Introduction to common defects of metal materials
The main defects of cast products include segregation, pores, shrinkage and porosity, inclusions, cracks, cold barriers and other defects.
1, segregation
Segregation-the phenomenon of uneven chemical composition in the casting. Segregation makes the performance of castings uneven, and can cause waste products in severe cases.
Segregation can be divided into two categories: micro-segregation and macro-segregation.
Intragranular segregation (also known as branch segregation)-refers to the uneven chemical composition of each part of the crystal grain, which is a kind of micro segregation. In the crystallization process of the alloy that forms a solid solution, only when the atoms are fully diffused under very slow cooling conditions, can crystal grains with uniform chemical composition be obtained. Under actual casting conditions, the solidification rate of the alloy is relatively fast, and the atoms cannot be sufficiently diffused. In this way, the chemical composition of the grains grown in a dendritic manner must be uneven. In order to eliminate intragranular segregation, the casting can be reheated to high temperature and kept for a long time so that the atoms can fully diffuse. This heat treatment method is called diffusion annealing.
Density segregation (previously called specific gravity segregation)-refers to the uneven chemical composition of the upper and lower parts of the casting, which is a kind of macro segregation. When the density of the constituent alloying elements is very different, after the casting is completely solidified, most of the low-density elements are concentrated in the upper part, and the high-density elements are more concentrated in the lower part. In order to prevent density segregation, fully stir or accelerate the cooling of molten metal during pouring, so that elements of different densities cannot be separated in time. There are many kinds of macrosegregation. In addition to density segregation, there are positive segregation, inverse segregation, V-shaped segregation and band segregation.
2, stomata
In the process of metal solidification, the solubility of gas decreases sharply, and it is difficult to escape from the solid metal with a high degree of solidification and stay in the melt to form pores. Different from the shape of shrinkage cavities, the stomata are generally round, oval or long, distributed individually or in series, with smooth inner walls. Common gases in the hole are H2, CO, H2o, CO2, etc. According to the position where the pores appear in the ingot, they are divided into internal pores, subcutaneous pores and surface pores. The existence of pores reduces the effective volume and density of the ingot. Although it can be compressed and deformed after processing, it is difficult to weld, resulting in defects such as skinning, blistering, pinholes, and cracks in the product.
3. Shrinkage and shrinkage
Metal shrinks in volume during the solidification process, the melt cannot be replenished in time, and shrinkage holes appear in the final solidification place, which is called shrinkage cavity or shrinkage porosity. Large and concentrated shrinkage cavities are called concentrated shrinkage cavities, small and scattered shrinkage cavities are called shrinkage porosity, and the shrinkage porosity that appears in the grain boundaries and between the dendrites is called microscopic shrinkage porosity.
The surface of the shrinkage cavity is mostly uneven, approximately jagged, and the shrinkage cavity between the grain boundary and the dendrites is often angular. Some shrinkage holes are often filled by the precipitated gas, and the hole walls are relatively smooth. At this time, the shrinkage holes are also pores. Often accompanied by low melting point substances. Shrinkage holes appear in the central area of the section. The shrinkage holes on the head of the seat are mostly tapered, with uneven inner surfaces or coarse crystalline structures. The intermittent shrinkage cavities located in the middle are mostly irregularly shaped pores. The interior is sometimes filled with gas precipitated during metal solidification, and the surface is relatively smooth. It is often difficult to weld and form delamination and bubbles in subsequent processing. The vicinity of shrinkage cavity is also easy to cause stress concentration and cracks during processing.
Shrinkage porosity is often distributed near the center of the section or the entire section, and sometimes appears near the shrinkage cavity, with small scattered pores distributed in grain boundaries or dendrite gaps. Some small shrinkages are difficult to detect with the naked eye, and can only be detected with the help of a sub-microscope or water pressure test. Porosity results in a non-compact metal structure, which greatly reduces the mechanical properties and corrosion resistance of the alloy.
The size of shrinkage cavity and shrinkage porosity area is related to the solidification shrinkage coefficient of the alloy, the fluidity of the metal liquid, the width of the crystallization temperature range, the cross-sectional size of the ingot, the casting temperature and the solidification conditions. The larger the solidification shrinkage coefficient of the alloy, the larger the size of the ingot section, the more serious the shrinkage cavity will be. The narrower the alloy's crystallization temperature range and the better its fluidity, the more concentrated the shrinkage cavity. Conversely, the wider the alloy's crystallization temperature and the wider the crystallization transition zone during solidification, the easier it is to form shrinkage porosity.
The main reasons for shrinkage cavity and shrinkage porosity are: unreasonable smelting process, low casting temperature, poor feeding, and cut-off; high cooling strength and fast casting speed: unreasonable mold design, too low and humid heat preservation cap: The alloy crystal has a wide range of temperature skin and poor fluidity.
4. Inclusion
Metal or non-metallic objects that have an obvious interface with the substrate and differ greatly in performance are called inclusions.
According to the nature of inclusions, it can be divided into two types: metallic inclusions and non-metallic inclusions. Metal inclusions refer to the primary crystals of various metal compounds that are insoluble in the base metal and the unmelted high melting point pure metal particles and foreign dissimilar metals; non-metallic inclusions include oxides, sulfides, carbides, fluxes, slag, coatings, and furnace linings Debris and silicate, etc.
According to the different sources of inclusions, endogenous inclusions and exogenous inclusions can be divided. Endogenous inclusions may exist in a free state or in a state of combining with the base metal to form a compound, or they may be a combination of various impurities.
The primary crystals or pure metals of high-melting-point metal compounds precipitated in the endogenous inclusions are mostly regular particles, blocks, flakes or needles, and their distribution is extremely uneven. The low melting point metal compounds are often precipitated along the grain boundaries or between the dendrite axis in the form of beads, spheres, networks or films. During pressure processing, inclusions with good plasticity can be elongated and deformed along the processing direction, and inclusions with poor plasticity remain in the shape of casting or break into smaller particles, which are distributed in intermittent chains along the processing direction.
Foreign inclusions are peeled off from the furnace lining and tools during the production process. They are usually thick and have uncertain shapes. Because it has a completely different chemical composition and organization from the matrix, it can be found according to different colors and corrosion conditions during fracture or cutting.
5.Crack
The cracks produced in the metal solidification process are called hot cracks; the cracks produced after solidification are called cold cracks. Cracks destroy the integrity of the metal. Except for a few that can be removed by timely processing, they will usually expand along the stress concentration area during subsequent processing and use, and eventually lead to cracks.
Hot cracking is when the ingot has not completely solidified or has solidified and there is a small amount of low-melting phase between the grain boundaries and dendrites, due to the liquid, solid shrinkage and solidification shrinkage of the metal are hindered, when the shrinkage stress exceeds the current metal strength or line It is formed when the shrinkage is greater than the elongation of the alloy. According to the different locations, thermal cracks can be divided into surface cracks, central cracks, radial cracks and lateral transverse cracks. Thermal cracks mostly extend along the grain boundary, with irregular twists and branches, often with branches, and there may be an oxide film in the crack or a slight oxidation color on the surface.
The factors that affect thermal cracking include the nature of the alloy (the alloy's solidification shrinkage coefficient and high temperature strength skin, etc.), pouring process and ingot structure. Certain elements and insoluble low melting point impurities in the alloy can significantly increase the tendency of hot cracking. The cooling rate of semi-continuous ingots is higher, so it has a much greater tendency to hot cracking than iron mold ingots. Increasing the casting speed during casting will also increase the tendency of hot cracking. From the perspective of ingot structure, the larger the cross-sectional size, the easier it is. Thermal cracking occurred.
Cold cracking is when the ingot is cooled to an elastic state with a lower temperature. If there is a large temperature difference between the inside and outside of the ingot, the shrinkage stress may be concentrated in some weak areas. Once the stress exceeds the strength and plasticity limit of the metal, the ingot will appear cold cracking. The characteristics of cold cracks are mostly trans-crystalline cracks, and most of them extend in a straight line. The cracks are regular, straight and straight. Cold cracks often develop from hot cracks.
The direct cause of casting cracks is the existence of casting stress. The causes are: inappropriate casting temperature, fast speed, excessive or low cooling rate, uneven cooling; improper continuous casting process; alloy itself has hot brittleness and strength Poor; unreasonable selection of covering agent or lubricant; poor design, deformation or improper installation of molds, crucibles, brackets, casting pipes, etc.
6, cold partition
The appearance of wrinkles or layered defects on the surface of the ingot, or the appearance of metal discontinuity inside the ingot is collectively called cold partition.
The outer surface of the cold-spaced ingot is uneven, the layers are not continuous, the cross section is layered, and there are often defects such as oxide film and associated gas holes in the middle.
According to the shape, the cold barrier can be divided into two types: crimped type and laminated type. When the casting temperature is low, the film condensate produced by the molten metal surface fails to fuse with the later poured metal, resulting in a corrugated cold barrier. Stacked cold partitions are more common. This is because the static pressure of the molten metal is greater than the surface tension of the metal and the strength of the oxide film. The molten metal breaks through the oxide film and enters the mold wall, but the strong, ground cooling makes the metal fluidity very fast As a result, it cannot be fused with the oxide film condensate to form a laminated cold barrier.
The cold partition is divided into surface cold partition, subcutaneous cold partition and central cold partition according to different parts of appearance.
The reason for the cold barrier: low casting temperature, high cooling water pressure, unstable pouring speed, large liquid level fluctuations, intermediate flow interruption, and poor feeding are important factors for the formation of cold barrier; severe surface cold barrier extends into the ingot , It also causes subcutaneous cold partition: unreasonable design of the inner wall of the mold and improper material selection can also lead to the appearance of cold partition.
Cold partition is one of the common defects of ingots, which affects the integrity of the metal surface and internal, and will affect the processing and use, and cause processing cracks and other surface defects in severe cases.
7 Uneven grains
The phenomenon of large differences in grain size at different parts of the ingot is called grain unevenness.
The common ones are: the centerline of the slab crystal deviates from the center, the thick columnar crystals on both sides, the direction difference is large, the columnar crystals are twisted, and the direction is disordered; the round ingots are severely eccentric, locally large columnar crystals, and local crystal grains are small; suspended crystals or Other abnormally coarse grains.
The main reasons: the inner wall of the mold is rough, the mold is deformed, and the lubricant coating is unevenly distributed; the cooling strength difference is large, the cooling water flow is uneven, the shooting angle is unreasonable, and the direction is disordered: long casting time, casting temperature Low, slow cooling, etc.
8. Other surface defects
Common surface defects of ingots include: scars, pitted surfaces, pits, burrs, longitudinal streaks, horizontal slubs, etc.
(1) Hemp noodles
Various irregularities on the surface of the ingot are called pitting. There are often granular protrusions and blisters on the pitted surface, accompanied by paint, covering agent, oxide and other dirt. The main reasons are low casting temperature and slow speed; the inner wall of the mold is not smooth or the covering agent is not good; the funnel is blocked and so on.
(2) Burr
The phenomenon of sharp metal protrusions on the surface, corners and corners of the ingot is called burr. The main reason is that the inner wall of the mold is not smooth; the quality of the hollow casting slab continuous casting mandrel is not good.
(3) Longitudinal streaks
The continuous or intermittent longitudinal strip protrusions or depressions on the surface of the ingot are called longitudinal striations.
The main reason is that the inner wall of the mold is drilled with metal or other oxides or grooves that produce abrasion; the assembly gap of the lining is large.
(4) The continuous casting billet with a stretch-stop process on the slub has large periodic irregularities on the surface, which is called slub.
The main reason is improper pulling and stopping process or deformation of crystallizer and mold.
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