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Coarse grains refer to defects that show that the grain structure is too coarse and unsuitable for application after mechanical engineering or fracture inspection. This kind of coarse grain structure may be spread over the whole casting or may occur in a part of the casting. . In essence, the coarse grain defect is a metallurgical defect. Based on years of production practice and refer to relevant materials, xinyuanzhu group talks about the causes and preventive measures of coarse casting defects.

1. Casting structure and process design

1.1 If the cross-section difference of the casting is too large, the thicker cross-section will cool slowly and cause the grains to be coarse. Metals that are very sensitive to cross-sectional changes, such as gray cast iron, are more likely to produce such defects.
An effective way to prevent such defects is to avoid excessive disparity in the cross-sectional dimensions of castings, but this approach is sometimes beyond the reach of foundry workers. Therefore, as far as casting itself is concerned, it is possible to reduce the occurrence of such problems and reduce the severity of such defects by setting cold iron, controlling the pouring temperature, or selecting a suitable pouring system. The use of cold iron can speed up the cooling rate of the thicker section of the casting; too high pouring temperature will make this kind of problem more serious and should be avoided; by adjusting and revising the pouring system design, the lower temperature of the molten metal can be located on the section of the casting. Thick parts, and design the most effective riser at the thick section of the casting to minimize the size of the riser.
1.2 For castings with holes, process designers sometimes do not use cores that help reduce the effective cross-sectional size, and make the uncored cross-section too thick to produce this defect. Therefore, in process design, the thickness should be as thick as possible. A sand core is set in the section.
1.3 In some cases, the section of the casting is not too thick, but because a narrow recess or core forms a heat sink section in the casting, the result is the same as the thick section. E.g. It may be necessary to set a core at a columnar umbilicus in the deeper part of the casting, and this will cause slow cooling. In the case that the design cannot be modified, unless the metal temperature can be lowered or the gate can be re-placed, the best solution is to install cold iron at the core or mold section.
1.4 Excessive machining allowance is left in process design, which not only increases the cost of cutting, but also cuts off the denser surface of the casting, and exposes the loose part with slower center cooling. This design is undesirable because it is unreasonable from the perspective of casting or machining. The solution is to change the design of the casting. If changes to the design are not allowed, the correct method is to use cold iron, control the pouring temperature and adjust the pouring system.
1.5 Inappropriate core design at thick section, incorrect core support, or other techniques that cause eccentricity, will cause changes in the section of the casting and cause coarse grains.

2. Pouring riser system
2.1 Failure to achieve sequential solidification The failure of the pouring system to achieve sequential solidification well is usually the cause of coarse grains. For castings with sharp changes in cross-section, attention must be paid to the number and location of ingates. In order to perform feeding, keeping hot molten metal in the action area of the feeder will reduce the cooling rate of the thick section to the extent that coarse grains are produced. Improper riser design, such as too long riser neck, improper riser pad design, or too large riser size, will cause excessive heat collection at the thicker section.
2.2 Pouring riser distribution that is easy to cause heat sinks   Similarly, in order to feed thick sections, it often causes excessive heat collection in local areas. For example, because the side riser can cause overheating of thick sections and slow down the cooling rate, it is sometimes inconvenient to use in actual operation. In actual production, a reasonable riser design is required to reduce the size of the riser as much as possible.
2.3 Local hot spots caused by the connection between the inner gate or riser and the casting The inner gate or riser neck is short, which is beneficial for feeding, but it will make the runner or riser too close to the casting. Slow down the cooling rate of this part. Enlarging the riser neck will cause problems for feeding. Therefore, the best measure is to adopt an effective riser design, reduce the size of the riser as much as possible, do not make the runner and the riser too close to the key section that is easy to form a coarse product, and set the runner and the riser appropriately. To achieve feeding.
2.4 Insufficient number of internal gates; too few internal gates, not only easy to cause sand washing, but also local hot spots and coarse grain structure. This phenomenon is common in all cast metals, even aluminum alloys with lower casting temperatures. In some cases, too few gates can cause shrinkage defects. This shrinkage defect may conceal the coarse grain defects caused by the same reason. In fact, when the coarse grain defect deteriorates seriously, it becomes a shrinkage defect. Therefore, the preventive measures for these two defects are often the same.

3. Molding sand
Only when the displacement of the mold wall caused by the molding sand is sufficient to increase the cross-sectional size of the critical section (the section that is easy to form coarse grains), the mold sting is a factor that causes coarse grain defects. Since the wall movement at the thick section may be the largest, this kind of defect is still possible, and the coarse grain defect produced at this time is related to the expansion of sand.

4. Core making
In production, the use of unbaked or air-hardened oil sand cores should be avoided, because this type of core may produce an exothermic reaction and cause excessive heat collection. This happens either in large castings, or in thick and large cores using adhesives with exothermic properties. In a sense, this core acts as a highly efficient insulator and slows down the cooling rate of molten metal to a dangerous degree.

5. Modeling
5.1 Lack of ventilation holes that can accelerate the cooling rate. For thicker castings, the cooling rate of the castings is related to the rate of heat dissipation through the molding sand. Sufficient exhaust will help the water vapor to be discharged quickly, thereby producing a cooling effect.
5.2 The situation where chill nails or cold iron are not set is usually due to carelessness.

6. Chemical composition
Essentially, the coarse crystal grains and the chemical composition of the metal are related to the cooling rate, so it is very important to choose this combination. If the cooling rate is difficult to adjust, the coarse-grained structure must be due to improper metal chemical composition. Due to the importance of metal components, each metal is briefly described as follows.
6.1 Gray cast iron and malleable cast iron   carbon equivalent is too high, the mathematical calculation of carbon and silicon effects can usually be summarized as: CE=C+1/3Si, coarse grains may be due to excessive carbon or silicon, or carbon silicon To. Compared with silicon, the effect of carbon is three times as great, so the change in the amount of carbon is much more dangerous than the change in the same amount of silicon. This effect of carbon and silicon affects both malleable cast iron and gray cast iron.
For malleable cast iron, the coarse grains do not appear as black, nor do they appear as pits that indicate virgin graphite, but in the form of general coarse grains. This is due to the high carbon or silicon content, or Both are too high. Phosphorus also affects the coarse grains. When wp=0.1%, the shrinkage cavity defect will be aggravated, especially the degree of coarse grain defects in the section where the cooling is slower.
6.2 Cast steel In the melting and deoxidation operation of cast steel, some elements that will delay the growth of grains are added. Therefore, compared with forged steel, cast steel is not easy to form coarse grains. Steel castings with coarse grains caused by composition can be refined by annealing or normalizing.
6.3 Aluminum alloy iron impurities will cause coarser grains and increased brittleness of aluminum castings. Most of these defects are caused by improper melting operations. In aluminum alloys, especially those requiring overheating, it is necessary to add an appropriate amount of refined alloy elements.
6.4 Copper alloys The defects of coarse grains in copper alloys are often covered by pinholes, pores or shrinkage. Copper alloys will cause coarse grains due to changes in composition, but usually pinholes, pores or shrinkage porosity always appear first.

7.melting
The small melting operation will have an impact on the remaining grain structure. For different cast metals, Xiaotong’s melting process must be adopted.
7.1 The cupola melting gray cast iron blast volume and coke imbalance, will cause excessive carbon increase. For example, the height of the bottom coke is too high and the reduction of the blast volume will cause excessive carbon increase. When the furnace lining is eroded, the carbon increase will be more serious. Because the diameter of the cupola becomes larger, in order to maintain the same carbon content, the blast volume needs to be increased. Melting at an excessively high temperature will increase the amount of carbon, which is the case if hot air smelting is used. According to experience, every increase of 55°C in the blast temperature will increase the carbon (mass fraction) by 0.10%. If oxygen is used to increase the temperature, it does not necessarily cause the same problem.
If the interval between tapping liquid is too long, or the liquid iron stays in the hearth for too long, it will also cause carbon increase. The production of low-carbon cast iron generally uses a shallower hearth, and shortens the interval of tapping molten iron, and tries to achieve continuous tapping of molten iron. Intermittent melting will cause excessive carburization, resulting in coarse-grained structure. In addition, melting is discontinued due to wind stop, almost without exception, causing fluctuations in carbon and silicon content. After stopping the wind, it usually takes 15 minutes to regain the original chemical composition.
7.2 Deviations in the weighing or batching of malleable cast iron will cause changes in the chemical composition; the amount of air blast in the furnace is not guaranteed, which will affect the control of the chemical composition; overheating of melting or smoke filling in the flame will cause carbon increase.
7.3 The use of dirty crucibles for brass and bronze, and the remaining condensed shells or thin metal layers from the last melting at the bottom and side walls of the crucible will cause pollution to the next melting. Therefore, the production should Avoid using waste materials from unknown sources, and prevent the incorporation of gas-producing raw materials into the metal furnace charge, such as wet, oil-contaminated or other dirty materials.
7.4 Overheating of molten aluminum due to improper control of the melting temperature of aluminum is a common cause of coarse aluminum alloy grains. Therefore, during production, the overheated aluminum liquid should be slowly cooled down to lower the casting temperature. In addition, carelessness or contamination of the charge during the batching process can also cause coarse grain defects.

8. Pouring
For all metals, too high a pouring temperature can easily cause coarse grain defects.

9. Other
9.1 Excessive cooling speed is not only related to design, pouring system and metal composition, but also related to other factors, such as low molding sand compactness, cold iron when needed, and long time interval between pouring and falling sand , and stacking hot castings together after falling sand.
9.2 Improper heat treatment is also one of the main reasons for the coarse grains of some metals.
9.3 Improper machining & emsp; Improper machining can make the actually dense castings look like they have coarse grain defects. The so-called improper machining refers to the improper grinding of the tool, the blunt tool, the wrong cutting speed or feed control, and the improper roughing method, etc., which will cause a porous appearance with certain damage. This appearance will make It is believed that the casting has the defect of coarse grains.

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