The Manufacturing Process of Steel Shots: Casting and Forging (Part 2)

　　2、 Casting steel shots:

　　Low to medium chromium cast steel shot has poor wear resistance and high breakage rate. Although it is cheap, its overall cost-effectiveness is not high, so it is not recommended here. High chromium cast steel pellets have excellent hardness and are a high-quality wear-resistant material widely used in dry ball mills in the cement industry. However, high chromium cast steel pellets have poor toughness and are prone to breakage in ball mills with diameters greater than 3 meters, and are also expensive.

　　The influence of casting steel shot material: Generally speaking, the harder the steel shot, the greater its wear resistance. To improve the wear resistance of steel shot, its hardness needs to be increased. However, as the hardness increases, the impact toughness of the steel shot will decrease. At the same time, the hardness of the steel shot should also take into account the material and hardness of the lining plate, and should not be too high (damaging the lining plate) or too low (not wear-resistant). Therefore, how to balance the appropriate hardness and good impact toughness of steel shot is the key to improving the wear resistance of steel shot.

　　The commonly used materials for steel balls are:

　　Low carbon alloy, high manganese steel, high chromium cast iron, high carbon high manganese alloy steel. Low carbon alloy steel shot has good toughness and low cost. Under the same working conditions, its service life is more than twice that of low chromium cast steel shot;

　　High manganese steel has good toughness, good processability, and low price. Its main feature is that under the action of large impact or contact stress, the surface layer will quickly undergo work hardening, and its work hardening index is 5-7 times higher than other materials, resulting in a significant improvement in wear resistance;

　　High chromium cast iron is a wear-resistant material with excellent wear resistance, but it has low toughness, is prone to brittle fracture, and is expensive.

　　High carbon and high manganese alloy steel is mainly an alloy structural steel containing chromium, molybdenum and other elements, with high hardness and good toughness. Its matrix structure includes martensite, bainite or bainite martensite composite structure. The hardness of the steel shot is around HRC60, and the impact toughness is ≥ 15J/CM2. Under the same working conditions, its service life is at least twice that of high manganese steel shot.

　　The quenching and tempering heat treatment of steel shot is crucial. After quenching and tempering heat treatment, not only is the overall toughness required to reach 12 joules/CM2, but also considerable plasticity and wear resistance are required. However, the general casting process can only reach 3-5 joules, resulting in a high breakage rate. Even for high chromium cast steel shot, it is recommended to use forgings when the working strength is high.

　　The manufacturing process of steel shot is also a key factor determining its service life. If there are defects on the surface or internal manufacturing of the steel shot, such as shrinkage, cracks, transgranular, etc., it will not only reduce the performance of the steel shot, but also cause the steel shot to break. Therefore, it is necessary to develop reasonable forging, casting, and heat treatment processes in the production of steel balls. For example, for high chromium cast iron steel balls, measures such as vertical casting and reasonable use of external cooling iron, as well as strict control of casting temperature, should be taken to ensure that the steel balls have a good solidification sequence and shrinkage conditions during casting, thereby obtaining a dense internal structure and reducing the phenomenon of coarse grains. The heat treatment process of steel shot determines the realization of its mechanical properties and the distribution morphology of carbides. Due to the presence of a certain amount of chromium and other alloying elements in the composition of steel shot, how to fully utilize the role of these alloying elements in forming hard point carbides is also a factor considered when formulating heat treatment processes. In short, developing reasonable production processes and strict quality inspection methods are prerequisites for achieving the performance of steel balls.

　　In summary, forgings are superior to castings, and the specific type of steel shot that is more suitable depends on their respective working conditions and economic strength.