The main characteristics and causes of their generation of forging defects
Forging changes the plastic shape of metal through external force, and the billet needs to be heated to the plastic deformation temperature range (usually 800-1250 ℃ for carbon steel) first. In the red hot state, metal grains are refined and reorganized, and plastic deformation is achieved through two methods: impact hammering or compression molding. Free forging is formed by repeated hammering between the upper and lower anvils, while die forging constrains metal flow through the mold cavity.
High temperature treatment can eliminate casting defects in metals, and the larger the forging ratio (cross-sectional area ratio before and after deformation), the higher the material density and strength. Contemporary industrial forging commonly uses a combination of press machines and intelligent molds, with a single forging forming accuracy of up to ± 0.1 millimeters.
The advantages of forging technology are:
Organizational optimization: eliminate as cast coarse grains and form a coherent fiber flow direction
Performance improvement: The tensile strength is increased by about 30% compared to castings, and the wear resistance is enhanced by 2-3 times
Defect control: Close the internal gaps of the material to prevent casting defects such as sand holes and air holes
Product lifespan: The service life of typical forgings is extended by 50% -200% compared to castings
Cost optimization: Forging can not only reduce the use of materials, but also reduce machining allowances,
Although the process has the above advantages, the production process needs to be strictly controlled, otherwise internal defects will seriously affect safety and usage requirements. Here are some common defects listed below for engineers to encounter specific analysis and queries
| Defects arising from the raw materials | |||
| Defect Name | major characteristic | The causes and consequences | |
| Capillary crack (fracture) | Capillary cracks located on the surface of the steel, with a depth of approximately 0.5 - 1.5 mm | During the rolling process of steel, the subsurface gas bubbles in the steel ingot are elongated and ruptured. If not removed before forging, they may cause cracks in the forged piece. | |
| rolling skin | There is a thin film that is prone to flaking on a certain area of the steel surface, with a thickness of approximately 1.5mm. It cannot be welded during forging and appears as a scar on the surface of the forged piece. | During the pouring process, due to the splashing of the molten steel, it cools and adheres to the surface of the steel ingot. During the rolling process, it is pressed into a thin wax and adheres to the surface of the rolled material, which is known as a scar. After forging, after acid cleaning for cleaning, the scar peels off, and pits appear on the surface of the forging. | |
| Crease (fold) | On the end face of the rolled material, there are opposite-directional creases at both ends of the diameter. The creases form an angle with the tangent of the arc, and there are oxide inclusions within the creases, along with decarburization around them. | The shape grooves on the rolls are not properly sized, or the burrs generated on the worn surfaces of the grooves are folded during the rolling process. If these burrs are not removed before forging, they will remain on the surface of the forged piece. | |
| Non-metallic inclusions | On the cross-section of the rolled material, there are elongated or fragmented non-metallic inclusions that are distributed discontinuously along the longitudinal direction. The former are sulfides, and the latter are oxides, brittle silicates. | This is mainly due to the chemical reactions that occur between the metal, the furnace gas, and the container during the smelting process; in addition, during the pouring process of smelting, due to the falling of refractory materials, sand, etc. into the molten steel, it causes this situation. | |
| foliated fracture | It often occurs in the central part of the steel. At the fracture or cross-section of the steel, there are some morphologies similar to those of broken slabs or tree bark. This defect is more common in alloy steel, especially in chromium-nickel steel and chromium-nickel-tungsten steel, and has also been found in carbon steel. | There are non-metallic inclusions in the steel, such as dendritic segregation, pores, and porosity. These defects are elongated along the longitudinal direction during the forging and rolling process, causing the steel fracture surface to be in a layered form. The layered fracture significantly affects the lateral mechanical properties of the steel, and during forging, it is prone to rupture along the layer boundaries. | |
| Component segregation zone | In some alloy structural steels, such as 40GrNiMoA and 38GrMoAlA, longitudinal microcracks or strip-like defects appear along the flow lines on the longitudinal microstructure of the forgings. The microhardness of the defect area is significantly different from that of the normal area. | The component segregation zone is mainly caused by the segregation of alloy elements during the production of raw materials. Minor component segregation zones have little impact on mechanical properties, while severe segregation zones significantly reduce the plasticity and toughness of the forgings. | |
| Bright stripes or bright bands | On the surface of the forged piece or the processed surface of the forged piece, there are bright stripes of varying lengths. Most of these bright stripes are distributed along the longitudinal direction of the forged piece. This defect is mainly found in titanium alloys and high-temperature alloy forged pieces. | This is caused by alloy element segregation. The bright stripes in titanium alloy forgings mostly belong to the areas with low aluminum and low vanadium segregation; the bright stripe areas on high-temperature alloy forgings mostly have higher contents of elements such as nickel, chromium, and cobalt. The presence of these bright stripes reduces the plasticity and toughness of the material. | |
| Insufficient level of carbide segregation | It often occurs in alloy steels with high carbon content such as high-speed steel and high-chromium cold-worked die steel. The key point is that there are a large number of carbides concentrated in certain local areas, causing the carbide segregation to exceed the permitted standard. | This is caused by the fact that the ledeburite eutectic carbides in the steel do not fully break and separate during the initial rolling and subsequent rolling processes. Severe carbide segregation can easily lead to overheating, overburning or cracking of the forging. | |
| white dot | On the longitudinal fracture surface of the steel billet, there are circular or circular-shaped silver-white spots. On the transverse fracture surface, there are fine cracks. The size of the white spots varies, ranging from 1 to 20mm or even longer. White spots are common in alloy structural steel and have also been found in ordinary carbon steel. | This is caused by the high hydrogen content in the steel and the large stress in the microstructure during phase transformation. When large steel billets are forged and then cooled rapidly, white spots are prone to occur. White spots are internal cracks that reduce the plasticity and strength of the steel. They are stress concentration points and are prone to cause fatigue cracks under alternating loads. | |
| Residual shrinkage cavities | During the low-magnification inspection of the forgings, irregular wrinkled-like gaps appeared, resembling cracks, and were of a dark brown or grayish-white color; under high magnification, there were a large number of non-metallic inclusions near the shrinkage cavities, and the material was brittle and prone to flaking off. | Due to the fact that the concentrated shrinkage cavities formed in the mold part of the steel ingot were not completely removed, they remained inside the steel billet during the roughing and rolling processes, resulting in… | |
| The coarse-grained rings on the aluminum alloy extruded rods | The aluminum alloy extruded rods supplied after heat treatment have coarse grains in the outer annular layer of their cross-section, which is called coarse grain ring. The thickness of the coarse grain ring gradually increases from the starting extrusion end to the end of the rod. | Mainly due to the presence of elements such as Mn and Cr in the aluminum alloy, as well as the friction between the metal and the inner wall of the extrusion die during the extrusion process, the surface layer of the bar material undergoes severe deformation. For billets with coarse grain rings, they are prone to cracking during forging. If they remain on the forged part, it will reduce the performance of the component. | |
| Aluminum alloy oxide film | On the microstructure of the forgings, the oxide film is distributed along the metal flow lines and appears as black short lines. On the fracture surface perpendicular to the longitudinal direction of the oxide film, the oxide film resembles a torn and layered structure; on the fracture surface parallel to the longitudinal direction of the oxide film, the oxide film is in the form of sheets or fine and dense dot-like structures. These oxide films can easily be seen on the flange or near the parting surface within the forgings. | During the melting process, there are no remaining oxide inclusions in the aluminum liquid. During the pouring process, these inclusions are incorporated into the metal liquid from the surface. During the deformation processes such as extrusion and forging, they are elongated and thinned, thus forming oxide films. The oxide films have little effect on the longitudinal mechanical properties of the forgings, but have a significant impact on the transverse properties, especially in terms of short transverse mechanical properties. By comparing according to the forging category and the oxide film standard, only those that are (unqualified) will be scrapped. | |
| ②Defects resulting from cutting and processing | ||
| Defect Name | major characteristic | The causes and consequences |
| Inclined | The end face of the billet is inclined relative to the axis of the billet, exceeding the permitted limit. | This is caused by the failure to press the bar stock tightly during cutting. The cut-off materials with an uneven edge are prone to bending when being upset, and it is difficult to position them during die forging, which may result in folding. |
| The end of the raw material is bent and has burrs. | During the cutting process, some of the metal was caught between the blades, resulting in sharp burrs. The end of the blank was also deformed and bent. | Due to the excessive gap between the blades or the dull edges, the workpieces with burrs are prone to folding during forging. |
| The end of the billet is concave or convex. | The central part of the metal at the end face of the billet has been pulled apart, resulting in protrusions or depressions on the end surface. | The gap between the blades is too small. The metal in the center of the billet is not cut but pulled apart, causing some of the metal to be removed. Such billets are prone to folding and cracks during forging. |
| End crack | This mainly occurs during the cutting of large-section materials. When cutting alloy steel or high-carbon steel at room temperature, such cracks can also appear. | Due to the excessively high hardness of the material and the excessive unit pressure on the blade during the cutting process, the forging will cause the end cracks to further expand. |
| Cracking of the convex core | When using a lathe for material cutting, there is often a raised core on the surface of the blank. If this is not removed, it may cause cracking around the raised core during forging. | Due to the small cross-sectional area of the protruding part and the fast cooling rate: the end surface area is large and the cooling rate is slow, thus causing cracks to form around the protruding core. |
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