Steel castings often have various casting defects such as blisters, pores, cracks, shrinkage porosity, shrinkage cavities, and inclusions, making production control difficult, especially for alloy steel castings cast in sand molds. Because the more alloying elements in the steel, the poorer the fluidity of the molten steel, the easier it is for casting defects to occur.
Casting process characteristics of cast steel
The melting point of cast steel is high, the molten steel is easy to oxidize, has poor fluidity, and has large shrinkage. Its volume shrinkage is 10 to 14%, and its linear shrinkage is 1.8 to 2.5%. To prevent steel castings from defects such as insufficient pouring, cold shut, shrinkage cavities and porosity, cracks, and sand adhesion, more complex technological measures must be taken:
1. Due to the poor fluidity of molten steel, to prevent cold insulation and insufficient pouring of steel castings, the wall thickness of steel castings must not be less than 8mm; the structure of the pouring system should be simple; dry casting or hot casting should be used; the pouring should be appropriately increased The temperature is generally 1520° to 1600°C. Because the pouring temperature is high, the superheat of the molten steel is large, the molten steel remains liquid for a long time, and the fluidity can be improved. However, if the pouring temperature is too high, it will cause defects such as coarse grains, thermal cracks, pores, and sand sticking. Therefore, for general small, thin-walled, and complex-shaped castings, the pouring temperature is about the melting point temperature of steel + 150°C; for large, thick-walled castings, the pouring temperature is about 100°C higher than its melting point.
2. Due to the large shrinkage of cast steel, to prevent shrinkage cavities and shrinkage porosity defects in castings, measures such as risers, cold iron, and subsidies are mostly used in the casting process to achieve sequential solidification.
3. To prevent shrinkage cavities, shrinkage porosity, pores, and cracks in steel castings, the wall thickness should be uniform, sharp corners and right-angle structures should be avoided, sawdust should be added to the casting sand, coke should be added to the core, and Hollow cores and oil sand cores are used to improve the concession and air permeability of the sand mold or core.
4. The melting point of cast steel is high, and the corresponding pouring temperature is also high. The interaction between molten steel and casting materials at high temperatures can easily produce sand-sticking defects. Therefore, artificial quartz sand with higher refractoriness should be used as the casting mold, and paint made of quartz powder or zirconium sand powder should be brushed on the surface of the casting mold. To reduce the source of gas and improve the fluidity of molten steel and the strength of the casting mold, most steel castings are cast in dry or quick-drying molds, such as CO2-hardened water glass quartz sand molds.
Common casting defects of steel castings
Various casting defects often occur in steel castings during the production process. Common defects include blisters, sand sticking, pores, shrinkage cavities, shrinkage porosity, sand inclusions, scarring, cracks, etc.
1. Trachoma defect
The sand holes are caused by the sand particles (chunks) washed down from the surface of the sand mold cavity by the molten metal, or the sand particles (chunks) falling into the mold cavity during the molding and closing operations, which have no time to float into the pouring system and remain inside or on the surface of the casting. Caused. There are small holes filled with molding (core) sand inside or on the surface of the sand hole defect, which is a common casting defect.
2. Sand sticking defect
The surface of the casting is fully or partially covered with a layer of mixture (compound) of metal (or metal oxide) and sand (or paint) or a layer of sintered molding sand, making the surface of the casting rough and difficult to clean. Sand sticking mostly occurs in the parts where the surface of the mold and core is strongly heated and is divided into two types: mechanical sand sticking and chemical sand sticking. Mechanical sand sticking is formed by the penetration of molten metal into the micropores on the surface of the casting. When the penetration depth is less than the radius of the sand grain, the casting does not form sand sticking, but the surface is rough. When the penetration depth is greater than the radius of the sand grain, mechanical sand sticking is formed. Chemical sticky sand is the product of the chemical interaction between metal oxides and modeling materials and is formed by being firmly combined with the casting.
3. Stomatal defects
Inside the casting, on the surface, or close to the surface, there are smooth holes of varying sizes. The shapes are round, long, and irregular, some are single, and some are clustered into pieces. The color is white or with a layer of dark color, sometimes covered with a layer of oxide scale. Due to different sources and formation reasons of gas, stomata also manifest in different forms, including intrusive pores, precipitated pores, and reactive pores. Intrusive pores are large in size and approximately pear-shaped in shape. They often appear in the upper part of the casting near the core wall or the pouring position. This is mainly caused by the gas generated in the sand core invading the metal and failing to escape. The small end of the pear-shaped pores The location indicates where the gas enters the casting. Precipitated pores are numerous and dispersed. They are generally located on the surface of castings. They often appear in almost all castings in the same furnace. This is mainly because the gas absorbed by the metal during the smelting process does not fully precipitate before solidification, forming many scattered small pores in the casting. Reactive pores are pores caused by chemical reactions at the metal-casting interface. Because they are evenly distributed and often appear 1-3è below the surface of the casting (sometimes under a layer of oxide skin), they are also called subcutaneous pores. And because these pores are mostly in the shape of elongated pinholes, with their long axis perpendicular to the surface of the casting, they can also be called pinholes.
4. Shrinkage and shrinkage defects
Irregular holes with rough surfaces are formed inside the thick section of the casting, at the hot joint, or at the center of the axis where it is finally solidified. The grains there are coarse and often contain dendrites. Large and concentrated holes are shrinkage cavities, while holes are small. The scattered ones are called shrinkage. It is mainly caused by the fact that the liquid shrinkage and solidification shrinkage produced by the casting during cooling and solidification is much greater than the solid shrinkage, and the liquid metal cannot be replenished at the final solidification place of the casting. E) Sand inclusion and scarring defects.
Sand inclusion refers to a layer of metal nodules or flakes on the surface of the steel casting. There is a layer of molding sand or paint sandwiched between the metal nodules and the casting. Scarring is caused by the liquid metal being washed away locally on the surface of the casting. A raised scar appears where a piece of sand has been removed or where stirring or boiling occurred, and the fallen sand is trapped in the scar or other parts of the casting. During pouring, the moisture on the surface of the wet mold cavity migrates to the inside of the sand mold due to the high-temperature baking of the molten steel, forming a moisture condensation area with low strength, which can easily cause delamination on the surface of the mold cavity and cause scarring and scarring of steel castings. Defects such as sand inclusions.
5. Crack defects
Cracks are divided into cold cracks and hot cracks. Cold cracking refers to cracks caused by the local casting stress being greater than the ultimate strength of the alloy when the casting is cooled to an elastic state after solidification; hot cracking refers to the cracking of the casting at the end of solidification or shortly after final solidification when the strength and plasticity of the casting are still very low. Below, cracks are caused by the obstruction of solid-state shrinkage of castings. White spots are microcracks caused by the evolution of hydrogen and the generation of high structural stress and thermal stress during rapid cooling of alloy steel castings with high hardenability.
1). Subcutaneous cracks
Subcutaneous cracks are hidden under the surface of the casting. The cracks are large and tortuous and can be discovered after machining. Subcutaneous cracks are also common thermal cracks.
2). External cracks
External cracks often appear at the hot joints where the two walls of the valve casting meet, such as the flange root and the raised surface of the outer wall of the valve body. External cracks can be seen with the naked eye. The cracks are relatively straight and perpendicular to the direction of stress. They are typical intergranular cracks. Since the surface of the casting solidifies early, the cracks develop from the outside to the inside. Therefore, it is generally difficult to repair external cracks by self-welding with molten metal.
3). Internal cracks
Internal cracks refer to thermal cracks formed inside thick hot joints of castings, which can be discovered during wave detection or radiographic inspection or can lead to leakage.
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