showing shrinkage in isolated hot-spot undersized chills; the chills become saturated with superheat from the metal resulting in fusion at the contact to the casting. As a guide, chill thickness should be between 0.6 and 1.0 times the thickness of the section to be chilled; the length and width should be at least twice the chill thickness. Figure 1b. Solidification pattern through centre of casting