Analysis of the causes of defects in ductile iron castings

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Defects in castings, such as inclusions, pores, and cracks (including gas holes, shut holes, cracks, cold shuts, etc.), often affect the mechanical properties, physical and chemical properties, and machinability of the castings, and determine the quality of these castings. Ductile iron parts can develop almost all types of casting defects, but due to their methods of production, crystallization patterns, casting properties, and differences from other casting alloys, they often exhibit some specific defects. So, what are the defects in ductile iron parts related to ballizing agents, or in other words, what defects in ductile iron parts are caused by ballizing agents? Research shows that almost all defects in ductile iron parts are related to ballizing agents. This mainly includes the following aspects: 1. Alienation of graphite balls – The alienation of graphite balls results in irregular shapes of graphite, such as clumped, tadpole-like, worm-like, horn-shaped, or other non-spherical forms. This is due to the local crystal growth patterns and growth rates deviating from normal growth laws as spherical graphite grows in a radial direction. When the amount of residual spheroidizing elements in the casting exceeds the appropriate range, for example, if the residual magnesium level is too high and exceeds the minimum amount required to maintain the spheroidal shape of the graphite, it will also affect the conditions under which the graphite crystallizes, leading to the formation of tadpole-shaped graphite. When there is an excess of residual rare earths, high-carbon equivalent molten iron tends to form fragmented graphite, and the areas where such fragmented graphite accumulates are generally referred to as \"gray spots\". The appearance of worm-like graphite is due to insufficient residual amounts of spheroidizing elements or the presence of excessive amounts of titanium and aluminum. 2 Graphite floating: In thick-walled ductile iron parts with a hypereutectic composition, a region with a high concentration of graphite often appears at the top of the pouring area; this is known as the \"graphite floating\" phenomenon. It occurs because of the difference in density between graphite and molten iron, causing the graphite that forms directly from the hypereutectic molten iron to be pushed upward by buoyancy. The degree of graphite floating is related to factors such as carbon equivalent, the type and residual amount of spheroidizing elements, the solidification time of the casting, and the pouring temperature. Magnesium can increase the eutectic carbon content in ductile iron. For molten iron with the same carbon equivalent, increasing the residual magnesium content reduces the occurrence of floating graphite, while an excessive amount of residual rare earths facilitates the formation of explosive graphite. 3 Reverse white cast structure: In general, the white cast structure in cast iron parts tends to appear in the surfaces that cool more rapidly, as well as at sharp corners and seams. Conversely, with the reverse white cast defect, the carbide phases appear in the central areas of the cross-sections of the castings, as well as in areas prone to heat accumulation. When the residual amount of spheroidizing elements is too high, it promotes the formation of inverse white cast iron defects. Rare earth elements are more effective than magnesium; both of them generally increase the degree of supercooling during the formation of ductile iron microstructure. 4 Subcutaneous pores: The subcutaneous pinholes mainly contain hydrogen, with small amounts of carbon monoxide and nitrogen as well. When the residual magnesium level is too high, it also increases the tendency to absorb hydrogen from the green shape, thereby raising the likelihood of the formation of subsurface pinholes. Furthermore, a longer residence time of the globular molten iron can also increase the number of pinholes. 5 Shrinkage holes and porosity: Shrinkage holes often appear in the areas of the casting that solidify last (at hot spots, at the junction between the riser neck and the casting, at internal corners, or at the junction between internal gates and the casting). They are holes that are hidden inside the casting or that are connected to its surface. Shrinkage, which appears macroscopically at heat spots, takes the form of tiny shrinkage pores, most of which are interconnected within each other. Regarding the spheroidizing elements, it is necessary to keep the residual magnesium and rare earths at low levels; this has a significant effect on reducing both macroscopic and microscopic shrinkage porosity, with the tendency to form porosity being almost proportional to the amount of spheroidizing elements. 6 Black slag: It generally appears in the upper part of the casting (at the pouring site), and is mainly divided into lump-shaped, rope-like, and fine black slag. The main component of black slag is magnesium silicate, which is formed by the reaction of MgO and SiO2 in molten iron, and it is influenced by their relative contents. Therefore, one of the measures to control black slag is to reduce the residual magnesium content (when 0.15% magnesium is added, the total amount of slag accounts for about 0.1% of the weight of the molten iron), and residual rare earths, due to their strong affinity for oxygen, have a significant effect in reducing black slag. 7. Spheroidization degradation: This occurs because the molten iron remains in that state for an extended period, causing the residual magnesium to decrease gradually. If the slag is not removed in time, sulfur can return to the molten iron, which reduces or even eliminates the graphite present in the solidified structure, resulting in graphite that appears irregular, worm-like, or flaky in shape. This reduction in spheroidization is related to a low content of rare earths in the spheroidizing agent, or to a low amount of this agent used. However, simply increasing its dosage is also not advisable, as it leads to higher levels of magnesium residue, which in turn increases the amount of slag and cementite; in thick sections, it can also cause the graphite spheres to transform into tadpole-shaped graphite. Production experience shows that a low sulfur content in the molten iron is the most effective way to prevent the degradation of spheroidization. Including the defects in ductile iron parts, almost all of them are related to the composition and amount of the ballizing agent. However, we cannot rely on the ballizing agent to solve many problems, let alone all of them; the effects of the balling elements as well as the amount of ballizing agent have both advantages and disadvantages. The ballizing agent is merely a very important factor in the system for ensuring stable production of ductile iron, and it can only enable stable balling treatment when used in combination with other accompanying measures.