The diameter and length of graphite nodules in ductile iron and their relationship with the quality of ductile iron parts
1. The diameter and length of graphite nodules in ductile iron and national standards
In conventional ductile iron, the diameter of graphite nodules is usually between a few microns and tens of microns. For example, for some common ductile iron parts, the average diameter of graphite nodules may be around 10-30 microns.
According to the GB/T 9441-2009 "Metallographic Inspection of Ductile Iron" standard, the size of graphite nodules is divided into 6 grades, with grade 1 being the largest, with an average diameter greater than 25μm; grade 6 being the smallest, with an average diameter less than 6.3μm. Different application scenarios have different requirements for the size of graphite nodules. For example, in some automotive parts with higher requirements, the graphite nodules may need to be smaller and more uniform in size to improve the mechanical properties and wear resistance of the castings.
Generally speaking, when the graphite nodules are 3-6μm in size, the comprehensive performance of ductile iron is relatively excellent. At this time, the graphite balls are small and evenly distributed, which can effectively disperse stress, hinder crack expansion, and improve the strength, toughness and ductility of the material. At the same time, the small-sized graphite balls have a large bonding area with the matrix, and the interface thermal resistance is small, which is conducive to heat conduction, and can make the surface hardness of the cast iron more uniform, improving wear resistance.
From the perspective of metallographic inspection, according to the GB/T 9441-2009 "Metallographic Inspection of Ductile Iron" standard, the graphite ball size of 5-7 can usually meet the requirements of the comprehensive performance of ductile iron in most application scenarios. In some specific ductile iron products, such as ductile iron pipes, the graphite size is generally 6-7, the spheroidization level is controlled to 1-3, and the spheroidization rate is ≥80%, which can enable the material to obtain good mechanical properties, ductility and corrosion resistance.
However, the optimal graphite ball size of ductile iron will also be affected by multiple factors such as specific application scenarios, production processes and material composition. In actual production, it needs to be adjusted and optimized according to specific circumstances.
2. What are the effects of the size of graphite balls in ductile iron on the performance of castings?
The size of graphite balls in ductile iron has many specific effects on its performance, which are mainly reflected in the following aspects:
Mechanical properties
Strength: When the graphite balls are small and evenly distributed, they can disperse stress more effectively, hinder crack expansion, and make the strength of ductile iron higher. Because the bonding area between small-sized graphite balls and the matrix is relatively large, the stress transmission is more uniform, and the material's ability to resist external forces is enhanced.
Toughness and ductility: Small graphite balls can make the matrix of cast iron more continuous. When subjected to external forces, the matrix can deform more evenly, thereby improving the toughness and ductility of the material. Large-sized graphite balls are equivalent to larger "defects", which are prone to cause stress concentration when subjected to force, resulting in premature fracture of the material and reduced toughness and ductility.
Wear resistance
When the graphite balls are small and evenly distributed, the surface hardness of the cast iron is more uniform. During the friction process, it is not easy to have excessive local wear, and the wear resistance is better. At the same time, small graphite balls have little cutting effect on the matrix, and the matrix can better support the load and resist wear. Wear grooves are easily formed around large graphite balls, which accelerates the wear of the material.
Corrosion resistance
Small and uniform graphite balls help to form a denser oxide film and improve the corrosion resistance of ductile iron. Because the small graphite balls are evenly distributed, the microstructure of cast iron is more uniform. In a corrosive environment, it is not easy to form local corrosion cells, thereby slowing down the corrosion rate. Large graphite balls may cause uneven microstructure of cast iron, and the interface between graphite balls and matrix is easy to become the starting point of corrosion, reducing corrosion resistance.
Processing performance
Graphite balls are small, and during the processing, the impact on the tool is small, the surface quality of the processing is good, and the tool wear is relatively small, which is conducive to improving processing efficiency and reducing processing costs. Large graphite balls may increase the roughness of the processing surface, and even cause defects such as edge collapse, affecting processing accuracy and surface quality.
3. The influence of the size of graphite balls of ductile iron on the wear resistance of castings is as follows:
Small graphite balls: Graphite balls with small size and uniform distribution can make the matrix of cast iron have good continuity and more uniform hardness. During the friction process, it can effectively support the load, and it is not easy to have local stress concentration and excessive wear, and the wear resistance is good. At the same time, small graphite balls have little splitting effect on the matrix, which can reduce the formation of wear grooves, thereby improving the wear resistance of the material.
Large graphite balls: Large-sized graphite balls are equivalent to "defects" in the material, which are easy to cause stress concentration when subjected to force. During the friction process, wear grooves are easily formed around large graphite balls, which accelerates material wear. In addition, large graphite balls will make the microstructure of cast iron uneven, resulting in large differences in surface hardness and reducing overall wear resistance.
4. What factors affect the size of graphite balls in ductile iron?
Chemical composition
Carbon and silicon content: Carbon is the main element in the formation of graphite. With high carbon content, the number of graphite balls increases and the size may become smaller. Silicon can promote graphitization, and an appropriate amount of silicon can make the graphite spheres small and uniform. However, if the silicon content is too high, it will reduce the fluidity of the molten iron and affect the growth of the graphite spheres.
Spheroidizer and inoculant: Spheroidizer can make graphite crystallize into spheres, and the appropriate amount of spheroidizer added can ensure the roundness and size uniformity of the graphite spheres. Inoculant can increase the graphite core, increase the number of graphite spheres and reduce their size.
Melting process
Iron liquid temperature: If the iron liquid temperature is too high, the graphite spheres will grow and easily cause spheroidization decline. If the temperature is too low, the fluidity of the iron liquid is poor, which is not conducive to the uniform growth of graphite spheres.
Melting time: Properly extending the melting time can make the iron liquid composition more uniform, which is conducive to the uniform growth of graphite spheres. However, too long melting time will cause the iron liquid to oxidize, affecting the formation and growth of graphite spheres.
Casting process
Pouring temperature: If the casting temperature is too high, the graphite spheres will have more time to grow during the solidification process, resulting in larger size. If the pouring temperature is too low, the filling capacity of the molten iron is poor, which may cause defects such as cold shut and insufficient pouring, affecting the distribution and size of the graphite balls.
Pouring speed: If the pouring speed is too fast, the flow of molten iron in the mold cavity will be turbulent, which will cause the graphite balls to be washed and collided, affecting their growth and distribution. If the pouring speed is too slow, the molten iron will cool for a long time in the runner, which may form graphite balls in advance, resulting in uneven graphite ball sizes.
Cooling speed
Fast cooling speed and large supercooling will increase the nucleation rate of graphite balls, increase the number of graphite balls and reduce their size. If the cooling speed is slow, the graphite balls have more time to grow and their size will increase. In actual production, the cooling speed can be adjusted by controlling factors such as casting materials and wall thickness.
