Castings shrink during cooling after solidification, and some alloys also undergo a phase change with shrinkage or expansion in the solid state. If the size of a part of a casting or casting changes, and if it is not hindered by freedom, stress, deformation, or cracking will occur. The volume change of molten metal in the process of solidification and cooling is constrained by the outside world or itself. The stress caused by the deformation is the casting stress of the casting, which is the main reason for the defects such as the deformation and crack of the casting. First, the casting stress classification 1 according to the casting stress formation causes can be divided into three categories: 1 thermal stress: casting parts of different thickness, in the solidification and subsequent cooling process, the cooling rate is different, causing the same time the various parts of the amount of shrinkage Inconsistent, the various parts of the casting restrict each other, and the resulting stress; 2 solid phase change stress: the solid phase transformation of the alloy, due to different casting cooling conditions, they reach the phase transition temperature at different moments, and the degree of phase change is also different The resulting stress; 3 mechanical impediment to stress: the casting shrinkage is subject to mechanical strains such as molds, cores, box stops, and cores. 2 According to the existence time of casting stress can be divided into two categories: 1 temporary stress ((temporary stress): casting internal stress may be temporary, when the cause of the stress is eliminated, the stress disappears, called temporary stress; 2 residual Responsive stress: The stress that remains after the cause of the stress is removed. Second, the risk of casting stress Casting stress and deformation of castings on the quality of the castings are very harmful. Casting stress is the main cause of deformation and cracking in the production, storage, processing and use of the casting, which can reduce the performance of the casting. For example, when the working stress of a machine component is in the same direction as the residual stress, the stress is superimposed and may exceed the strength limit of the alloy and fracture occurs. Castings with residual stress can be deformed after being placed for a long period of time or after being machined to make the parts lose their precision. When there is internal stress in the casting, if the internal stress exceeds the yield point of the alloy, the casting is often deformed. Deformed castings may be scrapped due to insufficient machining allowance. To do this, it is necessary to increase the machining allowance. In mass production of flowing water, deformed castings are often scrapped when they are machined. In addition, flexural deformation will also reduce the dimensional accuracy of the castings, especially for castings that require high precision, and it is especially important to prevent deformation. Third, the factors that affect the casting stress castings in the solidification and cooling process, the stress is the thermal stress, phase change stress and mechanical barriers to the algebraic sum. This stress value is greater than the strength of the metal at this temperature, the casting will crack Mechanically impeded stresses generally disappear after the casting has fallen, and are temporary stresses. Residual stress is often thermal stress and phase change stress. Residual stress is related to the following factors 1. Metal properties 1 The greater the modulus of elasticity of the metal, the greater the residual stress in the casting. For example, the residual stress of cast steel, white iron and ductile iron is higher than that of gray cast iron One of the reasons for the large size of Figure 3-8 is related to the elastic modulus of the metal. 2 The residual stress of the casting is proportional to the free line shrinkage factor of the alloy. Figures 3-8 are linear expansion curves for several materials from 0-600°C. When the other conditions are the same, the residual stress is 50% larger than that of the ferritic stainless steel. 3 The thermal conductivity of the alloy directly affects the temperature difference between the thickness of the casting. Alloy steel has lower thermal conductivity than carbon steel. Therefore, alloy steel has large residual stress under the same conditions. The effect of phase transformation on residual stress is manifested in the following two aspects: the change in specific volume caused by phase change; the phase change thermal effect changes the temperature distribution in various parts of the casting. (2) Casting properties The greater the coefficient of heat storage of the cast, the greater the cooling rate of the casting, the greater the temperature difference between the inside and the outside of the casting and the greater the stress generated. Metal molds tend to cause greater residual stresses in castings than sand molds 3. Casting conditions Increasing the pouring temperature is equivalent to increasing the temperature of the casting mold, delaying the cooling rate of the casting, and making the temperature of each part of the casting uniform, thereby reducing the residual stress. 4. Casting structure The greater the difference in wall thickness of the casting, the greater the temperature difference between the thick and thin walls when cooling, the greater the thermal stress caused IV. Prevention and Elimination of Casting Stress In the solidification and cooling process of general castings, there are different casting stresses. Since casting stress is the source of deformation and cold cracking, it is necessary to reduce and eliminate the casting stress. The main way to reduce the casting stress is to reduce the temperature difference of each part of the casting during the cooling process in the process of formulating the casting process according to the structural characteristics of the casting, to improve the concession of the mold and the core, and to reduce the mechanical obstruction. The following specific measures can be used 1 For alloys, select the alloy material with small elastic modulus and shrinkage factor under the premise that the parts can meet the working conditions. 2 In the aspect of casting mold, in order to make the temperature distribution of the casting during the cooling process uniform, cold iron can be placed in the thick part of the casting, or the sand with large heat storage coefficient can be used, and the special thick part of the casting can also be forcedly cooled, that is, in the cooling of the casting. During the process, the coolers previously buried in the mold are blown with compressed air or water vapor mixture to accelerate the cooling rate in the thick parts, and the sand layer in the thick-walled parts of the castings can also be thinned. Using the principle of simultaneous solidification, the temperature difference between the various parts of the casting during the solidification process is as small as possible; the temperature of the casting mold is increased, the entire casting is cooled slowly, so as to reduce the temperature difference in various parts of the casting mold; and the concession of the casting mold and the core is improved. To avoid the mechanical obstruction of the casting during the cooling process after solidification; In order to improve the concession of the mold and the core, the compactness of the sand mold should be reduced, or appropriate amount of wood chips, coke, etc. should be added in the molding sand, and the shell type or resin should be used. Sand type, the effect is particularly significant Using fine sand and paint, can reduce the friction of the mold surface 3 Pouring conditions, the location of the inner gate and the riser should be favorable for the uniform temperature distribution of the various parts of the casting, and the layout of the inner gate should consider simultaneously the requirements of uniform temperature distribution and minimum resistance. Castings must have sufficient cooling time in the mold, especially when using water blasting, it can not be boxed prematurely and the water explosion temperature cannot be too high. However, for some complex castings, in order to reduce the resistance of the molds 4 to improve the casting structure, to avoid greater stress and stress concentration, casting wall thickness difference should be as small as possible, thick and thin wall joints to be a reasonable transition, the heat section should be small and scattered. In order to prevent deformation, in the design of the casting, the wall thickness is uniform and the shape is simple and symmetrical. For thin, long, large and thin deformed castings, the pattern can be made into the shape opposite to the deformation direction of the casting. After cooling, the deformation of the casting is just opposite to the opposite shape. Refer to the CS method. Fifth, to eliminate the method of casting residual stress The residual stress in the casting can be eliminated by the following methods 1 artificial aging. The heat treatment temperature and holding time for removing residual stress by artificial aging should be differently specified according to the nature of the alloy, the casting structure, and the cooling conditions. However, the general rule is to heat the casting to the elasto-plastic state and hold it at this temperature. Time, so that the stress disappears, then slowly cool to room temperature Determine the heat treatment specification should pay attention to, in the casting temperature and cooling process seeks uniform temperature throughout, in order to avoid excessive temperature difference produces additional stress, resulting in casting deformation or cold cracking. For this reason, the temperature of the castings should be increased, and the cooling rate should not be too fast. However, in order to increase the production efficiency, the heating and cooling rates should not be too small, and the holding time should not be too long. It is necessary to formulate higher production rates according to specific conditions. Figure 3-9 Best heat treatment specification for efficiency without generating significant additional thermal stress. When determining the heat treatment specification for a certain alloy casting, many same-sized ring specimens can be cast from the same alloy. The ring is provided with a notch of the same size, and the wedge-shaped iron is wedged at the notch to put the ring in a stress state. 9) The sample is then placed in a furnace and annealed according to different specifications. After the annealing, the wedge iron is removed. According to the size of the notch, the degree of stress reduction is known. The specification that the wedge iron can be freely removed from the gap is the best heat treatment specification 2 natural aging. Placing castings with residual stress in open space, the stress gradually disappears naturally after several months to more than half a year, and the stress relief method is called natural aging. The existence of residual stress in the castings inevitably causes the lattice to undergo a sharp change, and the atomic potential energy on the distorted lattice is high and extremely unstable. Under long-term exposure to constantly changing temperatures, atoms have enough time and energy to exchange energy with their conditions, the energy of the atoms tends to balance, the crystal lattice becomes reverted, the casting deforms, and the stress is relieved. Although the cost of this method is low, the biggest drawback is that it takes too long and is inefficient. Modern production is rarely used. 3 Resonance aging. The principle of resonance aging is to adjust the vibration frequency so that the casting can obtain considerable vibration energy under the excitation force with a resonant frequency. In the resonance process, the alternating stress and the residual stress are superimposed, and the casting is locally yielded to produce plastic deformation, so that the residual stress in the casting gradually relaxes and disappears. At the same time, the atoms on the distorted crystal lattice also gain more energy, so that the lattice distortion recovers and the stress disappears. The exciter is mainly composed of a shaking table and a control box. During work, the vibrator is firmly clamped to the middle or end of the workpiece ((J, the part is mounted on the vibrating table 1). The main process parameters are resonance frequency, dynamic stress and excitation time. 1 Determination of resonant frequency. Adjust the frequency of the vibrator. When the frequency of the vibrator coincides with the natural frequency of the workpiece, the amplitude reaches the maximum value. At this time, the frequency is the resonance frequency. 2 The maximum benefit can be obtained when the dynamic stress approaches 35Pa 3 The exciter time should be based on the original conditions of the casting and the actual conditions in the treatment process. Heavy castings take longer to process Resonance aging has significant advantages: a short time, low cost, low power, 735.5WQ horsepower) vibrator can handle more than 50t castings, energy saving, no pollution, the mechanism is light, easy to operate, the casting surface does not generate oxide scale, no Damage to the casting's dimensional accuracy. This method is particularly effective for box and frame castings, but it has poor results for discs and thick castings and needs to be further improved. VI. Formation and prevention of hot and cold cracking 1. Hot cracking and hot cracking. At the late stage of solidification, the solid phase skeleton has formed and begins to shrink. As the shrinkage is hindered, stress and deformation occur in the casting. When the stress or deformation exceeds the strength limit or deformability of the alloy at this temperature, the casting generates thermal cracks. Hot cracks are divided into external cracks and internal cracks. The crack that can be seen on the surface of the casting is called an external crack. It has a wide surface and a narrow interior, sometimes running through the entire section of the casting. Outer cracks often occur at the corners of castings, where there is a sudden change in the thickness of the section, or where the partial condensation is slow and where tensile stresses are applied during solidification. Internal cracking occurs at the site of the final solidification within the casting, often also near the shrinkage hole or at the end of the shrinkage hole. Most of the external cracks can be observed with the naked eye, and small external cracks can only be found by magnetic flaw detection or other methods; internal cracks need to be checked by X-ray or ultrasonic flaw detection. The presence of any form of hot cracks in the castings severely impairs their mechanical properties. When used, cracks will propagate to fracture the castings and cause accidents. Therefore, no hot cracking is allowed in any casting. Cracking is easy to find. If the casting alloy has good welding performance, the casting can still be used after repair welding. If the welding performance is poor, the casting is scrapped. The inner crack is hidden inside the casting and is not easy to find, so it is more harmful 2 factors affecting thermal cracking. The following are the main factors affecting thermal cracking 1 cast alloy properties, chemical composition of the hot embrittlement area. The greater the hot embrittlement area of ​​the alloy, the greater the hot cracking tendency, and the size of the hot embrittlement zone is related to the chemical composition of the alloy. The generation of thermal cracks is also closely related to the properties of the intercrystalline layer. When there is a fusible third phase on the grain boundary and spread as a liquid film, the hot cracking tendency increases significantly; if it is spherical, the hot cracking tendency decreases significantly. The coarser the crystal grains, the more obvious the direction of the columnar crystals, and the greater the tendency to generate thermal cracks. This is due to the fact that the crystal grains are coarse and the bonding force between the crystals is low, and the intergranular strength of the columnar crystals is lower than that of the equiaxed crystals. Some alloy steel castings tend to have coarse columnar crystal structures and are prone to hot cracking. The greater the shrinkage of the alloy, the more likely it is that hot cracking will occur. Gray cast iron undergoes graphitized expansion during the solidification process, so the gray cast iron is not prone to thermal cracking, whereas the malleable cast iron and cast steel parts have a high tendency to hot cracking. 2 The impact of casting. The size of the mold resistance is mainly determined by the concession of the mold, the concession of the wet type is better than that of the dry type, and the wet cracking tendencies of the castings produced by the wet type are small. All organic agent sands have a good concession, so it can reduce the tendency of castings to produce hot cracks. The tendency of hot cracking of castings is also related to the moment of mold retreat. For example, clay sand heated to above 1250 °C has a good concession. If the compressive strength of the sand caused by the maximum compressive strength at the moment, just coincides with the moment when the casting solidification is about to end, and the possibility of hot cracking is the greatest. Therefore, the use of clay sand to make cores for thin-walled parts should pay attention to improving the concession of cores. The box box and the core of the sand box are too close to the casting, it will increase the shrinkage resistance of the casting, and the casting is likely to produce hot cracks. 3 Casting conditions. The temperature near the riser is high, the cooling rate is slow, and it is easy to produce a concentrated deformation, so it is easy to form a thermal crack. Therefore, when the principle of sequential solidification is adopted to prevent the shrinkage of the casting, the characteristics of the alloy and the possible defects (A cracks comprehensive consideration. The arrangement of the pouring riser may also cause the mechanical obstruction of the shrinkage of the casting, resulting in castings Cracks in castings can also hinder casting shrinkage and cause thermal cracking There is no simple rule for the effect of the pouring process on hot cracking tendency. Increasing the pouring temperature can reduce the hot cracking tendency of thin-walled castings. This is because, on the one hand, it increases the time of the heat acting on the mold material at a high temperature, causing it to lose strength and improve the concession of the mold. On the other hand, the casting shrinkage speed and the degree of concentrated deformation are reduced. However, for thick and large castings, the casting temperature is too high, which will make the castings coarse, and the intercrystalline bond strength decreases, increasing the tendency of hot cracking. Pouring speeds affect hot cracking by changing the temperature distribution of the casting. For thin-walled parts, speed up the filling speed to prevent local overheating; for thick-walled castings, it is required to reduce the casting speed as much as possible 4 casting structure effects. The structural design of castings is unreasonable, and stress concentration is easily generated at the sharp corners, and thermal cracking is easily generated in these parts. For example, if the castings intersect at right angles, if the fillets at the intersections or intersections of the two walls are too small, hot cracks are likely to occur at the intersections. If arc-shaped transitions are used, hot cracks can be eliminated. Uneven casting thickness, different cooling rates in different places, the temperature distribution is very uneven, most of the thickness of a large concentration of deformation, easy to produce hot cracks. Incorrect pouring openings, or due to structural reasons, casting shrinkage is seriously hindered, then increase the shrinkage stress of the casting, resulting in excessive concentrated deformation at the hot section, prone to thermal cracking 3 ways to prevent hot cracking. Based on the above discussion, it can be seen that all the factors that can reduce the hot cracking tendency can be formulated according to its measures to prevent the production of hot cracking of the casting. Therefore, the following aspects can be solved. 1 alloy composition and smelting process: on the premise of not affecting the use of castings, the alloy composition can be properly adjusted, or choose the hot cracking tendency of the alloy, for example, use of alloys close to eutectic composition, can also be reduced Harmful impurities in small alloys The content of S" P in the steel should be reduced as much as possible, because S is particularly sensitive to the effect of thermal cracking. Therefore, the S content in the charge should be strictly controlled, and the desulfurization and dephosphorization should be strengthened in the melting process, and the deoxidation of the alloy should also be improved. Processes to improve the effect of deoxidation, for example, the use of a comprehensive deoxidizer can reduce inclusions, can also improve the shape and distribution of inclusions in the castings, thereby improving the crack resistance; refine the primary crystal structure, incubate the alloy to refine Grains, eliminating columnar crystals, adding a small amount of hunger in steel No. 20 can refine the crystal column, eliminate the Weishi structure, and add a small amount of Ce to the alloy steel to eliminate the columnar crystals and distribute the sulfides evenly. Casting method, refinement of primary crystal, increase of mechanical strength, ultrasonic vibration can make grain refinement of steel casting, solidification of metal under the action of rotating magnetic field can also refine grain, and can eliminate hot section. 2 mold: improve the sand and sand core collapsibility. In the case of slush sand, some wood chips may be added to improve the breakability of the core; in case of wet sand type instead of dry sand type, a thin-walled hollow core or a loose material is added in the core (-crushed coke, straw rope; reduction; Possible impediments caused by the core and box file; spring sand should not be too hard; use paint to smooth the cavity surface to reduce frictional resistance between the casting and the mold 3 Casting conditions: reduce the mechanical resistance of the casting system to casting shrinkage, avoid hot cracking, and reduce the temperature difference in various parts of the casting. Specific measures are that the sprue is set in the thin part of the casting or introduced in multiple sprues to avoid excessive flow of the molten metal in each sprue, so that the temperature of each part of the casting tends to be uniform, and local deformation of the casting is prevented. The use of cold iron to eliminate the harmful effects of the hot section, placing cold iron at the intersection of the wall of the casting and the wall, speeding up the cooling at this point, eliminating hot spots and reducing the concentration of deformation; casting thin-walled parts, in order to slow down the solidification rate and reduce thermal cracking Tendency, usually requiring higher pouring temperatures and faster casting speeds, and vice versa for thick-walled parts 4 Casting structure: The irrational design of the casting structure is one of the causes of thermal cracking. When designing castings, care should be taken to ensure that the intersection of the two walls is rounded; avoid cross-sections between the two walls and stagger the intersecting walls; when unequal thickness cross-sections are to be used on castings, each part of the castings should be made as narrow as possible. Obstruction occurs. For example, the spokes of the pulleys are formed into a curved shape; crack-proof ribs are provided at places where castings are prone to thermal cracking, and since the crack-guarding ribs are thin, the solidification is rapid, and the strength is high, and the strength at the cracked portion of the casting is strengthened, and the crack is prevented. The ribs can be removed during cleaning. If they do not affect the use, they may not be removed. In short, there are many factors that affect the production of thermal cracks. They are complex and intricate. They must be analyzed according to specific conditions. The main contradictions must be identified and appropriate measures must be taken to effectively prevent and eliminate thermal cracking of castings. 2. Cold cracking 1 Cold cracking formation. Cold cracking is caused by the fact that the stress in the casting exceeds the strength limit of the alloy. Cold cracking often occurs at locations where the casting is stretched, especially where stress concentration occurs and where casting defects occur. The factors that affect cold cracking and the factors that affect casting stress are basically the same. The composition and smelting quality of the alloy have an important influence on cold cracking. For example, although elements such as C.Cr and Mn in steel can increase the strength of steel, they reduce the thermal conductivity of the steel. Therefore, when the content of these elements is high, the cold cracking tendency of steel is increased; P increases the cold brittleness of steel. When the mass fraction of P is greater than 0.1%, the impact toughness of the steel decreases sharply and the cold cracking tendency increases. When the molten steel deoxidizes insufficiently, the oxide inclusions accumulate on the grain boundary, which reduces the impact toughness and strength of the steel and promotes cold cracking. The formation of non-metallic inclusions in the casting increases, the tendency of cold cracking also increases. The microstructure and plasticity of the casting also have a great influence on the cold cracking. For example, low-carbon nickel-chromium-acid-resistant stainless steels and high-manganese steels are all austenitic steels, and they are prone to large thermal stress, but nickel-chromium-acid-resistant stainless steels are not prone to cold cracking, while high-manganese steels are prone to cold cracking. This is because low-carbon austenitic steels such as nickel-chromium-resistant stainless steels have low yield limits and high plasticity. Casting stresses often quickly exceed the yield limit, causing plastic deformation of the castings, while high manganese steels have high carbon content. When the casting cools, brittle carbides precipitate on the austenite grain boundaries, which seriously reduces the plasticity and easily forms a cold crack. Cold cracking of steel castings can be used after welding. Some alloys have poor weldability and die casting, and these alloy castings have to be scrapped when they appear cracked. 7 Cold crack prevention measures. The reason for the cold cracking and deformation of the casting is that the cooling rate of the various parts of the casting during cooling is inconsistent. Therefore, the above-mentioned methods for preventing the casting from generating casting stress can be used to prevent deformation and cold cracking of the casting. In addition, from the process to prevent deformation can also take the following measures: 1 to increase the mold stiffness, increase the weight of the weight can reduce the deflection of the casting. 2 control casting boxing time. Boxing too early, the casting temperature is high, cooling in the air will increase the temperature difference between inside and outside, resulting in deformation and cracking. Prolonged boxing time can avoid cracking and reduce deformation, but for some complex castings, cold cracking can be caused by poor mold or core collapsibility. For the important castings that are easily deformed, the box can be beaten early and put into the furnace slowly. 3 Take anti-deformation measures. The pre-deformation amount equal to the amount of residual deformation of the casting is made on the pattern, and the casting is produced according to the pattern. After the cooling and deformation of the casting, the size and shape just meet the requirements. 4 Set anti-distortion bar. Anti-deformation ribs can withstand part of the stress and prevent deformation. After the heat treatment of the casting, the anti-deformation tendons are removed. 5 Change the structure of castings, use curved spokes instead of straight spokes to reduce resistance and prevent deformation Sanding Belt
Abrasive
belt is a belt-shaped tool that can be ground and polished, which is made by
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Abrasive
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