Development and production of Bainite Ductile Iron grinding ball material (1)
Abstract: The heat treatment process of step isothermal quenching of casting waste heat can be used to produce low alloy (Mo, Mn, Cu, CR) ductile iron grinding ball materials; its matrix structure is lower bainite with a small amount of martensite, retained austenite and carbide, impact value is more than 14J/CM2, hardness is 52 ~ 59hrc, hardenability is good, service performance is higher than other materials grinding ball.
Keywords: casting; ductile iron; bainite; grinding ball
(Guilin University of Electronic Technology, Mechatronics and Transport Engineering, Guangxi, Guilin 541004)
The grinding ball is the most easily worn part in the use of a ball mill. It is widely used in the milling equipment of metallurgy, mine, electric power, building materials, and other industrial fields. Therefore, increasing the service life and reducing the production cost of wearing parts has been paid attention to by domestic and foreign research and production departments. At present, the main wear-resistant materials for grinding balls in China are forged steel, medium manganese ductile iron, low alloy white cast iron, and high chromium white cast iron. Although the properties of high chromium white cast iron are good, the production cost is high, and it needs to be melted in the electric furnace, which makes it difficult to popularize a lot; low alloy white cast iron has poor properties and is seldom used at present; the production cost of medium manganese ductile iron is low, but its performance is unstable, the breakage rate is high, will gradually lose the market.
Bainite ductile cast iron has high strength, hardness, toughness properties, and good casting properties. Since its appearance in the 1970s, it has been hailed as one of the great achievements in iron metallurgy in the past 40 years, it will be a competitive material in the 21st century. Therefore, it is of great significance to develop and produce high-performance bainite ductile iron grinding ball material.
1.Performance requirements for grinding balls
According to the working conditions of the grinding ball, the following performance requirements are proposed for the material of the grinding ball:
(1) requires good casting performance to prevent casting defects, reduce the generation of the crack source.
(2) the reasonable matching of strength, hardness, and toughness is required, so that the grinding ball has good impact resistance, wear resistance and impact fatigue spalling resistance. The selection of hardness is mainly based on the hardness of the grinding medium. The hardness of coal powder: the medium is low, the lower limit of grinding ball is generally 45 ~ 48HRC. Toughness selection is mainly based on the size of the ball mill and the diameter of the ball.
(3) good heat treatment performance, reasonable heat treatment process makes the structure uniform, small, good hardenability, improve the performance, reduce the internal stress of heat treatment as far as possible, prevent the grinding ball internal crack source.
2.The test results were discussed and analyzed
2.1 Selection of chemical composition
Carbon can stabilize austenite, delay bainite transformation and have a decisive influence on the stability of retained austenite. In addition, carbon is a graphite forming element; high carbon content can prevent cementite precipitation. But high carbon content makes the graphite float, so it must be controlled at 3, between 5% and 317%.
Silicon is a negative segregation element, which can raise the austenitizing temperature, does not have the dragging action to the bainite transformation, promotes the bainite transformation entirely, is advantageous to the performance enhancement, silicon or the graphitization formation element, can refine the graphite ball, increases the ferrite quantity, the transformation of bainite into fine-needle bainite is promoted. Thus the mechanical properties of ductile iron are improved. If the silicon content is too high, the toughness decreases sharply. So the range of silicon is 2. 5% ~ 3 between 6%.
Manganese can shift the C curve to the right and improve the hardenability obviously. The amount of Mn should be controlled at 0. Manganese content can be increased by increasing silicon content (2.5% ~ 3.9%) and strengthening inoculation. 6% ~ 0. 8% ductile iron obtained ideal bainite structure and properties.
Molybdenum is the most commonly used alloying element in the production of bainite ductile iron. Molybdenum is a carbide-forming element. The best advantage of molybdenum is good hardenability. 25% ~ 0. 5%.
Copper is used to improving the hardenability of ductile iron and has the effect of refining graphite balls. 5% ~ 0. 8%.
The main function of chromium is to improve the hardenability and mechanical properties of ductile iron. 15% ~ 0. 35% ./nPhosphorus is a positive segregation element, easy to form phosphorus eutectic in the grain boundary, greatly reduce the impact value, should be controlled at 0, below 0.7%.
Sulfur is easy to combine with spheroidizing elements, resulting in spheroidizing instability, which should be controlled at 0, below 0.3%. The exact composition is shown in Table 1.
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