Probe Into the Causes of Coarse Grain Defects in Castings and Effective Preventive Measures in Practice!
Coarse grains of a casting are defects that show excessively coarse grain structure and are not suitable for application when subjected to mechanics or fracture inspection. Such coarse grains may be distributed throughout the casting or may occur in the casting. Local. In essence, a coarse grain defect is a metallurgical defect. Based on many years of production practice and referring to relevant materials, the author talks about the causes of coarse grain defects and the preventive measures.
1.Casting Structure And Process Design
(1) Excessive cross-sections of castings will cause coarse grains at thicker sections due to slow cooling of thicker sections. Metals that are very sensitive to cross-section changes, such as gray cast iron, are more likely to have such defects. An effective way to prevent such defects is to avoid excessive disparity in the cross-sectional dimensions of the casting, but this approach is sometimes beyond the power of the foundry worker. Therefore, in terms of casting, such problems can be reduced by setting cold iron, controlling the pouring temperature, or by selecting a suitable pouring system, and reduce the severity of such defects. The use of cold iron can speed up the cooling rate of thicker sections of castings; excessively high pouring temperatures will make these problems more serious and should be avoided; by adjusting and correcting the design of the pouring system, the molten metal with a lower temperature is located on the sections of the castings. For thick parts, design the most effective feeder at the thick section of the casting to minimize the size of the feeder.
(2) For perforated castings, the process designer sometimes does not use a core that helps reduce the effective cross-sectional size, and the section without a core is too thick to cause this defect. Therefore, when designing the process, A sand core is set in a thick section.
(3) In some cases, the cross section of the casting is not too thick, but the heat sink cross section is formed in the casting due to a narrow depression or core, and the result is the same as the thick section. E.g. A core may be required at a cylindrical umbilical in the deep part of the casting, which will cause slow cooling. In the case that the design cannot be modified, unless the metal temperature can be lowered or the gate is repositioned, the best solution is to install cold iron at the core or mold section.
(4) The machining allowance is too large when designing the process, which not only increases the cost of cutting, but also cuts out the denser casting surface, and exposes the loose parts with slower center cooling. This design is not desirable because it is unreasonable from the perspective of casting or machining, and the solution is to change the design of the casting. If design changes are not allowed, the correct approach is to use cold iron, control the pouring temperature, and adjust the pouring system.
(5) Inappropriate core design at thick sections, incorrect core support, or the use of other techniques that cause eccentricity will cause changes in the cross-section of the casting and cause coarse grains.
(1) Failure to achieve sequential solidification The pouring system fails to achieve sequential 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 perform shrinkage, maintaining hot molten metal in the action zone of the riser will reduce the cooling rate of the thick section to the extent that coarse grains are generated. Improper riser design, such as the riser neck is too long, improper riser pad design, or the size of the riser is too large, it will cause excessive heat collection at thicker sections.
(2) The distribution of the riser that is prone to cause heat sinks. Similarly, in order to compensate for thick sections, excessive heat collection is often caused in local areas. For example, because the side riser can cause overheating of a thick section and slow down the cooling rate, it is sometimes inconvenient to use in actual operation. In actual production, a reasonable riser design should be adopted to reduce the size of the riser as much as possible.
(3) The joint of the inner gate or the riser and the casting causes a short neck of the inner gate or the riser in the local thermal section, which is beneficial for shrinkage, but it will make the runner or the riser too close to the casting. Slowed down the cooling rate of this part. Increasing the riser and neck will also cause problems for filling. Therefore, the best measure is to adopt an effective feeder design, to reduce the size of the feeder as much as possible, not to make the runner and the feeder too close to the key section that is easy to form coarse grains, and to set the runner and the feeder properly To achieve cramping.
(4) The number of gates is insufficient. The number of gates is too small, which is not only easy to cause sand blasting, but also to cause local thermal nodes and coarse grain structure. This phenomenon is common in all cast metals, even in aluminum alloys with lower pouring temperatures. In some cases, too few gates can cause shrinkage defects. Such shrinkage defects may mask coarse grain defects caused by the same reason. In fact, when the coarse grain defect is severely deteriorated, it becomes a shrinkage defect, so the preventive measures for these two defects are often the same.
Only when the displacement of the molding 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 moulding is a factor causing the coarse grain defects. Since the wall movement at the thick section may be the largest, this kind of defect is still possible. At this time, the coarse grain defect is related to the sand expansion.
The use of unsintered or air-hardened oil sands cores should be avoided in production, as this type of core may produce an exothermic reaction, which can cause excessive heat collection. This occurs either in large castings or in thick large cores with adhesives with exothermic properties. In a sense, this type of core acts as a highly efficient thermal insulator and slows down the cooling rate of the molten metal to dangerous levels.
(1) Lack of ventilation holes that can accelerate the cooling rate. For thicker casting sections, the cooling rate of the casting is related to the rate of heat dissipation through the molding sand. Sufficient exhaust will help the rapid release of water vapor, resulting in a cooling effect.
(2) The absence of chill pins or cold iron is usually caused by careless negligence.
In essence, the coarseness of 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 the inappropriate chemical composition of the metal. Due to the importance of the metal composition, each metal is briefly described below.
(1) The carbon equivalent of gray cast iron and malleable cast iron is too high, and the mathematical calculation of the carbon and silicon effects can usually be summarized as: CE = C + 1 / 3Si, the grain size may be due to excessive carbon or silicon, or excessive carbon To. Compared with silicon, the effect of carbon is almost three times, 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 are neither black nor pits that represent primary graphite, but are presented in the form of ordinary coarse grains, which is due to excessive carbon or silicon content, or Both are too high. Phosphorus also affects grain size. When wp = 0.1%, the shrinkage defects will be aggravated, especially the degree of coarse grain defects at the section where the cooling is slower.
(2) Cast steel slabs In the melting and deoxidizing operations of cast steel, some elements that delay grain growth are added. Therefore, compared with forged steel, cast steel is less likely to form coarse grains. Steel castings with coarse grains due to composition can be refined by annealing or normalizing.
(3) The impurities of aluminum alloy and iron will make the aluminum castings coarser and more brittle. Most of these defects are caused by improper melting operation. In aluminum alloys, especially those that require overheating, it is necessary to add an appropriate amount of refined alloy elements.
(4) Copper alloys / copper alloys are often covered by pinholes, pinholes or shrinkage due to coarse grains. Copper alloys cause coarse grains due to composition changes, but usually pinholes, pores, or shrinkage always appear first.
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