Steel Casting Smelting
Castings are made of cast steel, similar to cast iron, but better than cast iron. Casting in the casting process prone to defects in the pores, the angle is not accurate and other shortcomings in the long-term use of the phenomenon may occur in the chassis fracture.
Steel castings should be used after heat treatment. Because of the existence of stomata, cracks, shrinkage and shrinkage, coarse grain, uneven structure and residual internal stress, the strength of the castings, especially the plasticity and toughness, are greatly reduced.
For the refinement of grain, uniform organization and the elimination of internal stress, steel castings must be normalized or annealed. Normalized steel, its mechanical properties than the high after annealing, the cost is low, so the application of more. But because the normalizing treatment will cause more annealing of the large internal stress, only for carbon content of less than 0.35% of the steel castings. Because the low-carbon steel castings of plastic, cooling is not easy to crack. To reduce the internal stress, cast steel in the normalizing, it should be high temperature tempering. For the carbon content of ≥ 0.35%, the structure of complex and easy to produce cracks in the steel castings, can only be annealed. Cast steel should not be quenched, or easily cracked.
Cast steel smelting generally used flat furnace, electric arc furnace and induction furnace. The furnace is characterized by large capacity, can use scrap as raw material, can accurately control the composition of steel and can melt high-quality steel and low alloy steel, used for smelting high quality, large steel castings with molten steel.
Three-phase electric arc furnace opening and shutdown operation is convenient, to ensure that the composition and quality of molten steel, the requirements of the furnace is not very strict, easy to heat up, it can refining high-quality steel, high-grade alloy steel and special steel, Common equipment for steel castings.
In addition, the use of frequency or medium frequency induction furnace, can smelt a variety of high-grade alloy steel and carbon content of very low steel. Induction furnace melting speed, alloying elements burning small, less energy consumption, and the high quality of molten steel, that is, less impurities, less mixed, suitable for small cast steel workshop used.
Casting process characteristics of cast steel
1, due to the poor fluidity of molten steel, in order to prevent the cast steel produced cold and pouring, cast steel wall thickness can not be less than 8mm; pouring system structure is simple, and cross-sectional size than cast iron; Or hot casting; appropriate to improve the pouring temperature, generally 1520 ° ~ 1600 ℃, because the pouring temperature is high, the superheat of molten steel is large, keep the liquid for a long time, the mobility can be improved. But the pouring temperature is too high, will cause coarse grain, hot cracking, pores and sticky sand and other defects. So the general small, thin-walled and complex shape of the casting, the pouring temperature is about the melting point of steel temperature + 150 ℃; large, thick castings casting temperature than its melting point of about 100 ℃ or so.
2, due to the shrinkage of cast steel much more than cast iron, in order to prevent castings shrinkage, shrink loose defects, in the casting process mostly use riser, cold iron and subsidies and other measures to achieve the order of solidification. In addition, in order to prevent the steel castings from shrinkage, shrinkage, pores and cracks, the wall thickness should be uniform, to avoid sharp and right angle structure, in the molding sand with sawdust, in the core plus coke, and The use of hollow core and oil core, etc. to improve the sand or the core of the concession and permeability.
3, the melting point of cast steel is high, the corresponding pouring temperature is also high. High temperature molten steel and mold material interaction, easy to produce sticky sand defects. Therefore, the use of refractory high artificial quartz sand to do the mold, and in the casting surface brush by the quartz powder or zirconia powder made of paint. In order to reduce the gas source, improve the flow of molten steel and casting strength, most of the cast steel with dry or quick-drying type to cast, such as the use of CO2 hardened water glass sand.