In essence, the coarseness of grains and the chemical composition of metals are related to the coordination of cooling rate, so it is very important to choose this coordination. 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 summarized as follows.
(1) The mathematical calculation of the effect of carbon and silicon can be summarized as: CE = C + 1 / 3Si, the grain coarseness may be caused by the excess of carbon or silicon, or the excess of carbon and silicon. Compared with silicon, the effect of carbon is three times that of silicon, so the change of carbon content is much more dangerous than that of silicon. This effect of carbon and silicon affects both malleable cast iron and gray cast iron. For malleable cast iron, the grain coarseness is neither black nor pitting representing primary graphite, but in the form of general grain coarseness, which is due to high carbon or silicon content, or both. Phosphorus also affects the grain size. When WP = 0.1%, shrinkage defects will be aggravated, especially in the section with slow cooling.
(2) In the melting and deoxidizing operation of cast steel, some elements which can delay the grain growth are added. Therefore, compared with forged steel, cast steel is not easy to form coarse grain. The steel castings with coarse grain due to composition can be refined by annealing or normalizing.
(3) The impurity of aluminum alloy iron will make the grain of aluminum casting coarse and increase the brittleness. Most of these defects are caused by improper melting operation. In aluminum alloys, especially those that require overheating, it is necessary to add some fine grain alloy elements.
(4) The defects of coarse grains in copper alloy are often covered by pinholes, pores or shrinkage porosity. Because of the change of composition, the copper alloy will result in the coarseness of grains, but pinholes, pores or shrinkage porosity usually appear first.