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The essential characteristics of chrome iron powder castings affect the quality of castings

2019-11-13
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Formation mechanism The molten steel contains more alkaline oxide MnO, and the core is made of quartz sand. The MnO reacts with SiO in the quartz sand to form a compound with a lower melting point of MnO # SiO. Chemical sticky sand. Chromite powder, preventive measures: Use chromite resin sand as the core, sand type olivine water glass sand, and evenly coat a layer of magnesia powder on the mold and core. Due to the coarse grains, the low thermal conductivity of high manganese steel makes the molten steel solidify slowly. In the solidification process of steel, dendrites grow thick and easily grow into columnar columnar crystals, which sharply reduces the plasticity and impact toughness of high-manganese steel. Inoculation treatment: When smelting, add a certain amount of molybdenum and chromium elements for inoculation treatment. Because the carbides and nitrides of these elements can play a foreign core role in the crystallization process of steel, so that the grains are refined. Reasonable control of pouring temperature: The test found that the grain size of high manganese steel is closely related to the pouring temperature. When the pouring temperature is high, the molten steel accumulates a large amount of heat, has a slow solidification rate, and has coarse grains after crystallization. Otherwise, the grains are fine. Therefore, for high manganese steels with good fluidity and low thermal conductivity, it is best to use a lower casting temperature in order to obtain finer grains and higher mechanical properties.

Casting cracking and preventing casting production is a complicated process, and every link is very important. Although falling sand and cleaning cannot determine the essential characteristics of the casting, it also affects the quality of the casting. In order to prevent the occurrence of cracking, the following should be done:

    1) The casting boxing time should be reasonably formulated and carefully implemented, not in advance, and the castings should not be placed in the hall after boxing. The as-cast microstructure of high-manganese steel is austenite and carbides. Due to the presence of carbides, the strength of the steel is not high at this time and the brittleness is very large. Therefore, the boxing and handling processes must not collide, and watering must be avoided to prevent stress and The chilling caused cracking of the casting.

     2) Before the casting is heated in the kiln, the easy-to-cut riser of the small casting is knocked off with a hammer. When the casting riser of the large casting needs gas cutting, due to local sudden heating, great stress is generated, and cracks often occur at the root of the riser. Therefore, 5/6 had to be cut off, and the remaining amount was removed after water toughening treatment. At the same time, pay attention to the cutting process, no molten steel must flow to the casting, otherwise the casting will crack.

     3) Before the casting is heated in the kiln, the inner cavity and surface sand need to be cleaned up, and the flash and burrs should be removed. If it is too thick, it can be removed by gas cutting, but an appropriate margin must be left. If conditions permit, it is best to use a wheel cutter.

     4) After the water toughness treatment of the casting is completed, the riser margin must be removed under water, and the water surface at the cutting site must be flowed (12 water pipes can be set to spray water) to ensure cold cutting. At this time, a margin of 6mm7mm is still required. The remaining amount on the non-machined surface is finally removed by carbon arc gouging, and the grinding wheel is polished.


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