The relationship between the shrinkage rate of steel castings and the generation of crack defects
The 63rd training of Lao Lu foundry furnace workers was registered on April 26. The content of this training is ductile iron, white cast iron, vermicular cast iron, cast steel smelting and heat treatment training
I. Causes of shrinkage rate and crack defects in steel castings
1. Causes of shrinkage rate
Physical properties of metals
Thermal expansion and contraction: During the solidification and cooling process of steel castings, the thermal motion between atoms is weakened and the atomic distance is reduced due to the decrease in temperature, resulting in the shrinkage of the metal volume.
Crystal structure transformation: During the cooling process of cast steel, the crystal structure will change. The specific volumes of different crystal structures are different. For example, when austenite is transformed into ferrite and pearlite, it will be accompanied by volume shrinkage.
Influence of solidification process
Liquid shrinkage: From the pouring temperature to the solidification temperature, the liquid metal shrinks in volume due to the decrease in temperature. The higher the pouring temperature, the greater the liquid shrinkage.
Solidification shrinkage: During the solidification stage, liquid metal turns into solid state. Since the density of liquid metal is lower than that of solid metal, volume shrinkage will occur. The solidification shrinkage rate is related to the chemical composition of cast steel. For example, cast steel with high carbon content has relatively large solidification shrinkage rate.
Casting structure and size
Wall thickness: Thick-walled castings cool slowly, solidify for a long time, and it is relatively difficult to replenish liquid metal, so the shrinkage rate may be large. Moreover, defects such as shrinkage cavities and shrinkage porosity are easily formed in thick walls, which further affect the shrinkage rate.
Shape complexity: For castings with complex shapes, the shrinkage rate will change due to the uneven cooling rate of each part and the mutual restriction during shrinkage. For example, for castings with thin-walled and thick-walled connections, the thin-walled part cools quickly and solidifies first, limiting the shrinkage of the thick-walled part, resulting in abnormal shrinkage rate.
Casting conditions
Stiffness: The mold has high rigidity and poor yield, which will hinder the shrinkage of the casting, cause stress inside the casting, and reduce the actual shrinkage rate. However, in this case, the casting is prone to defects such as cracks.
Thermal conductivity: Good thermal conductivity of the casting will accelerate the cooling speed of the casting, causing the surface of the casting to shrink quickly and the inside to shrink slowly, which may lead to uneven shrinkage and affect the shrinkage rate.
Casting process parameters
Pouring temperature: Too high a pouring temperature will increase the superheat of the liquid metal, leading to increased liquid shrinkage, and will also increase the temperature of the casting, affecting the cooling speed and shrinkage process of the casting.
Pouring speed: If the pouring speed is too fast, the flow of the molten metal in the casting will be unstable, causing local overheating, affecting the solidification process and shrinkage uniformity; if the pouring speed is too slow, the molten metal may cool too quickly during the filling process, which is not conducive to the liquid metal's supplementation of shrinkage.
2. The causal relationship between crack defects and shrinkage in steel castings
Shrinkage stress causes cracks
Differences in cooling speed: During the cooling process of steel castings, different parts have different cooling speeds due to factors such as thickness and shape. The thick wall cools slowly, shrinks later, and is in a tensile state; the thin wall cools quickly, solidifies and shrinks first, and produces tensile stress on the thick wall. When this tensile stress exceeds the strength limit of cast steel, cracks will occur.
Phase change stress: Cast steel will undergo phase change when cooling, such as the transformation of austenite to martensite, and the volume will expand. If the surrounding solidified parts restrict this expansion, tissue stress will be generated. The superposition of tissue stress and shrinkage stress increases the possibility of cracks.
Shrinkage obstruction causes cracks
Hindered mold and core obstruction: If the mold and core have high rigidity and poor yield, it will hinder the shrinkage of steel castings. For example, when using metal mold casting, the rigidity of the metal mold is large, and the resistance encountered when the casting shrinks is large, which is easy to cause stress concentration inside the casting. When the stress exceeds the strength of the material, cracks will appear.
Unreasonable casting structure design: If there are sudden changes in wall thickness and sharp angles in the casting structure, the shrinkage will be uneven. In these parts, the stress generated by shrinkage cannot be released smoothly and is easy to concentrate, resulting in cracks.
Shrinkage pore defects cause cracks
Shrinkage and shrinkage: If the liquid metal is not replenished enough during the solidification and shrinkage process of steel castings, shrinkage and shrinkage will be formed. These pore defects will weaken the effective bearing area of the casting and increase local stress. In the subsequent cooling or use process, the stress is concentrated around the pores, which is easy to cause cracks and expand.
2. How to reduce the shrinkage rate of steel castings?
Optimize chemical composition
Adjust the carbon content. On the premise of meeting the performance of the casting, appropriately increase the carbon content and use graphitization expansion to compensate for shrinkage.
Reasonably increase the silicon content, improve the fluidity of the molten steel, facilitate shrinkage compensation during solidification, and reduce the shrinkage rate.
Control the pouring process
Strictly control the pouring temperature. On the premise of ensuring the fluidity of the molten steel, try to reduce the pouring temperature and reduce liquid shrinkage.
Optimize the pouring speed and use a suitable pouring speed to ensure that the molten steel fills the mold smoothly to avoid gas entrapment and local overheating.
Improve the structural design of castings
When designing, try to make the wall thickness of the castings uniform, avoid sudden changes in wall thickness, and reduce the shrinkage difference caused by uneven cooling.
Set reasonable risers in the thick parts of the castings to provide sufficient liquid metal to supplement the shrinkage.
Select suitable casting materials and processes
Select casting materials with good yield, such as resin sand, to reduce the obstruction of the casting to the shrinkage of the casting.
Preheat the casting, reduce the temperature difference between the castings, make the cooling process uniform, and reduce the shrinkage stress.
Strengthen the control of the solidification process
Use appropriate cooling methods, such as setting cold iron in the casting, to speed up the local cooling speed of the casting, achieve sequential solidification, and promote the shrinkage compensation of liquid metal.
Appropriately vibrate or stir the castings to break the dendrites in the molten metal at the beginning of solidification, increase the fluidity of the liquid metal, and facilitate shrinkage compensation.
