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An Experimental Study on Oolitic Hematite Flocculation and Strong Magnetic Separation

Posted by: steel world 2021-10-19 Comments Off on An Experimental Study on Oolitic Hematite Flocculation and Strong Magnetic Separation

1. Foreword According to statistics, China consumes more than 3 million tons of metal wear-resistant materials each year, of which only 100,000 tons of liners are consumed by metallurgical mines. At present, wearing parts such as lining plates in various kinds of mine mills and other beneficiation equipment such as mine mills are generally made of ZGMn13 high manganese steel. Such wearing parts have to withstand a certain amount of impact and abrasive wear during use, so their material should have good abrasion resistance and a certain impact toughness. The impact toughness of ZGMn13 austenitic high manganese steel is very high (ak up to 200J/cm2), the original hardness does not exceed HB230, but under high impact load, the working surface layer can produce a hardening effect, and its surface hardness can reach HRC42- 48, while the center still maintains excellent toughness.

However, if the impact energy is not enough during service, the impact hardening effect of the austenitic high manganese steel surface cannot be fully produced, and the surface of the high manganese steel cannot reach high hardness, the work body will wear out quickly. At the same time, the yield limit of the high manganese steel (δ0.2 ) Is low (about 350Mpa), in use, especially in the early stage of use, the workpiece is prone to plastic deformation. In addition, there is a problem of hardness matching between the liner of the ball mill and the grinding medium (such as grinding balls). The hardness of the grinding medium should generally be higher than the hardness of the liner HRC3. However, the low-chromium cast iron and high-chromium cast iron mills currently used in many factories The hardness of the ball is much higher than the hardness of the high manganese steel plate. The above-mentioned shortcomings of high manganese steel under low impact load often result in excess toughness of the workpiece but insufficient wear resistance, rapid wear failure, and serious deformation, resulting in short life of the work body. The eutectic carbide of high chromium white cast iron with Cr>11% is hexagonal M7C3, and the hardness of (CrFe)7C3 is HRM501200-1800, which is higher than the eutectic carbide Fe3C3 (HRV50840-1100) of general white cast iron. At the same time, (CrFe)7C3 is an isolated phase during solidification, while austenite is a continuous phase, so the toughness is greatly improved compared with ordinary white cast iron, so it is the first choice for abrasive wear and cutting wear resistance.

There are many applications abroad, mainly used for large and medium-sized wear parts such as liners, hammers, grinding balls, and slurry pump flow parts under medium and low impact load conditions. A series of studies on the wear mechanism, fracture mechanism, fracture toughness (K1c value) and crack propagation mechanism of high chromium cast iron have been conducted at home and abroad. The results show that high chromium cast iron can adjust the size and shape of carbides, the amount of secondary carbides and Dispersion and matrix structure (martensite, austenite, sorbite), So as to adjust the performance and meet the requirements of work and use. In recent years, relevant domestic units have also carried out research on high chromium cast iron liners, whose wear resistance can reach more than twice that of high manganese steel under the same working conditions. However, the toughness of these materials is still too low (the impact value of the 10×10×55mm unnotched sample is less than 7.3J/cm2) and contains alloying elements such as molybdenum and copper, and the production cost is relatively high. Therefore, this type of high chromium cast iron still needs to be further improved and perfected. 2. Composition design of high chromium cast iron 1. Carbon and chromium The main function of carbon and chromium is to ensure the quantity and shape of carbides in cast iron. As the amount of C increases, carbides increase; as the ratio of Cr/C increases, the morphology of the eutectic carbide undergoes a process of decreasing the degree of continuity from continuous network → flake → rod shape, and the eutectic carbide crystal The type undergoes a process of change from M3C→M3C+M7C3→M7C3. Some data point out: when the eutectic carbide is unchanged and the Cr/C is 6.6~7.1, the crack propagation ability of the same chromium cast iron is the strongest. According to these principles, the amount of C should be set at 3.1~3.6%, and the amount of Cr should be set at 20~25%. The Cr in the matrix can also improve the hardenability of the material. 2. Nickel’s role is to increase the hardenability of high chromium cast iron, inhibit the transformation of austenite matrix to pearlite, and promote the formation of martensite matrix. 3. Tungsten’s role is to refine crystal grains, increase hardness, and increase wear resistance.

4. The role of high-efficiency rare earth composite modifier is to deoxidize and desulfurize, thereby inhibiting the segregation of inclusions at the grain boundary and improving the condition of the grain boundary; in addition, due to the segregation of rare earth elements and adsorption in the direction of preferential growth of carbides, carbonization The growth of the material is inhibited, so that it becomes uniform and isolated, while other metamorphic elements can form dispersed carbon and nitrogen compounds to prevent the growth of crystal grains and thereby refine the crystal grains. The above effects of the rare earth composite modifier not only improve the microstructure of the material, but also significantly increase the hardness of the material, especially the impact toughness. The dosage of this high-efficiency rare earth composite modifier is 0.2~0.5%. [next] 3. The structure and properties of high chromium cast iron 1.As-cast structure: sorbite + eutectic carbides and bar-shaped massive bar-shaped carbides. Hardness: HRC 48.6, 49.3, 46.0, 49.4, 51.7. Average hardness: HRC49. 2. Heat treatment state After the heat treatment of “normalizing air cooling + tempering air cooling”, the average hardness is HRC60. 5. The metallographic structure is martensite + eutectic carbides + bar-like massive rod-like carbides. 4. Trial production of liner castings 1. Melting process Melting is carried out in a 500kg acid intermediate frequency electric furnace (1) First, add scrap steel and pig iron to the 500kg acid intermediate frequency electric furnace to melt, and then add ferrochrome, tungsten iron, and nickel to adjust the composition of the molten iron. (2) Add ferromanganese and ferrosilicon within 5-10 minutes before tapping.

(3) Add 0.05% pure aluminum for deoxidation about 2 minutes before tapping. (4) The temperature of molten iron out of the furnace is controlled at about 1460~1500℃. (5) Flush 1.4kg of high-efficiency rare earth composite modifier into the bag for inoculation treatment. (6) Sprinkle an appropriate amount of heat-preserving slag-aggregating agent into the bag to cover it, and calm it for about 5 minutes to remove the slag. (7) The temperature of molten iron pouring is controlled at about 1360~1400℃. 2. Molding core-making process The molding process uses organic ester sodium silicate sand. Ingredients: lower box sand and core sand: raw sand (40/70 mesh) 100% + water glass 5% (by weight of raw sand) + organic ester 12% (accounted for) Water glass weight) + EZK type collapsing agent 2.5% (accounting for the weight of the original sand). Upper box sand: 100% raw sand + 4.5% water glass (by weight of raw sand) + organic ester 12% (by weight of water glass) without disintegrant. Sand mixing process: mix raw sand with crushing agent for 1 minute → mix with organic ester for 2 to 3 minutes → mix with water glass for 1 to 2 minutes → use time for sand molding sand: 25 to 30 minutes. Demoulding time: 0.5~1.5 hours. The coating adopts alcohol-based zircon powder coating, which requires full stirring and even brushing twice to occupy the fire and dry quickly. The riser adopts floating bead insulation sleeve. The trial-produced castings have good surface quality and no casting defects. 3. Heat treatment process After the casting is cleaned, heat treatment is carried out. The heat treatment is carried out in a trolley-type resistance furnace, and the heat treatment process is “normalizing air cooling + tempering air cooling”. The average hardness of the castings after heat treatment is HRC60.5, and the impact toughness is as high as 8.J/cm2 (10×10×55mm unnotched specimen). V. Installation trial The trial production liner installation operation test was carried out in a ф3.6×4m wet ball mill with a production rate of 115T/h in WISCO Jinshandian Iron Mine. Mohs hardness of iron ore F=7-8. The new high chromium cast iron lining board and the high manganese steel (ZGMn13) lining board are installed at intervals at the same time. The test began on July 4, 2001. After 5081 hours of use and 606,720 tons of iron stone were processed, the quality change of the new low-alloy steel liner was compared with that of the high-manganese steel liner under the same working conditions. It is known that the wear resistance of the high chromium cast iron liner is 2.6 times that of the high manganese steel (ZGMn13) liner. After starting the inspection, no cracks were found in the lining board. This shows that the toughness of this high chromium cast iron liner can meet the requirements of the mill. 6. Conclusion The new high-toughness and high-chromium cast iron liner (KmTBCr20NiWRe) does not contain expensive molybdenum and copper, and adopts the main effect rare earth composite modifier suitable for the characteristics of our country’s resources and more chromium. Its hardness reaches more than HRC60, and its impact toughness It is above 8J/cm2, and the wear resistance is 2.6 times that of ZGMn13 high manganese liner.

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