Steel castings often have various casting defects, such as sand holes, air holes, cracks, shrinkage porosity, shrinkage cavities, and inclusions. It is difficult to control production, especially for alloy steel castings cast in a sand mold. Because the more alloying elements in the steel, the poorer the fluidity of the molten steel, and the casting defects are more likely to occur.
1、 Casting Process Characteristics of Cast Steel
The melting point of cast steel is high, the molten steel is easy to be oxidized, the fluidity of molten steel is poor, and the shrinkage is large. The volume shrinkage rate is 10-14%, and the linear shrinkage rate is 1.8-2.5%. To prevent defects such as insufficient pouring, cold shut, shrinkage cavity, porosity, cracks, and sand sticking of steel castings, more complicated technological measures must be taken.
1. Due to the poor fluidity of the molten steel, the wall thickness of the steel casting shall not be less than 8mm to prevent cold shut and insufficient pouring of the steel casting; The structure of the gating system shall be as simple as possible; Adopt dry or hot casting mold; Properly increase the pouring temperature, which is generally 1520 °~1600 ℃. Because of the high pouring temperature, the superheat of molten steel is large, the liquid state is kept 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, hot cracking, porosity, 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. Due to the large shrinkage of cast steel, to prevent shrinkage cavity and porosity defects of castings, riser, chill, and subsidy are mostly adopted in the casting process to achieve sequential solidification.
3. To prevent shrinkage cavity, porosity, air hole and crack defects of steel castings, the wall thickness shall be uniform, sharp angle and right angle structures shall be avoided, sawdust shall be added to the molding sand for casting, coke shall be added to the core, and hollow core and oil sand core shall be used to improve the yield and permeability of sand mold or core.
4. The melting point of cast steel is high, and the corresponding pouring temperature is also high. Under high temperatures, the interaction between molten steel and mold materials is very easy to produce sand-sticking defects. Therefore, artificial quartz sand with high fire resistance shall be used as the casting mold, and the coating made of quartz powder or zirconium sand powder shall be brushed on the surface of the mold. 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 or quick-drying mode, such as CO2-hardened sodium silicate sand mold.
2、 Common casting defects of steel castings
Various casting defects often occur in the production process of steel castings. Common defects include a sand hole, sand sticking, air hole, shrinkage cavity, shrinkage porosity, sand inclusion, scab, crack, etc.
A) Sand hole defect
Sand holes are caused by the sand particles (blocks) flushed down from the surface of the sand mold cavity by the metal liquid, or the sand particles (blocks) falling into the mold cavity during molding and box closing operations without enough time to float into the pouring system and remain inside or on the surface of the casting. It is a common casting defect that there are small holes filled with mold (core) sand inside or on the surface of the sand hole defect.
B) Sand adhering defect
The casting surface is completely or partially covered with a layer of metal (or metal oxide) and sand (or paint) mixture (compound) or a layer of sintered molding sand, which makes the casting surface rough and difficult to clean. Sand sticking mainly occurs in the part where the surface of the mold and core is strongly heated, and it can be divided into mechanical sand sticking and chemical sand sticking. Mechanical sand sticking is formed when the metal liquid penetrates the micropores on the surface of the mold. When the penetration depth is less than the sand grain radius, the casting does not form sand sticking, but the surface is rough. When the penetration depth is greater than the sand grain radius, mechanical sand sticking is formed. Chemical sand sticking is the product of mutual chemical action between metal oxides and molding materials, which is firmly combined with the casting.
C) Blowhole defect
Inside, on or near the surface of the casting, there are smooth eyelets of different sizes, round, long and irregular shapes, and single or aggregated chips. The color is white or with a layer of dark color, sometimes covered with a layer of oxide skin. Due to the different sources and causes of gas formation, the forms of pores are also different, including invasive pores, precipitation pores, and reactive pores. Invasive pores are large in volume and pear-shaped in shape. They often appear near the core wall or pouring position on the upper part of the casting, mainly because the gas generated in the sand core invades the metal and cannot escape. The position of the small end of the pear-shaped pores indicates that the gas enters the casting from there. Precipitating pores are numerous and dispersed, generally located on the surface of castings, and almost all castings in the same furnace will appear. This is mainly because the gas absorbed by the metal in the smelting process is not completely separated before solidification, so many scattered small pores are formed in the castings. Reactive porosity is caused by a chemical reaction at the metal mold interface. It is also called subcutaneous porosity because it is evenly distributed and often appears 1-3 è below the casting skin (sometimes under a layer of oxide skin). Because most of these pores are elongated pinholes, and their long axis is perpendicular to the casting surface, they can also be called pinholes.
D) Shrinkage porosity defect
Irregular rough surface holes are formed in the thick section of the casting, at the hot spot or the axis and other final solidification places, where the grains are coarse, often with dendrites The large and concentrated holes are called shrinkage cavities, and the small and dispersed holes are called shrinkage porosity. This is mainly because the liquid shrinkage and solidification shrinkage of the casting during cooling and solidification are far greater than the solid shrinkage, and there is no metal liquid supplement at the final solidification place of the casting.
E) Sand inclusion and scab defect
Sand inclusion means that there is a layer of metal lump or sheet on the surface of steel casting, and a layer of molding sand or coating is sandwiched between the metal lump and the casting. Scab is caused by a raised scar on the part where the metal liquid washes away a piece of sand on the surface of the casting mold or where stirring or boiling occurs. The fallen sand is sandwiched 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 zone with low strength, which is easy to delaminate the surface of the mold cavity and cause defects such as scab and sand inclusion on the steel casting.
F) Crack defect
The cracks are divided into cold cracks and hot cracks. A cold crack refers to a crack caused by the local casting stress the casting is greater than the ultimate strength of the alloy when the casting is cooled to the elastic state after solidification; Hot cracking is a kind of crack caused by blocked solid shrinkage of castings at the end of solidification or shortly after final solidification when the castings are still in low strength and plasticity. White spot is a kind of micro crack caused by hydrogen precipitation and higher structural stress and thermal stress during rapid cooling of alloy steel castings with high hardenability.
1. Subcutaneous crack
Subcutaneous cracks are hidden under the casting skin. The cracks are large and zigzag and can be found after machining. Subcutaneous cracks are also common hot cracks.
2. External crack
External cracks often occur at the hot spot where the two walls of the valve casting meet, such as the flange root and the raised surface of the valve body’s outer wall. The external crack can be found with the naked eye. The crack is relatively straight and perpendicular to the stress direction. It is a typical intergranular crack. As the casting surface solidified earlier, the cracks developed from the outside to the inside, so it is difficult to get self-welding repair of the external cracks with feeding metal liquid.
3. Internal crack
An internal crack refers to the hot crack formed in the thick hot spot of the casting, which can be found by wave flaw detection or radiographic inspection, or pressure leakage.
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