Preface: Different processes can produce different forms of defects, but the same form of defects can also come from different processes. Because the causes of forging defects are often related to various factors such as the raw material production process and the post-forging heat treatment process, when analyzing the causes of forging defects, do not do it in isolation.
1. Lack of flame produced by raw materials
Capillary cracks (splitting):
Capillary cracks with a depth of about 0.5-1.5mm are located on the surface of the steel. When rolling the steel, the subcutaneous bubbles of the steel ingot are elongated and ruptured. If it is not removed before forging, it may cause cracks in the forgings.
A layer of a thin film that is easily peeled off locally on the surface of the steel, its thickness can reach about 1.5imn. It cannot be welded during forging, and it appears on the surface of the forging in the form of scars. The reason is that when pouring, the molten steel is splashed and condensed on the surface of the ingot, and it is pressed into a film and attached to the surface of the rolled material during rolling, which is scarring. After forging, it is cleaned by pickling, the scars are peeled off, and pits appear on the surface of the forgings.
On the end face of the rolled product, creases in opposite directions appear at both ends of the diameter. The crease is at an angle to the tangent of the circle, and there are oxidation inclusions in the crease and decarburization around it. It is due to the incorrect sizing of the groove on the rolling mill, or the burrs generated by the wear surface of the groove are rolled into folds during rolling. If they are not removed before forging, the surface of the forging will remain.
On the longitudinal section of the rolled material, elongated, or broken, non-metallic inclusions are distributed intermittently along the longitudinal direction. The former such as sulfide, the latter such as oxide, brittle silicate. It is mainly caused by the chemical reaction between the metal, the furnace gas, and the container during smelting; in addition, it is caused by refractory materials, sand, etc. falling into the molten steel during smelting and pouring.
Often found in the heart of steel. On the broken button or section of the steel, there are some appearances similar to the broken slate and bark. This kind of defect occurs more in alloy steel, especially in chrome-nickel mace and chrome-nickel-tungsten steel, and also in carbon steel. There are non-metallic inclusions, dendrite segregation, pores, porosity, and other defects in the steel, which are elongated in the longitudinal direction during the forging process, making the fracture of the steel lamellar. The layered fracture seriously reduces the transverse mechanical properties of the steel, and it is easy to fracture along the layer during forging.
Compositional segregation zone:
In some alloy structural steels, such as 40CrNiMoA, 38CrMoAlA, and other forgings at the longitudinal low magnification, strip or strip defects different from the streamline appear along the streamline direction, and the microhardness of the defect area is obviously different from that of the normal area. The composition segregation zone is mainly caused by the segregation of alloying elements in the raw material production process. The slight composition segregation zone has little effect on the mechanical properties, and the severe segregation will obviously reduce the plasticity and toughness of the forgings.
Bright bars or bands:
On the surface of the forging or the processed surface of the forging, bright bars of varying lengths appear. Most of the bright bars are distributed along the longitudinal direction of the forging. This defect mainly occurs in titanium alloy and superalloy forgings. due to the segregation of alloying elements. The bright bars in titanium alloy forgings are mostly low aluminum and low vanadium segregation areas, and the bright bars on high-temperature alloy forgings are mostly chrome, cobalt, and other elements. The presence of high bright bars reduces the plasticity and toughness of the material.
Carbide segregation grade unqualified:
It often occurs in high-speed steel, high-chromium cold-deformed dies steel and other alloy steels with high carbon content. It is characterized by the accumulation of more broken carbides in local areas, which makes the carbide segregation exceed the allowable standard. It is caused by the fact that the ledeburite eutectic carbides in the steel are not sufficiently broken and evenly separated during billeting and rolling. Severe carbide segregation can easily cause overheating, overburning, or cracking of forgings.
There are round or oval silver-white spots on the longitudinal fracture of the billet and small cracks on the transverse fracture. The white spots vary in size and are 1-20mm in length or longer. White spots are common in alloy structural steels and are also found in ordinary carbon steels. It is caused by a large amount of hydrogen in the steel and the large structural stress during phase transformation. White spots are prone to occur when large billets are cooled quickly after forging and rolling. White spots are cracks hidden inside, reducing the plasticity and strength of the steel. The white point is the stress concentration point, which is easy to causes fatigue cracks under the action of alternating load.
Shrinkage crater residue:
During the low magnification inspection of the forgings, there are unplanned wrinkle-like gaps, which resemble cracks and appear dark brown or gray; there are a large number of non-metallic inclusions near the remnants of the shrinkage cavities at high magnifications, which are brittle and easy to peel off. It is caused by the concentrated shrinkage cavity generated in the riser part of the ingot that is not removed cleanly and remains inside the billet during billet opening and rolling.
Aluminum alloy extruded rod Murakami’s coarse-grained ring:
The aluminum alloy extruded bar was supplied after heat treatment. Coarse grains appear in the outer ring of its cross-section, which is called a coarse grain ring. The thickness of the coarse-grained ring. It gradually increases from the beginning of the bar to the end of the extrusion. The main reason is that the aluminum alloy contains elements such as Mn, Cr, and the friction between the metal and the extrusion cylinder wall during extrusion, which causes the surface layer of the bar to deform violently. Billets with coarse-grained rings are prone to cracking during forging, and if left on the forging will reduce the performance of the part.
Aluminum alloy oxide film:
On the low magnification of the forging, the oxide film is distributed along the metal streamline in the shape of black short lines. On the fracture perpendicular to the longitudinal direction of the oxide film, the oxide film is similar to tearing and delamination; on the fracture parallel to the longitudinal direction of the oxide film, the oxide film is in the form of flakes or small and dense dots. The oxide film in the die forging is easy to see on the web or near the parting surface. The oxide inclusions that are not removed in the molten aluminum during smelting are involved in the molten metal from the surface during the casting process, and are elongated and thinned to become an oxide film during deformation such as extrusion and forging. The oxide film has little influence on the longitudinal mechanical properties of forgings and has a greater influence on the transverse, especially short transverse mechanical properties. According to the forging category and the oxide film standard, the unqualified ones will be scrapped.
2. Defects caused by blanking
The end face of the blank is inclined to the sleeve line of the blank, which exceeds the permitted value. It is caused by the bar material not being compressed during shearing. The obliquely cut billet is easy to bend when upsetting, difficult to locate during die forging, and easy to form folds.
Billet ends bent and burred:
When cutting the material, part of the metal is brought between the scissor’s gap, forming sharp burrs, and the end of the blank is bent and deformed. Because the gap between the scissors is too large, or the cutting edge is not sharp, the blank with burrs is easily folded during forging.
Concave or convex end of billet:
The metal in the central part of the end face of the billet is pulled off, so the end face has bulges or depressions. The reason is that the gap between the blades is too small, and the metal in the center of the billet is not cut but pulled, so that part of the metal is pulled away. Such billets are prone to folds and cracks during forging.
It mainly occurs when shearing large-section billets, but also when shearing alloy steel or high carbon steel in a cold state. Caused by too high material hardness and too much unit pressure on the blade during shearing. Forging will further expand the end crack.
When the lathe is blanking, the convex core is often left on the end face of the billet. If it is not removed, it may cause cracks around the convex core during forging. Due to the small cross-section of the convex core, the cooling is fast; the end face area is large and the cooling is slow, which leads to the formation of cracks around the convex core.
Gas cut cracks:
Generally located at the end face or end of the billet, the crack is rough. It is caused by insufficient preheating before gas cutting, resulting in the formation of large thermal stress.
Grinding wheel cutting cracks:
When the superalloy is cut with a grinding wheel in the cold state, it often causes cracks in the end face. Such cracks are sometimes not visible to the naked eye until they have been heated. The thermal conductivity of the superalloy is poor, and the large amount of heat generated by the grinding wheel cutting cannot be quickly conducted, resulting in large thermal stress on the cutting section, and even tiny cracks. When heated, a large thermal stress is generated again, so that the tiny cracks expand into cracks visible to the naked eye.
3. Defects due to heating
The phenomenon of coarse grains is caused by excessive heating temperature. The characteristic of carbon steel overheating is the appearance of Widmanstatt structure; tool steel is characterized by primary carbides, and some alloy structural steels such as !8Cr2Ni4WA, 20Cr2Ni4A. After overheating, in addition to coarse grains, there is also MuS precipitation along the boundary, for the latter It is not easy to eliminate by the usual heat treatment method. The reason is that the heating temperature is too high or the time is too long, or the overheating caused by the thermal effect of deformation is not considered, which will reduce the mechanical properties of the steel forgings, especially the plasticity and the impact stiffness. In general, overheating of steel forgings can be eliminated by annealing or normalizing.
The billet of aluminum alloy and barrel alloy has a rough surface similar to the skin when upsetting, and even cracks in severe cases. Due to the overheating of the billet, the grains are coarse. The aluminum alloy with a coarse grain ring is damaged, and this phenomenon also occurs when it is upsetting.
Wilcoxon alpha or beta brittleness:
After the (α+β) titanium alloy bad material is overheated, its microstructure is characterized by the precipitation of a phase along the coarse original β grain boundaries and grains in the form of thick strips. The thick strip-like α phase precipitated in the crystal. Each is arranged in a certain direction, that is, the so-called Widmandelstein a phase is formed. Since the heating temperature exceeds the β transformation temperature of the (α+β) titanium alloy, the tensile plasticity index of the titanium alloy forgings with Widmanners α phase is significantly reduced, which is the so-called β knee heat treatment can not eliminate the β bile.
Overburning of steel forgings:
The grains in the overfired part are particularly coarse, the oxidation is particularly serious, and the surface between the cracks is light gray-blue. Immediately after carbon steel and alloy structures are overfired, grain boundaries are oxidized and melted. Immediately after the tool is fired, the grain boundary melts and fishbone-like ledeburite appears. Caused by excessive furnace temperature or excessive residence time of the billet in the high-temperature area. Oxygen in the furnace penetrates between the grains along the grain boundary, and oxidation or the formation of a fusible oxide eutectic destroys the connection between the grains.
Overburning of aluminum forgings:
The surface is black or dark black, and sometimes there are chicken skin-like bubbles on the surface. After the aluminum alloy billet is over-fired, grain boundary melting, triangular grain boundary, or the remelting ball will appear in its microstructure. As long as one of these phenomena exists, it is overburned. When the heating temperature of the aluminum alloy billet is too high, the strengthening phase melts. After cooling down, special shapes such as grain boundary thickening, triangular grain boundary, or remelting ball can be seen in the microstructure.
Generally, it is cracked along the cross-section of the bad material, and the cracks spread from the center to the surrounding. Such cracks are mostly caused by the heating of superalloy and high alloy steel ingots and steel. Due to the large size of the blank, poor thermal conductivity, and too fast heating rate, the temperature difference between the center and the surface of the blank is large, and the resulting thermal stress exceeds the strength of the blank.
Cracks appear on the surface of the steel forging. Under high magnification inspection, copper is distributed along the grain boundaries. This defect is prone to occur when heating sodium material in a furnace that has heated copper material. The copper oxide scraps remaining in the furnace are reduced to free copper by iron when heated. The molten copper atoms diffuse along the austenite grain boundaries at high temperatures, weakening the connection between the grains.
Some shiny facets like naphthalene crystals appear on the fracture of the steel forging. Such defects are easily seen in alloy structural steels and high-speed steels. Due to the high heating temperature or the high final forging temperature, the deformation is not large enough. The essence of naphthalene fracture is overheating. Therefore, the plasticity and toughness of steel forgings will be reduced.
It is a defect that occurs after severe overheating of alloy structural steel. The stone-like fracture is observed in the quenched and tempered state and is characterized by the appearance of the fibrous cone-shaped fracture matrix, some gray-white facets with a metallic luster like cement. It cannot be eliminated by heat treatment, so it is an impermissible lack of flame. When the heating temperature is too high, a large amount of MnS is dissolved. When the MnS melted in the steel is cooled, it precipitates on the coarse austenitic grain boundaries with extremely fine points, which weakens the bonding force of the grain boundaries. After the toughness is enhanced, the steel will break along the austenite grain boundaries when it is broken, and some dull gray-white superheated small planes will be formed on the fracture. Forgings with stony fractures should be scrapped.
Low magnification coarse crystal:
Low-magnification coarse grains are another reflection of alloy structural steel forgings after overheating. It is characterized by the appearance of polygonal grains visible to the naked eye on the acid-infiltrated low-magnification test pieces of the forgings. In severe cases, these polygonal grains look like snowflakes. shape. The superheated austenite grain boundaries are relatively stable and can be eliminated by the usual heat treatment. The recrystallization is only carried out in the coarse austenite grains, and several new small grains are generated in a canonize grain. Because the grain boundaries of the small grains are thin or the orientations are not very different, the original Austenstein coarse grains, that is, low-magnification coarse grains, are still seen at low magnifications.
The phosphorus content of the surface layer of the steel is significantly lower than that of the inner layer, and the hardness value is lower than required. The amount of cementite in the superficial layer decreases on the high magnification structure. When heating high carbon steel in an oxidizing atmosphere, it is easiest to decarburize steel with a large amount of silicon. The carbon in the lower layer of steel is oxidized at high temperatures. The depth of the decarburization layer is 0.01-0.6mm, depending on the composition of the steel, the composition of the furnace gas, the temperature, and the length of the heating time. Decarburization reduces the strength and fatigue properties of the parts and weakens the wear resistance.
The carbon content of the surface or part of the surface of the forging heated by the oil furnace is obviously increased, and the hardness is increased. Some thicknesses reach 1.5-1.6mm. When the billet is heated in the oil furnace, the spray intersection area of the two nozzles cannot be fully burned, or the nozzles are poorly atomized and spray oil droplets, which will increase the carbon on the surface of the forging. Carbonized forgings are easy to hit when cutting.
Heart cracking due to lack of heat penetration:
The core crack often occurs at the head of the bad material, and the crack depth is related to heating and forging, and sometimes the crack runs through the entire billet in the longitudinal direction. It is caused by the low plasticity of the heart due to insufficient heat preservation time. High-temperature alloys have poor thermal conductivity. If the cross-sectional size of the billet is large, attention should be paid to giving sufficient holding time.
4. Defects caused by forging
Longitudinal fissure on the surface of the bulging belly:
During free stewing, irregular longitudinal cracks are generated on the bulging surface of the blank due to tensile stress. Due to the friction between the contact surface of the blank and the anvil, uneven deformation occurs and the belly is bulging.
Shiyu crack (longitudinal internal crack):
Such cracks often occur in the drawing process of low-plastic high-speed steel and high-chromium steel. Shiyu cracks are distributed diagonally along the cross-section of the forging, and the depth of expansion in the fir direction is not -, and in severe cases, it can penetrate the entire length of the blank. In the process of drawing and lengthening by repeatedly turning 90°. If the feeding time is too large, the maximum alternating shear will be generated on the diagonal of the blank cross-section. When the shear stress exceeds the allowable value of the material, cracks will be generated along the diagonal direction.
Longitudinal strip cracks:
It mainly occurs when the material is drawn from a circle and pressed into a square, or when the blank is pulled back and rounded after drawing. In the cross-section, cracks appear in the middle part as strips, and the depth of crack propagation in the longitudinal direction varies, which is related to the forging operation. When the blank is pressed or rounded with a flat anvil, there is tensile stress in the horizontal direction of the blank. This tensile stress increases along the surface of the blank to the center and reaches a maximum value at the center. When it exceeds the strength of the material, it will form a longitudinal inner crack.
The four strands of the billet were sporadically appeared after being drawn. Corner cracks mostly occur in the drawing process of high-speed steel and high-chromium steel billets. After the bad material is drawn into a square, the temperature of the edge part drops, and the difference between the mechanical properties of the edge part and the body part increases. The edges and corners are cracked due to the tensile stress generated by the metal flow.
Internal transverse cracks:
Strip cracks appear in the height direction on the longitudinal section of the blank. When the high-speed steel and high-chromium steel billets are elongated, if the feed ratio is less than 0.5, such cracks are prone to occur. When the feed ratio during drawing is less than 0.5, tensile stress will be generated in the axial direction of the broken material. When it exceeds the tensile strength of the material at a weak point in the billet, transverse cracks are induced there.
Cracks appear radially at the edge of the punched hole. It occurs more when chrome steel is punched. It is caused by the fact that the punching core is not preheated, the preheating is insufficient, or the deformation of one punching is too large.
Duplex forging cracks:
Cracking along the interface of alpha and gamma phases or the lower strength alpha phase during die forging of austenitic, ferritic, or semi-martensitic stainless steel billets. It is caused by too much excess α phase (more than 12% of α phase in austenitic and ferritic stainless steel, and more than 10% of α phase in semi-martensitic steel) and high heating temperature.
Parting surface crack:
Die forgings have cracks along the plastic parting surface, which are often revealed after trimming. There are many non-metallic inclusions in the raw material, and there are residual or loose shrinkage cavities, which are caused by extrusion into the parting surface during die forging.
Cracks that appear parallel to the parting plane at the root of a die forging rib or boss with L-shaped, U-shaped, and H-shaped cross-sections. Due to too much bad material, after the ribs are full, there is a lot of excess metal on the web. When the die forging is continued, the excess metal on the web flows violently to the flash, resulting in large shear stress at the root of the ribs. When it exceeds the suppression strength of the metal, a piercing rib is formed.
Flow-like fine-grained regions appear on the forgings at low magnifications in the lateral direction. It is mostly found in titanium alloys and high-temperature alloy forgings forged at low temperatures. Due to the high sensitivity of titanium alloys and superalloys to chilling, in the process of die forging, the difficult deformation area near the contact surface gradually expands, resulting in strong shear deformation at the boundary of the difficult deformation area. As a result, strong directionality is formed, which degrades the performance of the forging.
A structure in which ferrite or other matrix phases are distributed in a band shape in a forging. Mostly appear in hypereutectoid steel, austenitic-ferritic stainless steel, and semi-martensitic steel. Because it reduces the transverse plasticity index of the material due to the forging deformation in the coexistence of the two phases, it is easy to crack along the ferrite band or the transformation boundary of the two phases.
Improper distribution of forging streamlines:
Streamline disturbances such as streamline disconnection, backflow, and eddy current convection appear at low magnifications of forgings. Due to improper mold design, unreasonable billet size and shape, and poor selection of forging methods.
Folding on the outer gauge is similar to cracking, and the folded outer streamline is bent on the low magnification test piece. If it is a crack, the streamline is cut. On the high magnification test piece, the bottom of the crack is sharp and thin, the bottom end of the fold is round and blunt, and the oxidation on both sides is serious. Folds are formed when the oxidized surface metals come together during the forging process. Folding on free forgings is mainly caused by too small amount of feeding time, too large pressing amount or too small radius of anvil fillet when the dry drawing is long; folding on-die forgings is mainly caused by convection of metal during die forging formation or backflow.
The grains in some parts of the forging are particularly coarse, while other parts are small, resulting in uneven grains. Heat-resistant steels and superalloys are particularly sensitive to uneven grains. The initial forging temperature is too high and the deformation is insufficient so that the deformation degree of the local area falls into the critical deformation; or the final forging temperature is too low, so that the superalloy billet is locally work-hardened, and the grains of this part grows seriously during quenching and heating. Grain inhomogeneity can cause long-lasting performance and decreased fatigue performance.
Casting tissue residue:
If the cast structure remains, the elongation and fatigue strength of the forgings are often unqualified. On the low magnification specimen, the streamlines of the residual cast structure are not obvious. Even dendrites can be seen. It mainly occurs in forgings that use ingots as billets. Due to insufficient forging ratio or improper forging method, this defect reduces the performance of forgings, especially the impact toughness and fatigue properties.
Insufficient local filling:
The phenomenon of insufficient filling at the top or edge of the convex part of the forging mainly occurs in the ribs, shoulder corners, etc. of the die forging, which makes the contour of the forging unclear. Insufficient heating of the blank, poor metal fluidity, unreasonable design of pre-forging dies and blank-making dies, and small tonnage of equipment may all cause this defect.
Insufficient die forging:
All dimensions of the forging in the direction perpendicular to the parting surface area increased, that is, exceeding the dimensions specified on the drawing. This defect is most likely to appear on hammer die forgings. The resistance of the flash bridge is too large, the tonnage of the equipment is insufficient, the volume or size of the blank is too large, the forging temperature is too low, and the wear of the die bore is too large.
The upper half of the die forging is dislocated relative to the lower half along the parting surface. The forging die is not installed correctly or the gap between the hammerhead and the guide rail is too large, or there is no lock or guidepost with dislocation balance on the forging die.
Fish scale scars on the surface:
The local surface of the die forging is very rough, and there are scaly scars. Such surface defects are most likely to occur when swaging austenitic and martensitic stainless steels. Due to improper selection of lubricants, poor quality of lubricants, or uneven application of lubricants, local mold sticking is caused.
5. Defects due to trimming
Cracks are generated at the split face during edge trimming. Due to the low plasticity of the material, cracks are caused during edge trimming. When the trimming temperature of magnesium alloy die forgings are too low, and the trimming temperature of copper alloy die forgings is too high, such cracks will occur.
After trimming, burrs larger than 0.5mm will be left around the parting surface of the die forging. If correction is required after trimming, the residual burrs will be pressed into the forging body to form a fold. Excessive clearance of the trimming die, excessive wear of the cutting edge, or inaccurate installation and adjustment of the trimming die can cause residual burrs.
There is an indentation or pressure injury on the local contact surface of the die forging and the punch. Because the shape of the contact surface between the convex film and the mold and the mold satin part does not match. Or the pressing surface is too small.
Bend or twist deformation:
Bending or twisting deformation of cross forgings during edge trimming. It is easy to occur on slender, flat, and complex-shaped die forgings. Caused by the contact surface of the trimmed punch forging being too small, or uneven fusion occurs.
6. Defects caused by improper cooling after forging
The cracks are smooth and slender, sometimes in the form of mesh cracks. High magnification observation: martensitic structure appears near the crack, and there is no trace of plastic deformation. It mostly occurs on martensitic steel forgings. Due to the rapid cooling after forging, large thermal stress and structural stress are generated. Slow cooling in a sandpit or slag around 200°C can prevent such cracks.
Large, thin-walled, thin-ribbed frame members, warping deformation during post-forging cooling. It is caused by the interaction of residual stress generated in forging and stress caused by uneven cooling. Annealing immediately after forging can prevent such defects.
475℃ brittle crack:
Surface cracks appear when ferritic stainless steel is cooled too slowly after forging, and the residence time is too long in the temperature range of 400-520 °C. Due to the long residence time at 400-520 °C, a special substance is precipitated and brittleness is caused. Fast cooling at 400-520°C can prevent cracks.
Carbides are precipitated in the form of a network along the grain boundaries, which reduces the plasticity and toughness of the forgings. This defect is often seen in steel forgings with high carbon content. Due to the slow cooling after setting, the carbides can be folded along the grain boundaries, which makes the forgings easy to crack during quenching and deteriorates the performance of the parts.
7. Defects caused by heat treatment after forging
Hardness too high:
When the hardness of the forging is checked after heat treatment, the measured hardness is higher than that required by the technical conditions. It is caused by too fast cooling after normalizing, or the unqualified chemical composition of the steel.
The hardness of the forgings is lower than that required by the technical conditions. It is caused by the low quenching temperature, high tempering temperature, or severe decarburization of the surface caused by repeated heating.
Uneven hardness (with soft spots):
The hardness of different parts on the same forging varies greatly, and the hardness of local parts is low. It is caused by too much dry loading, too short holding time, or severe decarburization locally.
During heat treatment, especially during quenching, forgings are deformed. Due to unreasonable heat treatment process and improper cooling method.
Cracks at stress concentrations such as sharp corners of forgings. Unlike forging cracks, there is no oxidation and decarburization on the inner sidewall surface of quenching cracks. It is caused by defects such as no preliminary heat treatment, too high quenching temperature, too fast cooling rate, and inclusions inside the forging.
The fracture is dark gray or nearly black. In the microstructure, there is cotton wool-like graphite distributed on the non-uniform spherical pearlite. Most of them appear in high carbon tool steel forgings. Because the annealing time after forging is too long, or after multiple annealing treatments, the graphitization process of the steel and the precipitation of graphitic carbon are promoted.
8. Defects of forgings during cleaning
There are pits or pits on the surface of the forgings, which are in the shape of pine holes. Due to the deterioration of the pickling solution, the pickling time is too long, or there is acid residue on the forging.
It mostly occurs on martensitic stainless steel forgings, which are characterized by fine network cracks on the surface of the forgings, and the cracks extend along the grain boundaries in the microstructure. Since the residual stress on the forgings after forging was not eliminated in time, stress corrosion occurred during the pickling process, resulting in the formation of cracks.
Local overheating cracks:
Cracks appear when the surface is cleaned with a grinding wheel. It is prone to occur on ferritic stainless steel forgings. It is caused by local overheating caused by grinding with a grinding wheel. An air shovel can be used instead to clean up its surface defects.
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