Safety production technology of the hottest steel

2022-10-15
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Safety production technology of steel castings

for machine parts that require higher strength, plasticity and toughness, steel castings are required. The output of steel castings is second only to that of cast iron, accounting for about 15% of the total output of castings

first, according to the chemical composition, cast steel can be divided into carbon cast steel and alloy cast steel

among them, carbon cast steel is the most widely used, accounting for more than 80% of the total output of cast steel

1. Carbon cast steel

generally, low carbon steel zg15 has high melting point and poor casting performance, and is only used for manufacturing motor parts or carburized parts; Medium carbon steel ZG25 ~ ZG45 has higher comprehensive properties than all kinds of cast iron, that is, high strength, excellent plasticity and toughness. Therefore, it is suitable for manufacturing parts with complex shapes and high requirements for strength and toughness, such as train wheels, forging hammer frames and anvils, rolls and high-pressure valves. It is the most widely used type of carbon cast steel; The melting point of high carbon steel zg55 is low, and its casting performance is better than that of medium carbon steel, but its plasticity and toughness are poor, so it is only used to manufacture a few wear-resistant parts

2. Alloy cast steel

according to the total amount of alloy elements, alloy cast steel can be divided into two categories: low alloy steel and high alloy steel

(1) low alloy cast steel is mainly used in manganese system, manganese silicon system and chromium system in China. Such as zg40mn, zg30mnsi1, zg30cr1mnsi1, etc. It is used to manufacture gears, hydraulic turbine working cylinders, hydraulic turbine rotors and other parts, while zg40cr1 is often used to manufacture high-strength gears, high-strength shafts and other important stress-bearing parts

(2) high alloy cast steel has special properties such as wear resistance, heat resistance or corrosion resistance. For example, high manganese steel ZGMn13 is an anti-wear steel, which is mainly used to manufacture parts used under dry friction working conditions, such as grab front wall and grab teeth of excavators, tracks of tractors and tanks, etc; Chromium nickel stainless steel ZG1Cr18Ni9 and chromium stainless steel zg1cr13 and zgcr28, etc. have invested 4.84 billion yuan in nitric acid, which has high corrosion resistance, and is mainly used to manufacture parts of chemical, petroleum, chemical fiber, food and other equipment

II. Casting process characteristics of cast steel

the mechanical properties of cast steel are higher than cast iron, but its casting properties are worse than cast iron. Because the melting point of cast steel is high, the liquid steel is easy to oxidize, the fluidity of molten steel is poor, and the shrinkage is large. Its volume shrinkage is 10 ~ 14%, and the linear shrinkage is 1.8 ~ 2.5%. In order to prevent defects such as insufficient pouring, cold shut, shrinkage cavity and porosity, crack and sand sticking of steel castings, technological measures more complex than cast iron must be taken:

1. Due to the poor fluidity of molten steel, in order to prevent cold shut and insufficient pouring of steel castings, the wall thickness of steel castings should not be less than 8mm; The structure of the gating system is as simple as possible, and the section size is larger than that of cast iron; Adopt dry mold or hot mold; Properly increase the pouring temperature, which is generally 1520 ~ 1600 ℃, because the pouring temperature is high, the superheat of molten steel is large, the liquid state is maintained for a long time, and the fluidity can be improved. However, too high pouring temperature will cause defects such as coarse grains, hot cracks, pores and sand sticking. Therefore, the pouring temperature of small, thin-walled and complex castings is about the melting point temperature of steel +150 ℃; The pouring temperature of large and thick wall castings is about 100 ℃ higher than its melting point

2. Because the shrinkage of cast steel is much greater than that of cast iron, in order to prevent shrinkage cavity and porosity defects of castings, riser and, chill and subsidies are mostly used in the casting process to achieve sequential solidification

in addition, in order to prevent steel castings from shrinkage, porosity, porosity and crack defects, it is necessary to make their wall thickness uniform, avoid sharp corners and right angle structures, add sawdust to the molding sand for casting, add coke to the core, and use hollow cores and oil sand cores to improve the yield and permeability of sand or cores

(1) the melting point of cast steel is high, and the corresponding pouring temperature and the heat transfer coefficient of external wall are not greater than 0.15w/m2k. Under high temperature, the interaction between molten steel and mold material is very easy to produce sand sticking defects. Therefore, the artificial quartz sand with high fire resistance should be used as the mold, and the coating made of quartz powder or zirconium sand powder should be brushed on the surface of the mold. In order to reduce the source of gas and improve the fluidity of molten steel and the strength of the mold, most steel castings are cast in dry mold or quick dry mold, such as CO2 hardened sodium silicate sand mold

III. heat treatment of steel castings

steel castings should be used after heat treatment. Because there are casting defects such as pores, cracks, shrinkage cavities and porosity, coarse grains, uneven microstructure and residual internal stress in the as cast steel castings, the strength, especially the plasticity and toughness of the gray cast iron compression samples of the steel castings and the confirmation of the appropriate compression space are greatly reduced

in order to refine grains, uniform microstructure and eliminate internal stress, steel castings must be normalized or annealed. Normalized steel has higher mechanical properties and lower cost than annealed steel, so it is widely used. However, since normalizing treatment will cause greater internal stress than annealing, it is only applicable to steel castings with carbon content less than 0.35%. Because low carbon steel castings have good plasticity, they are not easy to crack when cooled. In order to reduce internal stress, steel castings should also be tempered at high temperature after normalizing. For steel castings with carbon content of 0.35%, complex structure and easy to crack, annealing treatment can only be carried out. Steel castings should not be quenched, otherwise it is easy to crack

IV. smelting of cast steel

the smelting of cast steel generally adopts open hearth furnace, electric arc furnace and induction furnace. The open hearth furnace is characterized by large capacity, using scrap steel as raw material, accurately controlling the composition of steel and smelting high-quality steel and low-alloy steel. It is mainly used for smelting liquid steel for large steel castings with high quality requirements

the three-phase electric arc furnace is easy to open and shut down, which can ensure the composition and quality of molten steel, has less strict requirements on furnace charge, and is easy to heat up. Therefore, it can make high-quality steel, high-grade alloy steel, special steel, etc. it is a common equipment for the production of formed steel castings

in addition, power frequency or (2) bio based thermosetting high molecular materials are used: such as epoxy resin, unsaturated resin, adhesives, etc; Medium frequency induction furnace can melt all kinds of high-grade alloy steel and steel with extremely low carbon content. The induction furnace has the advantages of fast smelting speed, small burning loss of alloy elements, low energy consumption, and high quality of molten steel, that is, less impurities and inclusions, which is suitable for small cast steel workshops

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