The coarse grain size of the casting refers to the defect that the grain structure is excessively large and unsuitable for application after mechanical inspection or fracture test. The coarse grain structure may be spread throughout the casting or may occur in the casting. Partial. In essence, coarse grain defects are a metallurgical defect. Based on years of production practice and reference to relevant materials, the author talks about the causes and preventive measures of coarse defects in castings.
1. Casting structure and process design
1) The difference in the section of the casting is too large, which will result in coarse grain size due to the slow cooling of the thicker section. Metals such as gray cast iron that are very sensitive to cross-section changes are more prone to such defects.
An effective way to prevent such defects is to avoid excessive disparity in the cross-section of the casting, but this approach is sometimes impossible for the foundry. Thus, as far as casting is concerned, the occurrence of such problems can be reduced by setting cold iron, controlling the pouring temperature or by selecting a suitable juice system to reduce the severity of such defects. The use of cold iron can speed up the cooling of thicker sections of castings; if the pouring temperature is too high, such problems will be more serious and should be avoided. By adjusting and correcting the design of the casting system, the molten metal with low temperature is located in the section of the casting. Thick parts and design the most efficient riser at the thick section of the casting to minimize the size of the riser.
(2) For perforated castings, the process designer sometimes does not use a core that helps to reduce the effective section size, so that the un-cored section is too thick to produce this defect, so in the process design, it should be as much as possible A sand core is placed in a thick section.
(3) In some cases, the section of the casting is not too thick, but the result is a thick cross section due to a narrow recess or core forming a heat sink section in the casting. E.g. At a columnar umbilical in the deep part of the casting, it may be necessary to provide a core, which will result in slow cooling. In the case where design modifications are not possible, the best solution is to place cold iron at the core or mold section unless the metal temperature can be lowered or the gate is re-ground.
(4) When the process design is over, the machining allowance is too large, which not only increases the cost of cutting, but also cuts off the surface of the dense casting and exposes the loose portion with slower central cooling. This design has no merit, because it is unreasonable from the perspective of casting or machining. The solution is to change the design of the casting. If the design is not allowed to change, the correct method is to use cold iron, control the pouring temperature and adjust the gating system.
(5) The design of the core at the thick section is not suitable, the core support is incorrect, or other techniques that cause the eccentricity are used, which will cause a change in the cross section of the casting, resulting in coarse grain.
2, pouring riser system
(1) Failure to achieve sequential solidification The gating system fails to achieve a good order of solidification, which is usually the cause of coarse grains. For castings with sharp cross-section changes, attention must be paid to the number and location of the gates. In order to compensate, the hot molten metal is maintained in the active area of the riser, which reduces the cooling rate of the thick section to the extent that coarse grains are produced. Improper design of the riser, such as the neck of the riser is too long, the design of the riser pad is not appropriate, or the size of the riser is too large, which will cause excessive heat accumulation in the thick section.
(2) Distribution of riser that is prone to heat sinks Similarly, in order to compensate for thick sections, excessive heat is often caused in local areas. For example, because the side riser causes overheating of the thick section and slows down the cooling rate, it is sometimes inconvenient to use in actual operation. In actual production, a reasonable riser design is required to minimize the size of the riser.
(3) The local hot junction or the riser neck is short at the junction of the inner gate or the riser and the casting, which is advantageous for the feeding, but the runner or the riser is too close to the casting. Slowed down the cooling rate of the part. Increasing the neck of the riser will also bring problems to the contraction. Therefore, the best measure is to adopt an effective riser design, to minimize the size of the riser, and not to make the runner and riser too close to the key section that is easy to form coarse grain, and properly set the runner and riser. To achieve the complement.
(4) Insufficient number of ingates The number of ingates is too small, which is not only easy to cause sand washing, but also causes local heat and coarse grain structure. This phenomenon is common in all cast metals, even in low-temperature aluminum alloys. In some cases, because the number of gates is too small, it can cause shrinkage defects. Such shrinkage defects may mask defects of coarse grains due to the same reason. In fact, when the coarse defects of the grains are seriously deteriorated, they become a shrinkage defect, and thus the prevention and control measures for these two defects are often the same.
3, molding sand
The type is a factor causing coarse grain defects only when the molding sand causes the displacement of the wall to be sufficient to increase the cross-sectional dimension of the critical section (the section where the coarse grains are easily formed). Since the wall movement at the thick section may be the largest, such a defect is still possible, and the resulting coarse defect of the grain is related to the sand expansion.
Unbleached or air hardened oil sand cores should be avoided in production because such cores may produce an exothermic reaction that causes excessive heat build-up. This can occur either in large castings or in thick, large cores with exothermic adhesives. In a sense, the core acts as a highly efficient insulator and slows the cooling of the molten metal to a dangerous level.
(1) Lack of vents that can accelerate the cooling rate. For thicker sections of the casting, the cooling rate of the casting is related to the rate at which heat is dissipated through the molding sand. Excessive venting will help the water vapor to drain quickly, creating a cooling effect.
(2) The case where the chilled nail or the cold iron is not set is usually caused by carelessness.
6, chemical composition
Essentially, the coarseness of the 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 grain structure must be due to improper chemical composition of the metal. Due to the importance of the metal composition, each metal is now briefly described as follows.
(1) The carbon equivalent of gray cast iron and malleable cast iron is too high. The mathematical calculation of carbon and silicon effect can be summarized as follows: CE=C+1/3Si, the coarse grain may be due to excessive carbon or excessive silicon, or excessive carbon and silicon. To. Compared with silicon, the effect of carbon is three times higher, so the change in carbon production is much more dangerous than 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 grain is neither black nor does it represent the ram of the primary graphite, but is presented in the form of coarse grains in general, due to excessive carbon or silicon content, or Both are too high. Phosphorus also has an effect on grain coarseness. When wp = 0.1%, the shrinkage cavity defects are increased, especially in the case where the cooling is slower.
(2) Cast steel In the melting and deoxidizing operation of the cast steel, some elements which retard the grain growth are added, so that the cast steel is less likely to form a coarse grain than the forged steel. Steel castings with large grain size due to the composition can be refined by annealing or normalizing.
(3) Aluminum alloys Iron impurities can make the cast aluminum parts coarse and brittle, and most of these defects are caused by improper melting operations. In aluminum alloys, especially those that require overheating, it is necessary to add an appropriate amount of fine grained alloying elements.
(4) Copper alloys The defects of coarse crystal grains in copper alloys are often covered by pinholes, pores or shrinkage. Copper alloys can cause coarse particles due to changes in composition, but pinholes, pores, or shrinkage usually occur first.
The small melting operation will have an effect on the remnant grain structure. For different cast metals, a small melting process must be adopted.
(1) Cupola melting gray cast iron The air volume and coke imbalance will cause excessive carbon increase. For example, a high base height and a reduced blast volume can cause excessive carbon addition. When the lining is eroded, the carbon increase will be more serious. Since the diameter of the cupola becomes larger, in order to maintain the same carbon content, it is necessary to increase the amount of air. Melting at too high a temperature increases the amount of carbon that can be encountered if hot air smelting is used. As a rule of thumb, for every 55 °C increase in blast temperature, 0.10% of carbon (mass fraction) is added. If oxygen is used to raise the temperature, it does not necessarily cause the same problem.
If the interval between the irons is too long, or if the iron stays in the hearth for too long, it will also cause carbon increase. The production of low-carbon cast iron generally uses a shallow furnace, and shortens the interval between the molten iron, as far as possible to achieve continuous iron.
Intermittent melting can cause excessive carbonation, resulting in coarse grained structure. In addition, the melting is interrupted by the wind, and the fluctuation of carbon and silicon content is almost invariably caused. After the wind is stopped, it usually takes 15 minutes to regain the original chemical composition.
(2) Malleable iron The deviation caused by the weighing or the batching of the charge will lead to the change of chemical composition; the amount of air in the furnace is not guaranteed, which will affect the control of chemical composition; the melting of the superheat or the burning of smoke in the flame will cause carbon increase.
(3) The use of dirty enamel in brass and bronze, and the presence of a thin layer of crust or metal remaining in the melting and melting of the previous furnace at the bottom and side walls of the crucible will cause pollution to the next melt, thus producing Raw materials of unknown origin should be avoided to prevent the incorporation of raw materials that generate gases, such as wet, oil-contaminated or other dirty materials, into the metal charge.
(4) Aluminum The aluminum liquid is overheated due to improper control of the melting temperature, which is a common cause of coarse grain of aluminum alloy. Therefore, the superheated aluminum liquid should be slowly cooled down in production to lower it to a lower pouring temperature. In addition, carelessness or contamination of the charge during the batching process can also cause coarse grain defects.
For all metals, too high a casting temperature can easily cause coarse grain defects.
(1) The cooling rate is too slow, in addition to the design, pouring system and metal composition, but also related to other factors, such as the low tightness of the molding sand, the time interval between the use of cold iron, pouring and falling sand when needed. Too long, and put the hot castings together after falling sand.
(2) Improper heat treatment is also one of the main reasons for the coarseness of certain metal particles.
(3) Improper Machining Inappropriate machining can make a dense molded part look like a grainy defect. Improper machining means that the tool is unreasonably ground, the tool is too blunt, the cutting speed or the feed control is wrong, and the roughing method is improper. These will cause a porous appearance with some damage, which will make the appearance It is believed that the casting has defects in coarse grains.