Chilled hard cast iron is also called Cold hard cast iron. It uses ordinary eutectic grey cast iron to chill through various cooling methods after melting so that austenite has no time to transform into pearlite precipitate graphite, and form Leysite. Body and cementite (Fe_3C), the surface of the chilled workpiece requires high hardness and high wear resistance. It often uses metal mold casting to make it cool faster and obtain a certain depth of white layer so that the casting has high hardness and wear resistance, but also has good toughness.
Chilled hard cast iron is characterized by being hard and brittle, and not easy to process, so its range of use is limited. Recently, in order to save alloy materials and meet the special wear resistance needs of some mechanical parts, relevant factories and mines have done a lot of experimental research work on chilled cast iron to effectively control the chemical composition, metallographic structure, white depth, hardness, etc., which is very meaningful for stabilizing product quality, improving the wear resistance of chilled cast iron and increasing the service life of products.
Cooling rate, chemical composition, process factors, etc. have a significant impact on the depth and hardness of the white and mottled layers, as well as the quality and wear resistance of cast iron castings. If there are special requirements, alloy elements such as chromium and nickel can be added to improve performance.
◆Cooling rate
The cooling rate has a direct impact on the microstructure of cast iron. When the same composition of molten iron is rapidly cooled, a white structure can be obtained; When slowly cooled, a gray texture can be obtained; When the cooling rate is between the two, a mottled structure with both cementite and free graphite is formed, and cold hard cast iron is made based on this basic principle. Usually, a metal mold is used to control its cooling rate, forming a white microstructure on the surface of the casting in contact with the metal mold. The center away from the contact surface forms a gray mouth, and there is a transitional zone between the white mouth and the gray mouth, which is called the pockmarked tissue. The size of the white zone determines the wear resistance of the casting, and the larger the white zone, the higher the wear resistance. The ratio of the depth of the Baikou area to the total depth of the Baikou area plus the Makou area is called the relative depth. The relative depth is generally 0.25-0.45. The relative depth affects the strength of chilled cast iron. Under the same conditions of white spot depth, the larger the relative depth, the smaller the mottled area, and the higher the strength of the corresponding cold hard cast iron. Therefore, it is required that the smaller the mottled area, the better.
◆Chemical Composition
The effect of alloying elements on the depth of the white layer in cast iron mainly depends on their influence on graphitization. Any element that promotes graphitization reduces the depth of the whitening process. Any element that hinders graphitization increases the depth of the coating.
The effects of various elements on the thickness of the hemp layer are also different. Tellurium, carbon, sulfur, and phosphorus reduce the thickness of the hemp layer, while chromium, aluminum, manganese, molybdenum, and vanadium increase the thickness of the hemp layer. Due to the different functions of each element, elements that can increase the depth of the white layer may not necessarily increase the hardness of the white layer.
The increase in hardness of the white layer caused by various elements is due to different reasons, such as the increase in the amount of carbides in carbon; Phosphorus relies on the formation of phosphorus eutectic; Nickel, manganese, chromium, etc. are refined due to their microstructure, increasing the dispersion of pearlite in white cast iron, and forming high hardness martensite carbide structures; Vanadium forms special carbides; Silicon and aluminum are due to the strengthening of the matrix. In production, controlling the depth of the white layer mainly relies on silicon and tellurium to adjust, while controlling the hardness of the white layer relies on carbon and alloying. Since the 1970s, the requirements for the mechanical properties of rolling rolls have become increasingly high. In addition to surface hardness, sufficient core strength and toughness are also required. China has extensively adopted chilled ductile iron rollers, while internationally, nickel hard cast iron and high chromium cast iron rollers have led to new developments in the production of chilled cast iron castings.
◆Process Factors
Mainly including molten iron temperature, overheating time, pouring temperature, inoculation treatment, and furnace material condition. Increasing the superheat degree of molten iron and increasing the superheat time will reduce the crystallization core, thus increasing the depth of the white spot. Inoculation treatment will increase the graphite core and reduce the depth of whitening. Lowering the pouring temperature will reduce the depth of the white mouth. Adding white iron to the furnace material or adding scrap steel will increase the depth of the white iron. In order to obtain a qualified white spot depth, the carbon equivalent of molten iron must be controlled first. When the carbon equivalent is 4.25% to 4.35%, a white spot depth of 5-9mm can be obtained. The suitable temperature for molten iron to be discharged from the furnace is 1370~1390 ℃. Sometimes the carbon equivalent of molten iron is appropriate, but if the temperature of the molten iron is too high or too low, it can also cause the depth of the casting to be too large or too small. In order to effectively control the depth of the white mouth and increase the strength of the gray mouth, pre-furnace inoculation treatment can be carried piston out.
Chilled hard cast iron, commonly used as rolls, train wheels, road wheels pulverizer parts, etc. What we are doing now are plungers and rollers,
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