Causes of coarse grain size defects in castings
Author: Date: 2021-4-15 13:56:14
In essence, the coarse grain size is related to the chemical composition of the metal and the coordination of the cooling rate, so the selection of this coordination is very important. If the cooling rate is difficult to adjust, then the coarse-grained structure must be caused by the improper chemical composition of the metal. Because of the importance of metal composition, each metal is described briefly as follows.
(1) The carbon equivalent of gray cast iron and malleable cast iron is too high, and the mathematical calculation of the carbon and silicon effect can usually be summarized as: Ce =C+1/3Si, and the coarse grain size may be caused by excessive carbon or silicon, or carbon and silicon. Carbon is three times more potent than silicon, so a change in the amount of carbon is much more dangerous than an identical change in the amount of silicon. This action of carbon and silicon affects both malleable and gray cast iron. For malleable iron, grain coarseness is presented in the form of a general grain coarseness, neither as black nor as the pitting that indicates primary graphite, due to excessive carbon or silicon content, or both. Phosphorus also has an effect on grain size. When WP = 0.1%, the shrinkage cavity defect will be aggravated, especially the coarse grain defect in the section where cooling is slow.
(2) In the melting and deoxidation operation of cast steel, some elements are added that will delay the grain growth. Therefore, compared with forged steel, cast steel is not easy to form grain bulging. Cast steel with coarse grain due to composition can be refined by annealing or normalizing.
(3) aluminum alloy iron impurities will make the cast aluminum grain coarse, brittleness increase, most of these defects are caused by improper melting operation. In aluminum alloys, especially those in which overheating is required, it is necessary to add an appropriate amount of granular alloying element.
(4) copper alloy copper alloy coarse grain defects are often covered by pinholes, pores or shrinkage. Copper alloy due to composition changes can cause grain bulky, but usually always appear first pinhole, porosity or shrinkage.