Carbon steel castings
Carbon steel castings have similar properties to cast iron, but better strength than cast iron. Carbon steel castings are prone to air hole defects and inaccurate angle positioning in the casting process, which may lead to shell fracture in long-term use.
One of the advantages of carbon steel castings is the flexibility of design. Designers have the greatest freedom to choose the shape and size of castings. Especially for the parts with complex shape and hollow section, carbon steel castings can be manufactured by the unique technology of core assembly. Its shape and shape change is very easy. The transformation speed from pattern to finished product is very fast, which is conducive to rapid quotation response and shorter delivery time. Perfect design of shape and quality, minimum stress concentration factor and the strongest overall structure all reflect the flexibility and technological advantages of carbon steel castings design.
1) Carbon steel castings have strong adaptability and variability in metallurgical manufacturing, and can be controlled by different chemical composition and structure to meet the requirements of various projects; mechanical properties and service properties can be selected in a wide range through different heat treatment processes, and have good welding and processing properties.
2) The isotropy of cast steel and the overall structure of carbon steel castings are strong, thus improving the engineering reliability. In addition, the advantages of lightweight design and short delivery time make it competitive in price and economy.
3) The weight of carbon steel castings can vary in a wide range. Small weight can be only tens of grams of investment casting precision, while large carbon steel castings can weigh several tons, tens of tons or even hundreds of tons.
(1) Inhomogeneous tissue. The first layer of liquid metal contacted with the mold wall after the injection of liquid metal into the mold quickly solidified into fine grains due to the fastest temperature drop. With the increase of the distance from the mould wall, the influence of the mould wall gradually weakens, and the crystals grow into columnar crystals parallel to each other along the direction perpendicular to the mould wall. In the central part of the castings, the heat dissipation has no significant directionality, and can grow freely in all directions until contact with each other, thus forming equiaxed crystal zone. It can be seen that the structure of the castings is not uniform. Generally speaking, the grains are relatively coarse.
(2) The organization is not compact. Crystallization of liquid metals takes the form of branch growth. The liquid metals between the branches solidify at last, but it is difficult to fill all the branches with liquid metal, which results in the general non-compactness of castings. In addition, the liquid metal in the injection mold can form loose or even shrinkage holes in cooling and solidification if the volume shrinkage is not adequately supplemented. Graphite in iron castings often appears in flake, spherical or other shapes of larger size, and can also be regarded as a kind of non-compact structure.
(3) Rough surface. Generally speaking, the surface is rough, and can not be compared with the machined surface, the shape is also more complex.
(1) Melting of cast steel. Casting steel must be melted in electric furnace, mainly in electric arc furnace and induction furnace. According to the difference of lining material and slag system, it can be divided into acid furnace and alkaline furnace. Carbon steel and low alloy steel can be melted in any kind of furnace, but high alloy steel can only be melted in alkaline furnace.
(2) Casting technology. Cast steel has high melting point, poor fluidity, easy oxidation and aspiration of molten steel. At the same time, its volume shrinkage rate is 2-3 times that of gray cast iron. Therefore, the casting performance of cast steel is poor, and it is easy to produce defects such as insufficient pouring, porosity, shrinkage, hot cracking, clay, deformation and so on. In order to prevent the above defects, corresponding measures must be taken in the process.
Molding sand for carbon steel castings should have high fireproof and anti-sticking properties, as well as high strength, permeability and yielding properties. Silica sand with large and uniform grains is usually used as raw sand; in order to prevent sticking sand, the surface of the cavity is coated with higher refractory paint; in the production of large parts, it is used more than sand or sodium silicate sand, which is faster than casting. In order to improve the strength and yield of the mould, various additives are often added to the sand.
In the design of gating system and riser. As cast carbon steel tends to solidify layer by layer and shrinkage is large, the pouring system and riser should be set up according to the principle of rigid sequential solidification to prevent shrinkage and porosity. Generally speaking, risers should be set for carbon steel castings. Chilled iron is also widely used. In addition, the bottom pouring system with simple shape and large cross-section area should be adopted as far as possible to make molten steel fill the mould quickly and smoothly.
(3) Heat treatment. Heat treatment of cast steel is usually annealed or normalized. Annealing is mainly used for carbon steel castings with W (C) 0.35% or especially complex structure. Such castings have poor plasticity, high casting stress and easy cracking. Normalizing is mainly used for carbon steel castings with w(C)<0.35%. This kind of steel has low carbon content, good plasticity and is not easy to crack when cooled.
4. Common defects
Although the defects of carbon steel castings in the casting process are similar to those of ingot casting, they are still technological defects. The common technological defects are blowhole, inclusion, shrinkage, porosity and cracks.
(1) Pore (bubble): Pore (bubble) is a void formed by excessive gas content in liquid metal, moist model and poor permeability. The porosity in castings can be divided into single dispersed porosity and dense porosity.
(2) Inclusions: Inclusions can be divided into two categories: non-metallic inclusions and metallic inclusions. Non-metallic inclusions are formed by chemical reaction between metals and gases in smelting or by mixing refractories, moulding sand and other inclusions in molten steel during pouring. Metal inclusion is a kind of inclusion formed by dissimilar metals which occasionally fall into molten steel and fail to dissolve.
(3) Shrinkage: Shrinkage is a defect caused by the volume shrinkage of liquid metal when it is cooled and solidified. Shrinkage holes are mostly located near the riser and the largest part of the section or the abrupt change of the section.
(4) Porosity: Due to poor melting and improper shape of the casting die, fine grain boundary cracks or fine voids are formed in the middle of the wall thickness of carbon steel castings, resulting in a loose structure. The bonding between these grains is rather weak (cloudy shadow is formed on the X-ray film).
(5) Cracks: Cracks refer to defects in molten steel caused by excessive impurities at low melting point and excessive internal stresses (thermal stress and structural stress) which cause local cracking of castings during cooling. At the abrupt change of section size, the stress concentration is serious and cracks are easy to appear.
In summary, the obvious feature of technological defects in carbon steel castings is complex shape. The main defects of carbon steel castings are fatigue cracks, including mechanical fatigue cracks and thermal fatigue cracks.
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