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

金屬在凝固過程中產(chǎn)生的裂紋稱為熱裂紋;凝固后產(chǎn)生的裂紋稱為冷裂紋。裂紋破壞了金屬的完整性，除少數(shù)可通過及時加工除去外，通常在以后的加工和 使用過程中會沿著應(yīng)力集中區(qū)域進一步擴大，最后導(dǎo)致破裂。

熱裂紋是在鑄錠尚未完全凝固或雖已凝固而晶界和枝晶間尚有少量低熔點相時，因金屬液態(tài)、固態(tài)收縮及凝固收縮受到阻礙，當收縮應(yīng)力超過了當時的金屬強度或線收縮大于合金延伸率時形成的。按出現(xiàn)的部位不同，熱裂紋可分為表面裂紋、中心裂紋、放射狀裂紋及側(cè)面橫裂紋等。熱裂紋多沿晶界擴展，曲折而不規(guī)則，常出現(xiàn)分枝，裂紋內(nèi)可能夾有氧化膜或表面略帶氧化色。

影響熱裂紋的因素有合金的本性(合金的凝固收縮系數(shù)和高溫強度等)，澆注工藝和鑄錠結(jié)構(gòu)等方面。合金中某些元素及不溶性的低熔點雜質(zhì)能明顯增大熱裂傾向。半連續(xù)鑄錠的冷卻速度較大因而比鐵模鑄錠熱裂傾向大得多，鑄造中加大鑄造速度也會增大熱裂傾向，從鑄錠結(jié)構(gòu)看，截面尺寸越大，則愈易發(fā)生熱裂。

冷裂紋是在鑄錠冷卻到溫度較低的彈性狀態(tài)時，若錠坯內(nèi)外還存在較大的溫差，則收縮應(yīng)力可能集中于某些薄弱區(qū)域。一旦應(yīng)力超過了金屬的強度和塑性極限，鑄錠將出現(xiàn)冷裂。冷裂紋的特征是多呈穿晶開裂，多呈直線擴展，裂紋較規(guī)則、挺拔平直。冷裂紋往往由熱裂紋發(fā)展而來。

鑄造裂紋產(chǎn)生的直接原因是存在鑄造應(yīng)力，引起的因素有：鑄造溫度不合適，速度快，冷卻速度過大或過小，冷卻不均勻；連鑄拉停工藝不當；合金本身有熱脆性，強度差；覆蓋劑或潤滑劑選擇不合理；結(jié)晶器、坩堝、托座、澆鑄管等設(shè)計不良，變形或安裝不當。

鑄造中冷熱裂紋如圖6、圖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.