Coarse grain size of castings refers to defects that show excessively coarse grain structure during mechanical or fracture inspection, which is not suitable for application. This type of coarse grain structure may occur throughout the entire casting or in local parts of the casting. Essentially, coarse grain defects are metallurgical defects. Based on years of production practice and reference to relevant materials, the author discusses the causes and prevention measures of coarse grain defects in castings.

 

1. Casting Structure and Process Design

1) Excessive differences in the cross-section of castings can result in coarse grains at thicker sections due to slow cooling. Metals such as gray cast iron that are highly sensitive to cross-sectional changes are more prone to such defects.

The effective method to prevent the occurrence of such defects is to avoid excessive differences in cross-sectional dimensions of castings, but this approach is sometimes beyond the reach of casting workers. Therefore, in terms of casting itself, the occurrence of such problems can be reduced and the severity of such defects can be reduced by setting chills, controlling pouring temperatures, or selecting appropriate pouring systems. The use of cold iron can accelerate the cooling rate of thicker sections of castings; Excessive pouring temperature can exacerbate such problems and should be avoided; By adjusting and correcting the design of the pouring system, the low temperature metal melt is located in the thicker section of the casting, and the most effective riser is designed at the thick section of the casting to minimize the size of the riser as much as possible.

(2) For castings with holes, process designers sometimes do not use cores that help reduce the effective cross-sectional size, resulting in a defect where the non core section is too thick. Therefore, in process design, sand cores should be set in thicker sections as much as possible.

(3) In some cases, the cross-section of the casting is not too thick, but due to a narrow depression or core forming a heat sink cross-section in the casting, the result is the same as a thick cross-section. For example, at a columnar nave in a deeper part of the casting, a core may need to be installed, which can cause slow cooling. In cases where the design cannot be modified, unless the metal temperature can be lowered or the gate can be re installed, the best solution is to set a cold iron at the core or cross-section of the mold.

(4) Leaving too much machining allowance during process design not only increases the cost of cutting, but also cuts off the denser surface of the casting and exposes loose parts with slower central cooling. This design has no merit because it is unreasonable from both a casting and machining perspective, and the solution is to change the design of the casting. If design changes are not allowed, the correct method is to use cold iron, control pouring temperature, and adjust the pouring system.

(5) Improper design of the core at thick sections, incorrect support of the core, or the use of other techniques that cause core deviation can cause changes in the cross-section of the casting, resulting in coarsening of the grains.

　　

2. Pouring and Riser System

(1) The failure to achieve sequential solidification in the pouring system is usually the cause of coarse grain size. For castings with sharp changes in cross-section, attention must be paid to the number and position of internal gates. In order to perform shrinkage, maintaining a hot molten metal in the action zone of the riser will reduce the cooling rate of the thick section to the extent of producing coarse grains. Improper riser design, such as excessively long riser neck, improperly designed riser pad, or excessively large riser size, can cause excessive heat accumulation at thicker sections.

(2) The distribution of gating and riser that is prone to heat sink is the same. In order to supplement the thick section, excessive heat accumulation is often caused in local areas. For example, because the side riser can cause overheating of the thick section and slow down the cooling rate, it is sometimes not convenient to use in practical operations. In actual production, it is necessary to reduce the size of the riser as much as possible through reasonable riser design.

(3) The connection between the inner gate or riser and the casting results in local hot spots. The shorter neck of the inner gate or riser is beneficial for shrinkage, but it can make the transverse gate or riser too close to the casting, slowing down the cooling rate of the area. Increasing the neck of the riser can also cause problems with shrinkage. Therefore, the best measure is to adopt effective riser design, minimize the size of the riser as much as possible, and not make the transverse sprue and riser too close to the critical section that is prone to forming coarse particles. The transverse sprue and riser should be appropriately set to achieve shrinkage.

(4) Insufficient number of internal gates not only leads to sand flushing, but also causes local hot spots and coarse grain structure. This phenomenon is common in all cast metals, even in aluminum alloys with lower pouring temperatures. In some cases, a small number of gates can lead to shrinkage defects. This shrinkage defect may mask defects with coarse grains caused by the same reason. In fact, when the coarse grain defect seriously deteriorates, it becomes a shrinkage porosity defect, so the prevention and control measures for these two types of 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 (which is prone to the formation of coarse grains), the micro size of the mold is a factor causing coarse grain defects. Due to the possibility of maximum wall movement at the thick section, this type of defect is still possible, and the resulting coarse grain defect is related to sand expansion.

 

4. Coremaking

Oil sand cores that have not been fully baked or air hardened should be avoided in production, as this type of core may produce exothermic reactions, resulting in excessive heat accumulation. This situation may occur in large castings or in thick large cores using adhesives with exothermic properties. In a sense, this type of core acts as an efficient insulator and slows down the cooling rate of the metal melt to a dangerous level.

 

5. Styling

(1) The lack of ventilation holes that can accelerate the cooling rate is related to the speed at which heat is dissipated through the molding sand for thicker casting sections. Adequate exhaust helps to quickly expel water vapor, resulting in a cooling effect.

(2) The absence of cooling nails or cold iron is usually due to carelessness and negligence.

 

6. Chemical composition

Essentially, the coarseness of grains is related to the coordination between the chemical composition of the metal and the cooling rate, so choosing this coordination is very important. If the cooling rate is difficult to adjust, then the coarse grain structure must be caused by improper chemical composition of the metal. Due to the importance of metal composition, each metal is briefly described as follows.

(1) The mathematical calculation of carbon and silicon effects in gray cast iron and malleable cast iron can usually be summarized as: CE=C+1/3Si, and coarse grains may be caused by excessive carbon or silicon, or excessive carbon and silicon. Compared to silicon, the effect of carbon is three times that of silicon, so changes in the amount of carbon are much more dangerous than changes in the same amount of silicon. The effect of carbon and silicon affects both malleable cast iron and gray cast iron. For malleable cast iron, the coarse grain size does not appear as black or as a rough surface representing primary graphite, but rather in the general form of coarse grain size, due to high carbon or silicon content, or both. Phosphorus can also have an impact on grain size. When wp=0.1%, it will aggravate the shrinkage defects, especially in the section where cooling is slow, which will aggravate the degree of coarse grain defects.

(2) In the melting and deoxidation operations of cast steel, some elements that can delay grain growth are added, so compared to forged steel, cast steel is less likely to form coarse particles. Steel castings with coarse grain size caused by composition can be refined through annealing or normalizing treatment.

(3) Aluminum alloy iron impurities can cause coarse particles and increased brittleness in aluminum castings, 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 refined grain alloy elements.

(4) Defects with coarse grains in copper alloys are often covered by pinholes, pores, or shrinkage porosity. Copper alloys can cause coarse particles due to changes in composition, but usually first appear pinholes, pores, or shrinkage porosity.

 

7. Melting

Small melting operations can have an impact on the remaining grain structure. For different cast metals, a similar melting process must be adopted.

(1) The imbalance between the blast capacity and coke of the cupola melting gray cast iron can cause excessive carbon addition. For example, excessively high bottom coke height and reduced blast rate can cause excessive carbon increase. When the furnace lining is corroded, the carbon increase will be more severe. Because after the diameter of the cupola increases, in order to maintain the same carbon content, it is necessary to increase the blowing rate. Melting at excessively high temperatures will increase carbon content, which can be encountered if hot air melting is used. Based on experience, every increase in blowing temperature of 55 ℃ will result in an increase of 0.10% carbon (mass fraction). If oxygen is used to increase temperature, it may not necessarily cause the same problem.

If the interval between molten iron and molten iron is too long, or if the molten iron stays in the furnace hearth for too long, it can also lead to carbon increase. The production of low-carbon cast iron generally adopts a shallow furnace hearth and shortens the interval time for molten iron to achieve continuous molten iron as much as possible.

Intermittent melting can cause excessive carburization, leading to the formation of coarse grain structure. In addition, the interruption of melting due to the cessation of wind almost invariably leads to fluctuations in carbon and silicon content. After stopping the wind, it usually takes 15 minutes to regain the original specified chemical composition.

(2) Deviations in the weighing or batching of malleable cast iron furnace materials can lead to changes in chemical composition; The lack of guaranteed air volume inside the furnace can affect the control of chemical composition; Melting and overheating or filling flames with smoke can cause carbon buildup.

(3) The use of dirty crucibles for brass and bronze, as well as the presence of residual solidified shells or thin metal layers from the previous melting at the bottom and side walls of the crucible, can cause pollution to the next melting. Therefore, it is necessary to avoid using waste materials from unknown sources in production to prevent the addition of raw materials that can generate gas, such as wet, oil contaminated, or other dirty materials, into the metal furnace material.

(4) Aluminum overheating caused by improper melting temperature control is a common cause of coarse grain size in aluminum alloys. Therefore, in production, the overheated aluminum liquid should be slowly cooled down to a lower pouring temperature. In addition, carelessness or furnace material contamination during the batching process can also cause coarse grain defects.

 

8. Casting

For all metals, excessive pouring temperature can easily cause coarse grain defects.

 

9. Other

(1) The slow cooling rate is not only related to design, pouring system, and metal composition, but also to other factors, such as low density of molding sand, not using cold iron when needed, long time interval between pouring and sand drop, and stacking hot castings together after sand drop.

(2) Improper heat treatment is also one of the main reasons for the coarseness of certain metal particles.

(3) Improper or improper machining can make actually dense castings appear to have coarse grain defects. The so-called improper mechanical processing refers to the improper grinding of cutting tools, excessive dullness of cutting tools, incorrect cutting speed or feed control, and improper rough machining methods. These can all lead to a porous appearance with some damage, which can make people believe that castings have defects with coarse grains.

 

Source: Hot Working Forum