Performance: the performance of a material in the course of use. Including mechanical properties, physical properties and chemical properties.
Technological performance: the performance of the material in the processing process. Including casting, forging, welding, heat treatment and cutting properties.
The metallic or non-metallic objects with obvious interface and different properties are called inclusions.
According to the nature of inclusions, they can be divided into metal inclusions and non-metallic inclusions. Metal inclusions refer to the primary crystals of various metal compounds and unmelted high melting point pure metal particles and foreign foreign metals that are insoluble in the matrix metal; non-metallic inclusions include oxides, sulfides, carbides, fluxes, slag, coatings, furnace lining debris and silicates.
According to the source of inclusion, it can be divided into endogenous inclusion and exogenous inclusion. Endogenous inclusions may exist in a free state or combined with the matrix metal to form a compound, or it may be the combination of a variety of impurities.
Most of the high melting point metal compounds or pure metals precipitated in the endogenetic inclusions are regular granular, massive, flaky or needle-like, and the distribution is extremely uneven. On the other hand, low melting point metal compounds often precipitate in the form of liquid beads, spheres, networks or thin films along the grain boundaries or between dendritic axes. During pressure processing, the inclusions with good plasticity can be elongated and deformed along the processing direction, while the inclusions with poor plasticity still maintain the shape of casting or break into smaller particles, which are distributed intermittently along the processing direction.
The foreign inclusion is the exfoliation from the furnace lining and tools in the production process, which is usually thick and indefinite in shape. Because it has a completely different chemical composition and structure from the matrix, it can be found according to different color and corrosion during fracture or cutting.
The crack produced during solidification of metal is called hot crack, and the crack produced after solidification is called cold crack. Cracks destroy the integrity of the metal, in addition to a few can be removed by timely processing, usually in the later processing and use process will be further expanded along the stress concentration area, and finally lead to fracture.
The hot crack is hindered by metal liquid, solid shrinkage and solidification shrinkage when the ingot has not been completely solidified or although it has solidified and there are still a small amount of low melting point phases between grain boundaries and dendrites. it is formed when the shrinkage stress exceeds the metal strength or the linear shrinkage is greater than the elongation of the alloy. According to the different locations, hot cracks can be divided into surface cracks, central cracks, radial cracks and lateral transverse cracks. Most of the hot cracks propagate along the grain boundary, tortuous and irregular, often branching, and there may be an oxide film or a slight oxidation color on the surface of the crack.
The factors affecting the hot crack are the nature of the alloy (solidification shrinkage coefficient and high temperature strength of the alloy, etc.), pouring process and ingot structure. Some elements and insoluble low melting point impurities in the alloy can obviously increase the tendency of hot cracking. The cooling rate of semi-continuous ingot is larger, so the hot cracking tendency of semi-continuous ingot is much higher than that of iron mold ingot, and increasing the casting speed will also increase the hot cracking tendency. From the structure of ingot, the larger the section size is, the easier it is to crack.
The cold crack is when the ingot is cooled to the elastic state with lower temperature, and if there is a large temperature difference inside and outside 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 crack. The characteristics of cold cracks are transgranular cracking, linear propagation, regular, straight and straight cracks. Cold cracks often develop from hot cracks.
The direct cause of casting crack is the existence of casting stress, which is caused by improper casting temperature, high speed, too large or too small cooling rate, uneven cooling, improper continuous casting process, poor thermal brittleness and poor strength of the alloy itself. unreasonable selection of covering agent or lubricant; poor design, deformation or installation of mold, crucible, bracket, casting pipe, etc.
The surface of the ingot is wrinkled or laminated, or the internal metal discontinuity is called cold insulation.
The outer surface of the cold spaced ingot is not smooth, the layer is discontinuous, the cross section is layered, and there are often defects such as oxide film and pores in the middle.
According to the shape, the cold partition can be divided into two types: folded type and cascading type. When the casting temperature is low, the thin film condensation shell produced on the liquid metal surface fails to fuse with the later poured metal, resulting in a wrinkled cold shield. Cascading cold insulation is more common, because the static pressure of the liquid metal is greater than the surface tension of the metal and the strength of the oxide film, the liquid metal breaks through the oxide film and enters the crystallizer wall, but the strong cooling reduces the fluidity of the metal very quickly. as a result, it can not be fused with the oxide film shell to form a laminated cold shield.
The cold compartment can be divided into surface cold compartment, subcutaneous cold septum and central cold septum according to the location where they appear.
The causes of cold isolation are as follows: low casting temperature, high cooling water pressure, unstable pouring speed, large fluctuation of liquid level, intermediate cut-off and poor feeding are important factors for the formation of cold isolation; serious surface cold isolation extends to the ingot, which also causes subcutaneous cold isolation; the unreasonable design of mold inner wall structure and improper material selection can also lead to the emergence of cold isolation.
Cold insulation is one of the common defects of ingots, which affects the integrity of metal surface and interior, and affects processing and use, and causes machining cracks and other surface defects in serious cases.
The phenomenon that there is a great difference in grain size in different parts of the ingot is called uneven grain.
The common ones are as follows: the crystallization center line of flat ingot deviates from the center, the two sides are thick columnar crystal, the direction is different, the columnar crystal is distorted and the direction is disordered; the round ingot is eccentric seriously, the local coarse columnar crystal is fine, and the suspended crystal or other abnormally coarse grains.
The main causes are as follows: rough inner wall of mold, deformation of mold, uneven distribution of lubricating oil paint, great difference in cooling intensity, uneven distribution of cooling water, unreasonable shooting angle, disordered direction, long casting time, low pouring temperature, slow cooling and so on.
The common surface defects of ingots include scars, hemp noodles, pockets, burrs, longitudinal stripes, transverse slubs and so on.
All kinds of unevenness on the surface of ingot are called hemp surface.