Casting properties of high chromium wear resistant cast iron
(1) with good fluidity, high chromium wear-resistant cast iron can be used for pouring thin and complex parts.
(2) It has large shrinkage, similar to that of carbon steel, and usually adopts riser feeding process similar to that of steel castings. The shrinkage of the model is 2%.
(3) High chromium wear-resisting cast iron has a tendency to crack, which is caused by large shrinkage, poor thermal conductivity, high modulus of elasticity and low plastic deformation. Therefore, the parts of high chromium wear-resisting cast iron should be cooled evenly in the process of cast iron so as to avoid cracking caused by excessive heat. At the same time, the hollow parts should have good concessionality to avoid cracking due to high mechanical resistance.
The foundry process of hammer head and lining plate is compared with the burning loss of various elements in cupola and medium frequency furnace. Because the burning loss of chromium in cupola is too large and other elements are difficult to control in the melting process, the medium frequency furnace is used to melt molten iron. The melting temperature is about 1450 C and the pouring temperature is between 1320 1360 C. If the pouring temperature is too high, the grain size of the castings will be coarser, which is unfavorable to the properties of the castings. The riser system should be considered in the casting process of hammer head and liner plate, so that the iron can be steadily punched and filled horizontally with good feeding.
The heat treatment of hammerhead and liner is an important process to obtain excellent wear resistance of high chromium cast iron castings, so the castings must be annealed and quenched after cleaning. The purpose of annealing is to eliminate the internal stress produced in the casting process, increase the plasticity and toughness of the casting matrix, make the composition or structure of the material uniform, and prepare for the subsequent heat treatment. The purpose of quenching is to obtain martensite structure of high chromium wear-resistant cast iron, which is a very important process to obtain high wear resistance of hammers and liners. Because of the existence of a small amount of pearlite in the matrix of high chromium cast iron, the wear resistance will be greatly reduced. The quenching process is to reheat the parts to austenitizing temperature, holding them for a period of time, and then cooling them in the air. Each of these three links is very important to the success of quenching. To achieve this, it is necessary to keep the temperature in the furnace heated evenly while heating up slowly. The holding stage is to make the whole part have enough time to austenite and reach equilibrium state, so that the supersaturated chromium and carbon in the cast matrix precipitate as secondary carbide (usually 2-4 h), so that the stability of austenite in the subsequent cooling will be reduced (that is, the MS point increased), if the high temperature does not decompose into pearlite, then at low temperature can become. Horse body. Therefore, when the austenite temperature decreases, the carbon content of the equilibrium is lower, and the low carbon martensite is obtained. As the hardness is low, the wear resistance is reduced. The austenitizing temperature is too high, and the equilibrium content of austenite is higher than that of carbon and chromium. Because of the high stability, more retained austenite will be obtained during martensitic transformation, which also affects the wear resistance. Therefore, the current heat treatment process is to use the stage heating up to 950~970 C, heat preservation 2~4h, and air cooling system. In order to eliminate stress, improve toughness and avoid cracks in the process of use, the parts after air quenching are generally tempered at 200 ~ 260 C for 2 ~ 4h.
Through the use of hammer metal crusher, when the size of material is not large, martensitic high chromium cast iron hammer can replace high manganese steel hammer head, service life can be more than three times the original, and in normal circumstances will not break, so the use of martensitic high chromium cast iron hammer head, can obtain very significant economic benefits.