Manufacturing technology of steel castings
1, casting steel production technology
For strength, ductility and toughness requests for higher mechanical parts, steel castings are required. The yield of cast steel is inferior to that of the cast iron, which accounts for 15% of the total output of the casting.
1, comply with the chemical composition of cast steel can be divided into two categories of carbon steel and alloy cast steel. Among them, the most widely used carbon cast steel, accounting for more than 80% of the total output of cast steel.
1, carbon cast steel
Individual, the higher the melting point of the low carbon steel ZG15, casting performance is poor, only for the production of motor parts or carburized parts; medium carbon steel ZG25 ~ ZG45, there is higher than that of the comprehensive performance of all kinds of cast iron, which has high strength, a wonderful plasticity and toughness, is suitable for making complex shape, strength and toughness of request high parts, such as train wheels, hammer frame and the anvil, roll and low pressure valves and other is carbon steel in operation of up to a class; the low melting point of high carbon steel of zg55 cast, the casting performance is better than that of carbon steel, but the plasticity and toughness poor only for making more than the number of wear-resistant parts.
2, alloy cast steel
Comply with the amount of alloy elements much, alloy steel can be divided into two low alloy steel and high alloy steel categories.
1) low alloy cast steel, China's important operation of the manganese, manganese silicon and chromium series. Such as ZG40Mn, ZG30MnSi1, ZG30Cr1MnSi1, etc.. Used to make gear, hydraulic machine working cylinder and turbine rotor and other parts, and ZG40Cr1 often operate to produce high strength gear and high strength shaft and other important parts.
2) high alloy cast steel, the existence of wear, heat resistance, corrosion resistance, etc.. Such as high manganese steel ZGMn13, is a kind of wear-resistant steel and important for production in dry operation under working conditions of friction parts, such as excavator grab the wall and grab Qian, man swallow hoop said Mu thumb caries Huan pomfret anchovy indistinct supper Connaught G1Cr18Ni9 and chromium stainless steel ZG1Cr13 and zgcr28 etc., of nitric acid corrosion resistance is high and important for production of chemical, petroleum, chemical and food equipment parts.
2, cast steel casting process characteristics
The mechanical properties of the cast steel are higher than that of the cast iron, but the casting performance is worse than that of the cast iron. Because of the high melting point of the cast steel, the steel liquid is easy to oxidize, the molten steel activity is poor, the shortening is big, the shortening rate is 10 ~ 14%, the line is shortened to 2.5% ~ 1.8. In order to prevent steel castings produced misrun and cold shut, shrinkage hole and shrinkage porosity, crack and sticky sand wrong fallacy must be prepared in a process by far more complex than the cast iron:
1, due to the difference of the liquid steel activities, in order to prevent steel casting cold insulation and misrun, casting wall thickness can not less than 8mm; the structure of the gating system strive to briefly, and section size than cast iron. The dry cast or hot mould; appropriate forward pouring temperature, usually 15 to 20 degrees to 1600 DEG C, due to the high pouring temperature, long molten steel overheating degree, link liquid at the moment, the activity can get improved. But the water temperature is too high, can stimulate the coarse grain, hot cracking, porosity and sand problems such as errors. Is individual small, thin wall and complex shape of the casting, the casting temperature is about the melting point of the steel temperature +150 degrees Celsius, large, thick wall castings pouring temperature than the melting point beyond the control of 100.
2, due to the cast steel is greatly reduced than cast iron, in order to prevent casting flash shrinkage hole, shrinkage porosity defect fallacy, in casting process on most of the riser and chill and allowances, the preparation, to achieve the progressive solidification.
In addition, in order to prevent steel casting shrinkage hole, shrinkage pores, cracks and wrong fallacy, should enable the uniform wall thickness, prevent the sharp corners and a right angle structure, in the mould with sand and sawdust, in the core and coke, and the empty heart core and oil core to improve sand mold or core concessions and permeability.
(1) the melting point of the cast steel is high, and the response of the casting temperature is also high. The molten steel and casting material under low temperature interactions, easy to produce sand with errors. In order to be used, the artificial quartz sand with a high degree of fire resistance should be used as the casting mould, and the coating can be brushed by the quartz powder or the zirconium sand powder. To cut gas source, forward activity of molten steel and mould strength, most steel castings with dry or drying to casting, such as the use of CO2 hardening water glass sand.
3, the heat treatment of cast steel
Cast steel parts should be in heat treatment after operation. Due to cast state of cast steel parts inside the pores, cracks, shrinkage and shrinkage porosity, coarse grain, organization and residual stress etc. casting wrong fallacy, the intensity and special steel casting is the plasticity and toughness decreases greatly.
For the fine grain and uniform microstructure and eliminate internal stress, steel castings must normalizing or annealing processing. Normalizing after processing of steel, its mechanical properties after annealing is high, the cost is relatively low, so the operation is more. But because the processing will stimulate internal stress than normalizing annealing, only suitable for cast steel with carbon content lower than 0.35%. Because the low carbon steel casting is good, it is not easy to crack when it is cool. In order to reduce the stress of steel castings in normalizing, should also be tempered at low temperature. For the complex structure of carbon steel castings is more than 0.35%, and easy to crack, only by annealing processing. Cast steel parts should not be quenched, otherwise it is easy to crack.
4, cast steel smelting
Steel smelting by individual furnace, electric arc furnace and induction furnace. Characteristics of an open - hearth furnace is large capacity, can dominate the scrap as raw material, can accurately control the composition of the steel and smelting of superior steel and low alloy steel, used for melting quality request high, large cast steel pieces used in liquid steel.
A three-phase electric arc furnace start-up and shutdown to dominate the convenience, security of the liquid steel composition and quality, to charge the request not very cruel, easily heating, so it can be refined