Causes of coarse grain defects in castings

In essence, the coarseness of grains and the chemical composition of metals are related to the matching of cooling rate, so it is very important to choose this kind of matching. If the cooling rate is difficult to adjust, the coarse-grained structure must be due to the improper chemical composition of the metal. Due to the importance of metal composition, each metal is described as follows.
(1) The mathematical calculation of carbon and silicon effect can be summarized as: CE = C + 1 / 3Si. The coarse grain may be caused by excessive carbon or silicon or excessive carbon and silicon. Compared with silicon, the effect of carbon is three times that of silicon, so the change of carbon is much more dangerous than that of silicon. This effect of carbon and silicon affects not only malleable cast iron but also gray cast iron. For malleable cast iron, the coarse grain does not appear to be black, nor to show the pitting of primary graphite, but in the form of general coarse grain, which is due to the high content of carbon or silicon, or both. Phosphorus also affects grain coarsening. When WP = 0.1%, the shrinkage defects will be aggravated, especially in the section with slow cooling.
(2) In the melting and deoxidizing operation of cast steel, some elements are added to delay the grain growth, so it is not easy to form coarse grain in cast steel compared with forged steel. Steel castings with coarse grain due to composition can be refined by annealing or normalizing.
(3) Iron impurities in aluminum alloy can make the grain of aluminum castings coarse and increase brittleness. Most of these defects are caused by improper melting operation. In aluminum alloys, especially those that require overheating, it is necessary to add appropriate amount of grain refinement alloy elements.
(4) The coarse grain defects in copper alloys are often covered by pinholes, pores or shrinkage porosity. Due to the change of composition, the grain size of copper alloy is coarse, but pinholes, pores or shrinkage porosity always appear first.
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