Metal material casting defect

Metal material casting defect

Segregation - The phenomenon of uneven chemical composition in the casting. Segregation makes the performance of the casting uneven, serious will cause waste.

Intracrystalline segregation (also known as dendrite segregation) - refers to the phenomenon of uneven chemical composition of various parts of the grain, which is a kind of microscopic segregation. Where the alloy forming a solid solution in the crystallization process, only under very slow cooling conditions, so that the atoms fully diffuse, in order to obtain a uniform chemical composition of the grain. Under the actual casting conditions, the solidification rate of the alloy is faster, and the atoms cannot be fully diffused, so the chemical composition of the grain that grows in a dendritic way is inevitably uneven. In order to eliminate the segregation in the crystal, the casting can be reheated to high temperature and held for a long time to fully diffuse the atoms. This heat treatment method is called diffusion annealing.

Density segregation (formerly known as specific gravity segregation) - refers to the phenomenon of uneven chemical composition of the upper and lower parts of the casting, which is a kind of macro segregation. When the density of the alloy elements is very different, after the casting is completely solidified, the elements with low density are mostly concentrated in the upper part, and the elements with high density are more concentrated in the lower part. In order to prevent density segregation, the liquid cooling of the metal should be fully stirred or accelerated during pouring, so that the elements of different densities are too late to separate.

Segregation gold image structure Figure 1:

Figure 1. The gray area on the edge is the anti-segregation area

In the solidification process of metal, the solubility of gas decreases sharply, and it is difficult to escape in the solid metal with a large degree of exposure and stay in the melt to form pores. Different from the shape of shrinkage porosity, porosity is generally round, oval or long, single or string distribution, the inner wall is smooth. Common gases in the hole are H2, CO, H2O, CO2 and so on. According to the position of pores in the ingot, it can be 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 peeling, blistering, pinholes and cracks.

The microstructure of stomatal morphology is shown in Figure 2

FIG. 2 During casting, the gas produced by the mold bottom and mold wall can not escape and form pores along the crystallization direction

During the solidification process, the volume of the metal shrinks, the melt cannot be replenished in time, and the shrinkage holes appear in the final solidification place, which is called shrinkage holes or shrinkage porosity. Large and concentrated shrinkage pores are called concentrated shrinkage pores, small and dispersed shrinkage pores are called shrinkage porosity, which appears in the grain boundaries and dendrites with the aid of a microscope is called microshrinkage porosity.

The surface of the shrinkage holes is mostly uneven, approximately serrated, and the shrinkage holes between the grain boundaries and dendrites are often angular. Some shrinkage holes are often filled with precipitated gas, and the wall of the holes is relatively smooth. At this time, the shrinkage holes are also porosity, and low melting point objects are often associated with the shrinkage holes.

The shrinkage holes all appear in the central area of the section. The shrinkage holes located in the head are mostly conical, and the inner surface is uneven or has coarse crystalline structure. The intermittent shrinkage holes located in the middle are mostly irregular sized holes, and the interior is sometimes filled with gas precipitated when the metal solidifies, and the surface is smooth, which is often difficult to weld and form layers and bubbles in the future processing. The vicinity of the shrinkage hole is also easy to cause stress concentration and crack in the processing.

Shrinkage is usually distributed near the center of the section or the entire section, and sometimes appears near the shrinkage holes, which are small dispersed holes distributed in the grain boundaries or dendrite gaps. Some small contractions are difficult to see with the naked eye and can only be detected with the help of a microscope or hydraulic test. The porosity causes the metal structure to be less dense, which greatly reduces the mechanical properties and corrosion resistance of the alloy.

The size of the shrinkage hole and the shrinkage 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 section size of the ingot, the casting temperature and solidification conditions. The larger the solidification shrinkage coefficient of the alloy, the larger the size of the ingot section, the more serious the shrinkage holes will be. The narrower the crystallization temperature range and the better the fluidity of the alloy, the more concentrated the shrinkage holes are. On the contrary, the wider the crystallization temperature of the alloy, the wider the crystallization transition zone during solidification, the easier it is to form shrinkage.

The main causes of shrinkage and porosity are: unreasonable melting process, low casting temperature, poor feeding and flow interruption; The cooling strength is large, the casting speed is fast; The mold design is unreasonable, the thermal cap is too low and wet; The alloy has wide crystallization temperature range and poor fluidity.

The microstructure of shrinkage porosity and shrinkage porosity are shown in FIG. 3 and FIG. 4.

Figure 4: There are scattered shrinkage holes in the cross section, accompanied by shrinkage loosens and small cracks

Metal or nonmetallic objects with obvious interface with matrix and great difference in performance are called inclusions.

According to the properties of inclusions, they can be divided into metal inclusions and non-metallic inclusions. Metal inclusion refers to the primary crystals of various metal compounds that are insoluble in matrix metals, the high melting point pure metal particles that are not completely melted, and foreign exotic metals. Non-metallic inclusions include oxides, sulfides, carbides, fluxes, slag, coatings, furnace lining debris and silicates.

According to the different sources of inclusion, it can be divided into internal inclusion and external inclusion. Endogenous inclusions may exist in the state of free or combined with the base metal to form compounds, or they may be combined with various impurities.

The primary crystals or pure metals of the high melting point metal compounds precipitated in the endogenous inclusions are usually in the form of regular particles, blocks, sheets or needles, and the distribution is very uneven. The metal compounds with low melting point are often precipitated along grain boundaries or dendrite axes in the form of liquid beads, spheres, networks or films. During pressure processing, the inclusions with good plasticity can be stretched and deformed along the processing direction, while the inclusions with poor plasticity still maintain the casting shape or break into smaller particles, and are distributed along the processing direction in a discontinuous chain shape.

External inclusions are the flaking material from the lining and tools in the production process, which is usually thick and variable in shape. 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.

The morphology of non-metallic inclusions in steel is shown in Figure 5

Figure 5 Non-metallic inclusions in steel

金属在凝固过程中产生的裂纹称为热裂纹;凝固后产生的裂纹称为冷裂纹。裂纹破坏了金属的完整性，除少数可通过及时加工除去外，通常在以后的加工和 使用过程中会沿着应力集中区域进一步扩大，最后导致破裂。

热裂纹是在铸锭尚未完全凝固或虽已凝固而晶界和枝晶间尚有少量低熔点相时，因金属液态、固态收缩及凝固收缩受到阻碍，当收缩应力超过了当时的金属强度或线收缩大于合金延伸率时形成的。按出现的部位不同，热裂纹可分为表面裂纹、中心裂纹、放射状裂纹及侧面横裂纹等。热裂纹多沿晶界扩展，曲折而不规则，常出现分枝，裂纹内可能夹有氧化膜或表面略带氧化色。

影响热裂纹的因素有合金的本性(合金的凝固收缩系数和高温强度等)，浇注工艺和铸锭结构等方面。合金中某些元素及不溶性的低熔点杂质能明显增大热裂倾向。半连续铸锭的冷却速度较大因而比铁模铸锭热裂倾向大得多，铸造中加大铸造速度也会增大热裂倾向，从铸锭结构看，截面尺寸越大，则愈易发生热裂。

冷裂纹是在铸锭冷却到温度较低的弹性状态时，若锭坯内外还存在较大的温差，则收缩应力可能集中于某些薄弱区域。一旦应力超过了金属的强度和塑性极限，铸锭将出现冷裂。冷裂纹的特征是多呈穿晶开裂，多呈直线扩展，裂纹较规则、挺拔平直。冷裂纹往往由热裂纹发展而来。

铸造裂纹产生的直接原因是存在铸造应力，引起的因素有：铸造温度不合适，速度快，冷却速度过大或过小，冷却不均匀；连铸拉停工艺不当；合金本身有热脆性，强度差；覆盖剂或润滑剂选择不合理；结晶器、坩埚、托座、浇铸管等设计不良，变形或安装不当。

铸造中冷热裂纹如图6、图7：

Internal stress cold crack

The surface of the ingot is wrinkled or stacked with defects, or the internal metal discontinuity phenomenon is referred to as cold insulation.

The outer surface of the cold isolated ingot is uneven, the layer is discontinuous, the cross section is stratified, and there are often defects such as oxide film and associated porosity in the middle.

The cold insulation can be divided into two types according to the shape of folding type and cascading type. When the casting temperature is low, the film coagulated shell generated by the metal liquid surface fails to fuse with the metal poured in later, resulting in a crease cold insulation. Laminated cold insulation is more common, this is 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 mold wall, but the strong cooling makes the fluidity of the metal quickly reduced, and the result can not be fused with the oxide film shell to form a laminated cold insulation.

According to the different parts of the cold partition surface cold partition, subcutaneous cold partition and central cold partition.

The reasons of cold insulation are as follows: low casting temperature, high cooling water pressure, unstable pouring speed, large fluctuation of liquid level, interrupted flow in the middle, poor feeding are important factors to form cold insulation; The severe surface cold insulation extends into the ingot and also causes subcutaneous cold insulation. The unreasonable structure design of the inner wall of the mold and improper material selection can also lead to the appearance of cold insulation.

Cold insulation is one of the common defects of ingot, affecting the integrity of the metal surface and interior, and affecting the processing and use, and causing machining cracks and other surface defects in serious cases.

The shape of cold insulation defects is shown in Figure 8.

he phenomenon of large grain size difference in different parts of the ingot is called grain heterogeneity.

The common ones are: the center line of the ingot is off the center, the two sides are thick columnar crystals, the direction is different, the columnar crystals are distorted, the direction is disordered; The ingot has severe eccentricity, local coarse columnar crystal and local fine grain. Suspended crystals or other abnormally coarse grains.

The main reasons are: the inner wall of the mold is rough, the mold is deformed, and the lubricating oil coating is uneven; The difference of cooling intensity is large, the distribution of cooling water is uneven, the Angle of injection is unreasonable, and the direction is disordered. Long casting time, low pouring temperature, slow cooling and so on.

Typical grain heterogeneity is shown in Figure 9

The grain is seriously uneven and the crystal direction is disorderly

The common surface defects of ingot are: scars, pitting, pitting, burrs, longitudinal streaks, horizontal bamboo joints, etc.

1. Hemp noodles

Various uneven phenomena on the ingot surface are called pitting.

There are often granular bumps and sand holes on the hemp surface, and there are associated with paint, covering agent, oxide and other dirt. The main reason is that the casting temperature is low and the speed is slow; The inner wall of the mold is not smooth or the covering agent is poor; Funnel blockage, etc.

2. Burr

The appearance of sharp metal bulges on the ingot surface, edges and corners is called burrs.

The main reason is that the inner wall of the crystallizer is not smooth. Hollow billet continuous casting core rod quality is not good.

3. Longitudinal streaks

The surface of the ingot is continuous or intermittent longitudinal strip convex or concave called longitudinal stripe

The main reason is that the inner wall of the mold is drilled with metal or other oxides or grooves that produce wear; The inner lining assembly gap is large.

4. Bamboo joints

The surface of continuous casting billet with pull and stop process has a large periodic concave and convex phenomenon called bamboo knot.

The main reason is improper drawing and stopping process or mold deformation.