Manufacturing Technology castings

　　For strength, ductility and toughness demanding machine parts, we need to use steel castings. Production of steel castings after the cast, accounting for 15% of total production castings.

　　First, according to the chemical composition of the cast can be divided into carbon steel and alloy steel two categories. Among the most widely used carbon steel, cast more than 80% of total production.

　　1, carbon steel

　　In general, the higher melting point ZG15 of low-carbon steel, casting poor performance, only for manufacturing electrical parts or parts carburizing; carbon steel ZG25 ~ ZG45, all kinds of cast iron has a higher overall performance, that is, high strength, excellent ductility and toughness, and therefore suitable for the manufacture of complex shape, strength and toughness requirements of parts, such as train wheels, rack hammer and the anvil, roll and pressure valves, carbon steel is the most widely used of a class; high-carbon steel ZG55 low melting point, its performance than carbon steel casting is good, but its poor ductility and toughness, for the manufacture of only a few wear parts.

　　2, Alloy Steel

　　Depending on how much of the total alloying elements, alloy steel can be divided into two major categories of low-alloy steel and high-alloy steels.

　　1) Low Alloy Steel, China's major application manganese, silicon manganese and chromium and the like. As ZG40Mn, ZG30MnSi1, ZG30Cr1MnSi1 like. It used to make gears, hydraulic press cylinder and turbine rotors and other parts, and ZG40Cr1 used to manufacture high-strength and high-strength gear shafts and other important force components.

　　2) high-alloy steel, with wear-resistant, heat-resistant or corrosion-resistant and other special properties. Such as high manganese steel ZGMn13, is a kind of anti-wear steel, mainly used in the manufacture of friction under dry working conditions, such as grab the front wall and grab teeth, tractors and tanks crawler excavators etc; chrome-nickel stainless steel ZG1Cr18Ni9 and chromium stainless steel ZG1Cr13 and ZGCr28 like, high corrosion resistance to nitric acid, mainly used for the manufacture of spare parts and equipment on the chemical, petroleum, chemical and food.

　　Second, the cast steel casting process characteristics

　　Cast iron mechanical properties than high, but its cast iron than poor performance. Because the melting point of the higher cast, easily oxidized liquid steel, molten steel illiquid, shrink large, the shrinkage rate of 10 to 14% linear shrinkage of 1.8 to 2.5%. To prevent the castings produced water shortage, cold shut, shrinkage and shrinkage, cracks and other defects sticky sand, we must take complex than cast iron process measures:

　　1, due to the poor mobility of molten steel, in order to prevent steel castings produced and poured cold shut insufficient, can not be less than the thickness of steel castings 8mm; strive gating system structure is simple, and the cross-sectional size larger than the cast iron; dry mold or hot mold; appropriately increasing the pouring temperature, typically 1520 ° ~ 1600 ℃, because the casting temperature, superheat of molten steel, long time remain liquid, liquidity can be improved. But the pouring temperature is too high, it will cause coarse grains, thermal cracking, porosity and sticky sand and other defects. They are generally small, thin-walled castings and complex shapes, its pouring temperature of about the melting point of steel + 150 ℃; large, thick-walled castings pouring temperature higher than the melting point of about 100 ℃.

　　2, due to the contraction of the cast much higher than cast iron, cast appears to prevent shrinkage, shrinkage defects, mostly using riser and cold iron and subsidies and other measures in the casting process, in order to achieve solidification.

　　In addition, to prevent steel castings produced shrinkage, shrinkage, pores and cracks, should make it uniform thickness, avoid sharp corners and right-angle structure plus sawdust in the mold with sand, add coke type core, and hollow core and oil sand core sand or the like to improve the core concession and breathability.

　　(1) The cast of high melting point, corresponding its pouring temperature is high. High temperature molten steel and mold material interaction, easy to produce sticky sand defects. Therefore, we should adopt a higher refractoriness synthetic quartz sand mold made, and brush is made of quartz or zirconium powder sand powder paint was prepared in the mold surface. To reduce the gas source, improve mobility and strength of the mold steel, steel castings mostly dry-type or drying-type to casting, such as the use of CO2 hardened sodium silicate sand.

　　Third, heat-treated steel castings

　　After the heat treatment of steel castings shall be used. Because the internal cast member cast in the presence of pores, cracks, shrinkage and shrinkage, coarse grains, tissue heterogeneity and residual stress within the casting defects, the strength of steel castings, especially ductility and toughness greatly reduced.

　　For the grain refinement, uniform organization and eliminate stress, steel castings must be normalized or annealed. Steel after normalizing, the mechanical properties higher after annealing, the cost is low, so more often. However, due to normalizing annealing process can cause relatively large internal stress, apply only to the carbon content of less than 0.35% of steel castings. Because plastic Low-carbon steel casting is good, not easy to crack when cooled. To reduce stress, steel castings, after normalizing, tempering should be carried out. For the carbon content of ≥0.35%, complex structure and easy to crack the steel castings, can be annealed. It should not be hardening steel castings, or vulnerable to cracking.

　　Fourth, cast steel smelting

　　Cast steel smelting generally use open-hearth, electric arc furnace and induction furnace. Open hearth is characterized by large capacity, you can use scrap as raw material, can accurately control the composition of the steel and energy melting high quality steel and low alloy steel, used for melting high quality requirements of liquid steel with large steel castings.

　　Three-phase electric arc furnace and furnace shutdown easy to operate, to ensure the composition and quality of liquid steel, to charge less stringent requirements, easy to heat up, it could refine high-quality steel, high alloy steel and special steel, which is the production of molded Common equipment castings.

　　In addition, use of frequency or frequency induction furnace can melt steel and various high-low carbon content steel. Induction furnace melting fast, burning of alloying elements is small, less energy consumption, and high quality liquid steel, namely low impurity content, inclusions and less suitable for small-scale steel casting workshop using.