中文 青岛宏腾达机械制造公司 18765298988
| Defect name | Main characteristics | Causes and consequences | |
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Capillary crack(hair crack)
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Capillary cracks of about 0.5-1.5 mm deep on steel surface | When rolling steel, the subcutaneous bubbles of ingots are formed by rolling and breaking up. If it is not removed before forging, it may cause cracks in forgings. | |
| To scar | A layer of easily exfoliated film exists on the surface of steel, and its thickness can reach about 1.5mm. It can not be welded during forging and appears on the surface of forgings in the form of scars. | When pouring, the ingot surface is solidified due to the splash of molten steel. When rolling, it is pressed into thin wax and adhered to the surface of the rolled material, which is called scarring. After forging, it is cleaned by acid pickling, scaling and peeling, and pits appear on the surface of forgings. | |
| Folding (folding) | There are folds in opposite directions at both ends of the diameter on the end face of the rolled sheet. The fold is at an angle to the arc tangent. There are oxidized inclusions in the fold and decarbonization around it. | Incorrect sizing of grooves on rolls or burrs on the worn surface of grooves are rolled and folded during rolling. If the burrs are not removed before forging, the residual surface of forgings will be removed. | |
| Non-metallic inclusions | Non-metallic inclusions which have been elongated or broken but distributed intermittently along the longitudinal direction appear on the longitudinal section of the rolled steel. The former is sulfide, the latter is oxide and brittle silicate. | It is mainly due to the chemical reaction between metal and furnace gas and container during smelting and pouring. In addition, it is caused by refractories and sand falling into molten steel during smelting and pouring. | |
| Stratified fracture | It often appears in the axle of steel. On the fracture or cross section of steel, some morphologies similar to those of broken slab and bark appear. This defect is found in alloy steels, especially chromium-nickel steel, chromium-nickel-tungsten steel and carbon steel. | Non-metallic inclusions, dendrite segregation, porosity and other defects exist in steel. During forging and rolling, the steel fracture is lengthened along the longitudinal direction, which makes the steel fracture appear lamellar and lamellar, and the transverse mechanical properties of low-grade steel are serious. It is easy to fracture along the lamellar during forging. | |
| Compositional segregation zone | In some alloy structural steels, such as 40GrNiMoA and 38GrMoAlA forgings, there are strip or strip defects along the streamline, which are different from those along the streamline. The microhardness of defect zone is obviously different from that of normal zone. |
Compositional segregation band is mainly caused by segregation of alloy elements in raw material production process.
The slight segregation zone has little effect on the mechanical properties, while the low segregation zone significantly reduces the plasticity and toughness of forgings.
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| Bright stripes or ribbons | Bright strips of varying lengths appear on the surface of forgings or on the surface processed by forgings. Bright strips are mostly distributed along the longitudinal direction of forgings. This defect mainly occurs in titanium alloy and high temperature alloy forgings. | It is caused by segregation of alloying elements. The bright strip in titanium alloy forgings belongs to low aluminium and low vanadium segregation zone, while the bright strip in Superalloy Forgings belongs to high elements such as nickel, chromium and cobalt. The plasticity and toughness of materials decrease with the existence of bright stripes | |
| Unqualified carbide segregation grade | Often occurs in alloy steels with high carbon content, such as high-speed steel and high chromium cold-formed die steel. The nickel point is that there are more carbide aggregates in the local area, which makes carbide segregation exceed the allowable standard. | Ledeburite eutectic carbides in steel are not fully broken and evenly separated during billet opening and rolling. Serious carbide segregation can easily lead to overheating, overburning or cracking of forgings. | |
| White dot | Round or silver-white spots appear on the longitudinal fracture surface of the billet, and fine cracks appear on the transverse fracture surface. White dots vary in size, ranging in length from 1 to 20 mm or longer. White spots are common in alloy structural steels and common carbon steels. |
Due to the high hydrogen content in steel and the high stress in structure during phase transformation. It is easy to produce white spots when large billet is cooled quickly after forging and rolling.
White spot is a hidden crack in the steel, which reduces the plasticity and strength of the steel. White spot is the stress concentration point, and it is easy to induce fatigue cracks under alternating loads.
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| Shrinkage porosity | Irregular wrinkled cracks appear in forgings during low magnification inspection, which are like cracks and dark brown or gray-white. There are a lot of non-metallic inclusions near shrinkage hole residues in high magnification inspection, which are brittle and easy to peel off. | Because the concentrated shrinkage hole produced by the riser part of the ingot has not been removed completely, the remaining shrinkage hole in the ingot during billet opening and rolling is produced inside the ingot. | |
| Coarse-grained Ring on Aluminum Alloy Extruded Bar | After heat treatment, coarse grains appear in the outer ring of cross section of extruded aluminium alloy bar, which is called coarse grains ring. The thickness of coarse-grained rings increases gradually from the beginning of extrusion to the end of the rod. | It is mainly due to Mn, Cr and other elements in aluminium alloy and friction between metal and extrusion cylinder wall during extrusion, resulting in severe deformation of bar surface layer. Blanks with coarse-grained rings are prone to crack during forging. If left in the forging, the performance of the parts will be reduced. | |
| Aluminum alloy oxide film | At low magnification, the oxide film distributes along the metal streamline, showing a short black line. On the fracture perpendicular to the longitudinal direction of the oxide film, the oxide film is similar to the tear layer; on the fracture parallel to the longitudinal direction of the oxide film, the oxide film is in the shape of sheet or small dense point-shaped forgings, which is easy to be seen on the web or near the parting surface. | There is no oxide inclusion removed in the molten aluminium during smelting. During pouring, the surface is involved in the molten metal. During extrusion, forging and other deformation, the oxide film is elongated and thinned. Oxygen film has little effect on the longitudinal mechanical properties of forgings, but has great influence on the transverse mechanical properties, which is characterized by short transverse mechanical properties before age. Comparing with the forgings category and the standard of oxide film, unqualified forgings are discarded. | |
| Cutting oblique | The end face of billet is inclined to the axis of billet, which exceeds the allowable value. | When the bar is not compacted during shearing, the oblique billet is easy to bend when upsetting, and it is difficult to locate when forging, so it is easy to fold. |
| The end of billet is bent and burred | When cutting, part of the metal is brought into the gap between the scissors to form sharp burrs, and the end of the blank has bending deformation. | Because the gap between scissors is too large or the edge is not sharp, the burred blank is easy to fold during forging. |
| Sag or bulge at the end of billet | The central part of the billet end face is pulled apart, so there are bulges or depressions on the end face. | The clearance between blades is too small, the central part of the blank metal is not cut, but pulled, so that part of the metal is pulled off. It is easy to fold and crack when forging such billet. |
| End crack | It occurs mainly when shearing large section billets, and also when shearing alloy steel or high carbon steel in cold state. | The end crack will be further enlarged by forging due to the high hardness of the material and the high unit pressure on the blade during shearing. |
| Convex core cracking | The convex core is often left on the end face of the blank when the lathe is cutting. If it is not removed, it may cause cracking around the convex core during forging. | Cracks are formed around the convex core due to the small convex section, fast cooling, large end area and slow cooling. |
