1. Defect name: shrinkage
Definition and Characteristics
The upper plane collapses in the thick section of the casting or at the junction of the sections. Most of the shrinkage occurs at the thick section of the casting, and sometimes it also appears on the surface near the inner shrinkage cavity or shrinkage zone. There are often shrinkage cavities and porosity under the depression, and there is no obvious difference between the depression surface and the surrounding surface.
Identification method
Visual inspection with the naked eye. Note the difference between surface pores and open shrinkage cavities. The exposed shrinkage cavity is mostly funnel-shaped, with a rough surface, often accompanied by thick dendrites; when the surface pores occur at the thick section, the appearance is similar to the depression, and it is caused by the cavity gas of the low-permeability mold floating to the upper surface of the casting, or the gas and paint in the sand mold invade the surface of the casting to form. To distinguish between these two defects, it is often necessary to check whether the air permeability and ventilation of the sand mold are good, and sometimes the casting must be dissected to see whether the interior is subcutaneous pores, shrinkage cavities, or shrinkage porosity.
Cause
During the casting solidification and shrinkage process, the molten metal solidifies slowly at the thick section or hot joint, and the atmospheric pressure will sink the solidified shell on the surface of the casting with a certain degree of plasticity.
Prevention method
1. Modify the casting design to avoid sudden changes in section thickness, increase the fillet at the junction of thick and thin sections, and set process subsidies to improve sequential solidification conditions.
2. If possible, risers should be added, and cold iron or auxiliary runners should be set to ensure the correct solidification sequence and feeding.
3. Take measures related to the prevention of shrinkage cavities.
remedy
1. If it does not affect the performance, it can be filled with putty.
2. Parts that need to withstand gas or liquid pressure should be impregnated. After passing the sealing test, the surface should be filled with putty
3. The depression on the processing surface or working surface can be repaired by welding if the technical conditions permit.
2. Defect name: wrinkled skin
Definition and Characteristics
Irregular coarse-grained or wrinkled scars on the surface of the casting, and a shell with deep reticular grooves (elephant-like wrinkled skin). Usually appear on the upper surface and elevation of alloy steel thin-walled castings rich in chromium, silicon, manganese, and other easily oxidizable elements, thick-walled ductile cast; on the upper surface of iron parts, the inner wall of ductile iron centrifugal cast pipe, the cross-section changes greatly The upper surface of the magnesium alloy casting
Identification method
Visual inspection with the naked eye. Easy to distinguish from other surface defects. Elephant-like wrinkled skin has deep reticular grooves, which is a unique defect of ductile iron castings and magnesium alloy castings. It is easy to distinguish from general wrinkled skins according to the type of alloy.
Cause
1. The compounds (oxides, sulfides, silicates, etc.) formed when magnesium ductile iron is treated with magnesium are usually dispersed in the molten metal in the form of thin films, float slowly in the ladle, and enter the casting with the molten metal during pouring. type, float to the upper surface of the casting, and gather on the inner wall of the casting during centrifugal casting,
2. Magnesium alloy is very easy to oxidize. If there is no protection during smelting and pouring, a thicker oxide film will be formed on the surface of the molten metal, which will be injected into the cavity along with the molten metal. When the liquid flow narrows from wide to narrow in the cavity, the oxide film gathers on the upper surface of the casting to form elephant-like wrinkled skin.
3. The easily oxidizable elements in the alloy steel are oxidized during the pouring and filling process.
4. Generally, wrinkled skin is caused by the high viscosity of liquid metal, low pouring temperature or too slow pouring speed, gas generated by the reaction of metal oxide metal liquid and mold wall during pouring, and low temperature of the metal mold.
Prevention method
1. Use high-quality furnace materials to smelt nodular cast iron to reduce the sulfur content, gas, and inclusion content of the original molten iron; reduce the amount of magnesium added to reduce the residual magnesium of nodular cast iron under the premise of ensuring spheroidization; adjust the carbon content and carbon equivalent of molten iron Properly increase the spheroidizing temperature and strengthen the inoculation treatment to reduce the viscosity of molten iron and improve the fluidity of molten iron; use a ladle with good slag blocking effect, and the molten iron stays in the ladle for a sufficient time before pouring to make the slag and inclusions fully float up.
2. Use a protective atmosphere to smelt and cast magnesium alloys to prevent the oxidation of molten metal; the pouring position and pouring system settings should ensure that the molten metal fills the cavity quickly and stably, shortening the filling process; vacuum suction casting is used.
3. The easy-to-oxidize alloy steel is poured in a reducing or neutral atmosphere.
4. Adjust the composition of the alloy to improve its fluidity, appropriately increase the pouring temperature and pouring speed, prevent the molten metal from oxidizing during the pouring process, improve the composition of the molding sand and coating to prevent the chemical reaction between the mold wall and the molten metal, and let it stand in the ladle enough Time and strengthen slag blocking, properly increase mold temperature during metal mold casting 5. Set filter screen and slag collection bag in the gating system
Remedy
Castings with wrinkled skin defects are usually not scrapped. The wrinkled skin on the unprocessed surface can be polished off with a grinding wheel, and the wrinkled skin on the processed surface can generally be machined off.
3. Defect name: fire escape
Cause
1. The joint surface of the sandbox is not cleaned and the box is not properly sealed, which makes the fire defect prone to occur.
2. If the iron pressing or clamping is insufficient, a fire may easily occur.
3. The molten metal fills the mold violently, which can easily cause the core to break and cause fire defects
Prevention method
1. Guarantee the tightness of the surface and the strength of the core, and prevent the relevant holes in the core from leaking.
2. Ensure the weight of the weight, the strength of the box clamp, and the quantity are sufficient and choose a suitable location.
3. Control the pouring temperature, and the filling should be stable.
4. Defect name: veining
Cause
1. Silica sand undergoes phase transformation and expansion at 573C, and cracks are prone to occur on the surface of the mold during pouring.
2. When the sand core is drying, if the molding sand shrinks violently, it will easily cause the surface of the sand mold to crack.
3. If the core is pounded too hard, the thermal stress caused by the expansion will be large, and veins will easily occur.
4. The thermal deformation of the resin is large, and the core is easy to crack.
5. The pouring temperature is high, the pouring time is long, the resin sand expands rapidly, and the core is prone to cracks.
6. Local overheating in the core is easy to expand and crack
7. The paint is too thin or has poor performance. After the core is cracked, the paint melt is poorly filled.
Prevention method
1. Use regenerated sand in an appropriate amount, and control the particle distribution of the original sand in 3-4 sieve sizes.
2. Avoid heating up too fast, control the drying temperature at around 250C to ensure uniform drying and heating.
3. When molding and making cores, pay attention to controlling the compactness of the molding sand, and add 2% iron oxide powder to the molding sand.
4. Control the content of furfuryl alcohol in the resin to about 80% to prevent resin aging and increase the amount of resin added.
5. Control the pouring temperature and moderate the pouring speed.
6. Use special sand or thicken the thickness of the coating in places with local overheating
7. To ensure the quality and thickness of the coating, add 2 % iron oxide powder to the coating.
5. Defect name: not filled
Cause
1. In addition to the lack of liquid metal, the operation of the pourer is also very important.
2. The upper box is too short, the static pressure of the metal is small, and it is easy to be underfilled.
3. The gas volume of the core is large, which will increase the filling resistance of the liquid metal
4. These factors will reduce the fluidity of the metal and cause poor filling.
Prevention method
1. Before pouring, there should be enough molten metal in the ladle to fill the mold; the pouring speed should be appropriate, and the pouring operation should follow the process regulations.
2. Ensure that there is enough liquid metal pressure head to fill the cavity.
3. Strengthen the ventilation of the mold and control the gas generation of the molding sand and core sand.
4. The smelting is carried out strictly according to the smelting process, the slag and gas removal of the smelting is strengthened and the sulfur content in the molten metal is controlled.
Remedy
1. Before pouring, there should be enough molten metal in the ladle to fill the mold; the pouring speed should be appropriate, and the pouring operation should follow the process regulations
6. Defect name: rough surface
Definition and Characteristics
metal liquid slightly infiltrates into the gap between the sand grains on the surface of the sand mold, which is the early stage of mechanical sand sticking. It is characterized by the rough surface of the casting, and the depth of the rough layer is roughly the same as the size of the sand particles, but the metal on the surface of the casting is not fused with the sand particles. It mostly occurs on the surface of the wet type, and the dry type has no paint or the part where the paint layer is too thin. Often accompanied by defects such as trachoma, rat tail, sand inclusion, and scarring
Identification method
Visual inspection with the naked eye. Rough surface, generally non-sticky sand grains or only lightly marked local sticky sand layer
Cause
1. Molding sand and core sand are too coarse.
2. The compactness of the sand mold and sand core is low or uneven.
3. The coating quality of the mold and core is poor, the coating thickness is uneven, and the coating peels off.
4. The pouring temperature and pouring height are too high, and the metal hydraulic pressure is high.
5. The height of the upper box or the sprue cup is too high, and the hydrostatic pressure of the metal is large.
6. Molding sand and core sand contain too much clay, binder, or fusible additives, low refractoriness, and poor thermal conductivity.
7. There is too much-recycled sand in the mold and core sand, too much fine sand, dust, dead burnt clay, and iron-clad sand in the recycled sand, and the sintering temperature of the molding sand is low.
8. Castings are unpacked too late to form solid hot sticky sand, especially thick castings, and high melting point alloy castings.
9. Liquid metal has good fluidity and low surface tension. For example, the content of phosphorus and lead in the copper alloy is too high, and the content of phosphorus, silicon, and manganese in cast steel is too high.
10. The surface of the resin sand mold and core is not painted or the quality of the paint is poor, the thickness of the coating is uneven, the resin film between the sand particles is vaporized during pouring, and a capillary channel is formed. Or the metal vapor penetrates into the capillary channel,
11. The oxides and low-melting-point compounds in the molten metal react with the molding sand to generate slagging compounds, such as ferrous silicate and fayalite, which reduce the surface tension of the molten metal and improve its flow. The low-melting-point compound and molten metal penetrate into the gap between the sand grains through the capillary action mechanism, and in the process of penetration, the sand grains are continuously eroded to expand the gap between the sand grains, resulting in mechanical sand sticking or chemical sand sticking
12. Improper setting of gating system and riser, resulting in local overheating of molds and castings
Prevention method
1. Use fine-grained raw sand with high refractoriness.
2. When using reclaimed sand, remove fine sand, dead burnt clay, ash, metal oxides, scrap metal, iron-clad sand, and other harmful impurities to improve the quality of reclaimed sand. Regularly replenish the appropriate amount of new sand.
3. Water is a strong oxidizing agent, so the water content of the green sand should be strictly controlled, and an appropriate amount of carbon-containing materials such as coal powder, asphalt, and hydrocarbons should be added to form a reducing atmosphere in the sand mold. However, during high-pressure molding, the amount of carbon-containing materials added should be reduced to reduce the amount of gas generation
4. Use high-quality bentonite to reduce the clay content of clay sand.
5. The binder content in the molding sand should be appropriate and should not be too high, so as to improve the quality of sand mixing, ensure that the sand particles are evenly covered with a binder film, and have moderate air permeability. Avoid lumps in the molding sand.
6. Improve the compactness and uniformity of the sand mold
7. The gating system and riser settings should avoid local overheating of castings and molds, and the lower inner runner should avoid direct flushing of the mold wall.
8. Use anti-sticking sand coating, apply evenly, and increase the coating thickness appropriately in the parts prone to sticking sand. Coatings must not contain components that are prone to gas generation, oxidation, and reactions with molten metal and molding sand. Try not to use paint or surface sand that can form an easily peelable glass-like sand sticky layer at the interface of the casting mold by the reaction to solve the sticky sand problem (for example, in the iron casting mold sand and core sand by adding hematite powder, etc.).
9. Properly reduce the pouring temperature, pouring speed, and pouring height, reduce the height of the mold and the height of the sprue cup, so as to reduce the metal hydraulic pressure, static pressure, and thermal shock to the mold.
10. Strengthen the refining of high melting point alloys such as cast steel, purify the molten metal, reduce the degree of oxidation and gas absorption of the alloy liquid, and reduce the low melting point phase forming elements (such as sulfur and phosphorus in cast steel and cast iron, phosphorus in bronze, lead) content, control the content of elements that will reduce the surface tension of the molten metal and increase the vapor pressure of the molten metal (such as manganese and silicon in cast steel and cast iron, lead in bronze ).
11. For the Jue Ningtian-type friction wall castings, the bottom should be frosted in advance to speed up the cooling of the castings to prevent solid sand from sticking.
12. Adopt a smooth surface appearance and core box.
7. Defect name: lifting type/carrying box
Definition and Characteristics
The height of the casting increases at the parting surface, accompanied by thick fins
Identification method
Visual inspection with the naked eye. Note the difference from fins. The thickness of the simple fin is relatively thin, and the height of the parting surface of the casting does not increase
Cause
by the filling metal wave or the pressure generated by the expansion of the gas in the cavity under the heat of the filling metal wave due to the poor exhaust of the mold, causing the upper mold or core to lift up.
Prevention method
1. The weight of the pressing iron or the strength and quantity of the box clips and tight box bolts must be sufficient, the distribution should be uniform, and the tight box should be operated alternately or symmetrically at the same time.
2. After the casting is completely solidified, loosen the box or skim off the weight iron.
3. Reduce the pouring height of the ladle.
4. Before pouring, check whether the pressure iron is leaked or the box clip and box bolts are leaked.
5. When molding, the upper mold should be pierced with more ventilation holes and a sufficient number of air outlet risers should be set at appropriate parts.
Remedy
1. For single-piece or small-batch production of castings, excess metal can be removed by grinding or cutting.
2. During mass production, the castings with severe pick-up should be scrapped
8. Defect name: type leakage (leakage box)
Definition and Characteristics
Severe, shell-like imperfections are present in castings. Sometimes the shape of the casting is relatively complete, but the internal metal has been leaked, the casting is completely shell-shaped, and excess metal remains at the bottom of the mold
Identification method
A mold leak is a severe shell-like defect inside a casting, and the casting profile is usually complete.
Cause
1. The mold is opened too early during metal mold casting.
2. The strength of mold and core sand is low, the compactness of sand mold is not enough, the pouring temperature is too high or the pouring speed is too fast, and the height of the sprue is too high to make the metal liquid hydrostatic pressure too high, resulting in cracking of sand mold or sand core, and the liquid metal in the mold Leaking out of the mold from cracks or leaking into the inside of the sand core, resulting in mold leakage.
3. Collision with the sand mold during pouring, resulting in mold leakage.
4. Opening the box too early to shake out the sand, the molten metal that is not completely solidified in the casting breaks through the solidified shell and leaks out.
5. Forget to seal the assembly process hole in the sand mold or sand core, the gap between the sand core head and the core seat is too large or not sealed, the sand core wall is too thin, and cracks under the action of the metal liquid, causing the metal liquid to leak out of the mold Or leak into the sand core cavity.
6. The deformation and wear of the metal mold increase the gap between the parting surface, the gap between the metal mold and the metal core is too large, and the design of the exhaust plug, vent hole, and exhaust groove of the metal mold is unreasonable so that the molten metal flows from the parting surface, Core gap or exhaust channel leaking
Prevention method
1. Choose a suitable size sandbox and core bone to ensure that the sand mold and sand core have enough sand.
2. Avoid too high a sprue height, or install a buffer device in the gating system to reduce the pouring height of the ladle to reduce the dynamic pressure and static pressure of the molten metal in the mold.
3. The sand mold and sand core should be placed stably, with a flat and reliable supporting surface to prevent deformation.
4. Improve the strength of molding sand and core sand and the compactness of sand mold and sand core.
5. Avoid hitting the mold or subjecting the mold to severe vibration during and after pouring.
6. After the casting is completely solidified or when the strength of the solidified shell of the casting is sufficient, open the box and shake out the sand.
7. The assembly process holes on the sand mold and the sand core should be blocked before the mold is fitted; the core repair head should be moderate to ensure that there is a suitable gap between the core head and the core seat, and the gap should be blocked with filler.
8. Check whether the formwork and core box are severely worn or deformed and warped. If so, the formwork and core box should be repaired.
9. When casting metal molds, open the mold after the casting is completely solidified. Check whether the parting surface of the metal mold is worn, deformed, and tightly closed; check whether the gap between the metal core and the core guide hole is appropriate, the metal mold exhaust plug, exhaust Whether the hole diameter and gap of the hole and the exhaust groove are suitable. If it is not appropriate, it should be corrected.
10. Properly reduce the pouring temperature and pouring speed.
11. The inner runner should not go straight to the mold wall and the sand core, and the number of inner runners should be appropriately increased to disperse the metal liquid into the cavity, so as to prevent the mold from cracking due to local overheating
9. Defect name: damage (mechanical damage)
Definition and Characteristics
The casting is damaged, broken, and incomplete due to impact. It mostly occurs in the protruding parts such as casting ribs, bosses, edges, and corners of the casting, the connection part with the pouring riser, and the stress concentration and weak parts such as the mutation of the section.
Identification method
Visual inspection with the naked eye. The fracture is characterized by brittle fracture, sometimes with oxidized color, caused by mechanical damage, easy to identify
Cause
1. The structure of the casting is poor, the thickness of the section is very different, the transition is sharp, and there are slender bosses, ribs, etc.
2. The casting is damaged by the impact surface during handling, loading, and unloading.
3. The falling sand temperature is too high, and the vibration and impact are too severe.
4. Insufficient attention is paid to the brittleness of the casting structure and material, and the vibration, tumbling, and impact are too severe during cleaning.
5. The clamping force and cutting force of the casting are too large during machining.
6. The cross-section of the sprue, riser and air outlet riser is too large, there is no necking at the connection with the casting body or the size of the necking is too large, the fillet is too small, and the method of knocking out the pouring riser is improper, which will damage the casting body and lack flesh.
7. Poor casting strength and toughness.
8. There is large residual stress or cracks inside the casting.
Prevention method
1. Improve the casting structure, try to avoid weak structures such as cast ribs and slender bosses, and avoid excessive wall thickness differences and sharp corner transitions.
2. According to the wall thickness of the casting, correctly design the section size and necking size of the sprue, riser, gas outlet riser, and the connection part of the casting; easy-cut risers can be used for castings of brittle materials such as iron castings; knock out the sprue riser When cutting, the direction and method must be correct. Before knocking out, a gap should be cut with a grinding wheel.
3. The sand falling temperature of castings should not be too high, and the vibration and impact force should be moderate.
4. When cleaning the roller, thin-walled castings should not be mixed with heavy castings; vulnerable castings should not be cleaned by rolling; avoid impact when handling and loading and unloading castings.
5. Improve the mechanical properties of the alloy and reduce the residual stress of the casting. If necessary, thermal aging or vibration aging treatment should be carried out to eliminate residual stress before cleaning and machining
Remedy
1. Castings with severe damage should be scrapped.
2. Castings that are slightly damaged and do not affect the use can be repaired after sticking, filling, or welding
10. Defect name: Groove
Definition and Characteristics
groove is the surface of the sand mold cavity under the intense heat of the filling metal liquid, due to the combined effects of thermal stress, moisture migration, expansion, and strength reduction, etc., the surface of the sand mold is arched and cracked, and the unsolidified metal liquid drills into the cracks. Formed sand inclusion defects. Features: V-shaped dents with smooth edges on the surface of the casting, with a depth of about 2mm, sometimes up to 5mm or more. The grooves are usually branched and mostly occur on the upper or lower surface of the casting
Identification method
Visual inspection with the naked eye. The sand scarring that occurs at the inner and outer corners of the casting is called the inner corner sand scarring and the outer corner scarring, which should be distinguished from the corner burrs of the casting. The scab contains molding sand, and the surface of the casting is sunken after removal, and the burr has no such feature. According to these two points and the characteristics of sharp edges, sand inclusion scarring can be distinguished from sand swelling, sand washing, sand loss, sand sticking, and paint scarring. (Scar-like defects caused by paint peeling)
Cause
Under the action of the heat of the filling metal liquid, the sand layer that expands and arches on the upper or lower surface of the cavity is not cracked or has small cracks, making the gold
metal liquid failed to enter the cavity behind the arched sand layer, forming a groove.
Prevention method
1. Reduce the expansion stress of the sand mold: add flammable and easy-softening materials such as coal powder, asphalt, heavy oil, wood powder, and fiber materials to the molding sand to compensate for the expansion of the sand grains and reduce the expansion stress; all or part of the surface of the sand mold is made of high refractoriness, Low expansion sand (such as zircon sand) instead of silica sand.
2. Improve the wet strength of molding sand: use sodium bentonite or activated bentonite to prepare wet molding sand, increase the content of bentonite or clay in wet molding sand; use raw sand with dispersed particle size (such as four-screen sand), and appropriately increase the particle size of raw sand; control wetness The moisture content of the molding sand should not be too high; the sand should be evenly compacted to avoid excessive local compaction of the sand mold.
3. To improve the air permeability of the molded silicon, the upper mold should have more air holes to improve the ventilation of the sand mold.
4. Replace the wet type with the dry type and self-hardening sand type.
5. Properly reduce the pouring temperature and shorten the pouring time, so that the molten metal can fill the cavity quickly and evenly. For large plane castings, the pouring time should not be determined only based on the weight of the castings, but the surface area of the castings should also be considered; when one type of casting is used, the pouring time should be determined according to the weight and size of a single casting, not the total weight of the castings in the mold. time.
6. Cool the sand mold by blowing air during the pouring process.
7. When designing the casting process, place the large plane of the casting on the side. If necessary, it can be cast vertically, horizontally, or inclined
Remedy
Grooves generally do not render castings useless. For the unprocessed surface, the surface can be smoothed or filled with putty after shoveling off the excess metal; for the processed surface, the recessed area can be processed after shoveling off the excess metal
11. Defect name: foreign metal inclusions
Definition and Characteristics
The composition, structure, color and luster, and performance of the casting are different from that of the base metal, with irregular shapes and sizes of foreign metal or intermetallic compound impurities
Identification method
section inspection, metallographic inspection combined with non-destructive inspection (ultrasonic flaw detection or radiographic flaw detection).
Cause
1. Foreign metal impurities are mixed into the molten metal, or foreign metal impurities react with the molten metal of the casting body to form intermetallic compounds.
2. The metal charge or alloy additive is not completely melted and mixed in the molten metal.
3. The core bone is exposed or the core support is floating, and is fused by molten metal, but not completely fused into one.
Prevention method
1. Ensure that the furnace charge is clean and prevent foreign metals from being mixed.
2. Treat the alloy liquid with a small block master alloy, alloy additives, and treatment agents. The alloy melting and treatment temperature should be high enough and poured after the metal charge, alloy additives, and treatment agents are all melted; strengthen the stirring of the metal liquid, Promote rapid melting and dissolution of alloy additives and treatment agents.
3. When there is unmelted metal material or sediment in the molten metal at the bottom of the crucible, processing ladle, or ladle, do not pour
Remedy
According to the alloy microstructure and performance requirements, determine whether to scrap or not. If the technical conditions of castings permit, welding repairs can be carried out
12. Defect name: cold beans
Definition and Characteristics
usually located on the lower surface of the casting or embedded in the surface of the casting, the chemical composition is the same as that of the casting body, and the metal beads are not completely fused with the casting. There is oxidation on its surface, usually under or in front of the inrunner.
Identification method
Visual inspection with the naked eye. Cold beans usually appear on the lower surface of the casting, the chemical composition is the same as that of the casting body, the number is small, and there are no clusters or scattered distribution characteristics similar to sweating;
Cause
When the molten metal is injected into the cavity, splashing occurs, and the metal droplets splashed into the cavity in the early stage solidify rapidly and fail to fuse with the subsequently injected molten metal. Its influencing factors mainly include :
1. The design of the gating system is unreasonable, and the molten metal is sprayed or splashed when it is injected into the cavity from the inner runner.
2. The metal liquid stream injected into the cavity rushes directly to the mold wall, sand core or core support, causing splashing.
3. The sand core and sand mold have too much water, the paint is not dry, the cold iron is rusted or has oil stains, which makes the filling metal liquid boil
Prevention method
1. Improve the gating system, so that the molten metal flows into the cavity smoothly, and prevents jetting or splashing when the molten metal is injected into the cavity; the runner should not go straight into the mold wall, sand core, and core support.
2. Pouring carefully, the sprue cup should be aligned with the sprue cup to prevent the molten metal from splashing into the cavity from the open riser, gas outlet riser, and other openings; the sprue cup should keep a sufficient distance from the riser.
3. Control the moisture content of molding sand and core sand, the paint should be dried, and the cold iron and core support should be dry, rust-free, and oil-free, so as to prevent the molten metal from boiling in the mold
Remedy
Remove by shoveling, grinding, shot blasting, or drum cleaning. After the cold beans embedded in the surface of the casting are removed, the pits can be smoothed or filled with putty
13. Defect name: endosmosis (endosmosis bean)
Definition and Characteristics
A smooth, shiny bean-shaped metal exudate exists in a casting hole-like defect. Its chemical composition is inconsistent with the body of the casting and is close to the eutectic composition
Identification method
destructive testing (radiography or ultrasonic testing) is combined with section inspection. Infiltrates appear in the pore-like defects of castings, which are pea-like exudates, which are squeezed into shrinkage cavities or pores by the low melting point components in the alloy under the action of casting shrinkage stress or dissolved gas precipitation pressure during solidification. When the infiltration is exposed on the surface of the casting, it often appears in groups at the bottom of the riser or in the surface pores and open shrinkage cavities of the casting, which has the characteristics of “sweating”; in terms of chemical composition, the infiltration and the casting body different
Cause
1. During smelting, due to the humidity, rust, and oil pollution of the furnace material, the smelting temperature is too high and the time is too long, the alloy liquid is seriously inhaled and oxidized, and a large number of low-melting impurities that segregate near the grain boundaries are generated, and these impurities are precipitated under gas pressure, Under the action of casting solidification shrinkage stress and alloy phase transformation stress (such as graphitization expansion pressure in cast iron), the surface of the casting is extruded at the later stage of solidification, forming bean-shaped or sweat-bead-shaped metal nodules that are different from the chemical composition of the casting itself.
2. The heat treatment temperature is too high and the holding time is too long so that the low melting point phase distributed in the grain boundary is remelted. Under the action of the precipitated gas pressure, the remelted low melting point component is extruded from the surface of the casting to form a metal tumor. This phenomenon often occurs during heat treatment of die castings
3. The chemical composition of the alloy is unqualified or contains low melting point components, which reduces the eutectic temperature and generates a large number of low melting point eutectic phases. In the later stage of solidification or heat treatment, the low melting point eutectic phase is under the action of precipitated gas pressure and phase transformation stress. Extrusion casting surface.
4. The internal pressure of the casting that causes the infiltration includes: the dissolved gas precipitation pressure in the casting, the solidification shrinkage pressure of the casting, and the pressure generated by the eutectic graphitization when the cast iron solidifies.
5. High-phosphorus iron castings are prone to infiltration, and the phosphorus content of the infiltration is higher than that of the casting body.
6. When the content of elements that are easy to form low-melting point phases in the alloy (such as impurity elements sulfur and phosphorus in cast iron) is high, it is easy to produce infiltration in the casting
Prevention method
1. Eliminate hole defects in castings.
2. Furnace materials should be dry, rust-free, and oil-free. It is best to pre-press or melt waste and chips into secondary ingots.
3. Avoid excessively high smelting temperature or too long smelting time, strengthen refining, and slagging after melting to protect the surface of the molten pool, and reduce the air intake and inclusion content of the alloy liquid. Take other measures to prevent metal uptakes, such as vacuum or controlled atmosphere melting and pouring.
4. Reduce the content of low melting point phase-forming elements (such as phosphorus and sulfur in cast iron) in the alloy. Appropriately reduce the pouring temperature of castings
Speed up the solidification speed of castings.
5. Strictly abide by the heat treatment specification to prevent the heat treatment temperature from being too high.
6. Die castings are generally not suitable for heat treatment. The aging temperature of die-casting parts should not be too high. For die-casting parts that must be heat-treated, vacuum die-casting, oxygen-blowing die-casting, precision-speed dense die-casting, and other non-porous die-casting methods can be used for production.
7. Use a weak oxidizing or oxidizing atmosphere to quickly smelt bronze and strengthen refining measures, fully degas and remove slag, appropriately increase the zinc content to reduce the solidification temperature interval of bronze, and accelerate the solidification speed of castings
Remedy
The technical conditions of castings on the mechanical properties of castings and the requirements for defects such as pores, shrinkage cavities, and deformation, determine whether castings are scrapped or not. Important castings can be repaired by hot isostatic pressing or welding. For castings with large internal stress, aging or diffusion annealing can be carried out to prevent deformation defects associated with castings during heat treatment. Castings with deformation defects can be corrected by the thermal correction
14. Defect name: slag inclusion
Definition and Characteristics
Non-metallic inclusions are caused by molten slag on the surface or inside of castings. Irregular in shape, usually located on the upper surface of the casting, the casting surface below the sand core, or the dead corner of the casting. Slag inclusions often coexist with pores or shrinkage cavities (shrinkage porosity). In the casting section, the slag inclusion has no metallic luster
Identification method
Slag inclusions on the surface of castings are generally inspected by penetrating fluid or magnetic powder, and sometimes can be found with the naked eye; slag inclusions inside castings are generally inspected by a ray or ultrasonic waves, and sometimes they are exposed on the surface of castings after machining. After the casting is cleaned, the surface slag may fall off, leaving irregularly shaped holes on the surface of the casting. The slag inclusions are irregular in shape, distributed in large flakes or spots, and have no metallic luster.
Cause
1. When smelting, refining, or processing molten metal, the added flux and the formed slag are injected into the cavity together with the metal during pouring.
2. Secondary oxidation of molten metal during pouring. For example, due to continuous tumbling and splashing of nodular cast iron liquid during transportation, subcontracting, and pouring, magnesium, rare earth, silicon, manganese, iron, etc. are secondarily oxidized, resulting in metal oxidation together with sulfide and free graphite, float to the upper surface of the casting, or stay in the dead corner of the casting and the lower surface of the sand core, etc.
3. Due to the high sulfur content of cast steel and cast iron, improper manganese-sulfur ratio, poor deoxidation, desulfurization, slag removal, and degassing, the molten metal contains a large amount of sulfide, primary oxide and gas, and slag is formed in the casting after pouring Stomata.
4. Chemical reactions between components in the chemical composition of the alloy (such as C, Mn, S. Si. A1. Ti in cast steel and cast iron) or between these components and nitrogen and oxygen, the molten metal and its oxides Interfacial reaction with furnace lining, bag lining, sand wall or coating
Prevention method
1. During smelting, the furnace charge should be dry and clean, strengthen deoxidation, degassing, slag removal, purify the molten metal, and increase the temperature of the furnace and the treatment of the molten metal.
2. The ladle should be dry and clean. Before pouring, a slag collecting agent should be added to thicken the slag so that it can be easily removed; the molten metal should be left in the ladle for a period of time to facilitate the slag to float; the pouring temperature should be appropriate and should not be too high or too high Low; it is best to use bottom pouring bags and teapot bags for pouring to ensure that the slag stays in the ladle.
3. The sprue cup and sprue should be filled when pouring, and the slag collecting bag and filter should be set in the pouring system, and the non-turbulent pouring system should be used to pour quickly to prevent the secondary oxidation of the molten metal.
4. The processing surface or large plane of the casting should not be located on the top of the cavity as far as possible, and the slag collecting riser should be set at an appropriate position in the casting mold
5. Cast steel, gray cast iron, and nodular cast iron should reduce the sulfur content of the original molten iron, maintain an appropriate silicon-manganese ratio (w(Si) % >w(Mn) % +0.5%), and limit the content of Al. Ti. Under the premise of ensuring spheroidization, reduce the residual magnesium content of nodular cast iron as much as possible
Remedy
If the technical conditions of the castings permit, the defects can be dug out and then repaired by welding. If the defect is serious and welding repair is not allowed, it should be scrapped
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