Disgregation and defects in centrifugal casting
Centrifugal casting bimetal composite pipe defects and control using centrifugal casting, can obtain far better than gravity casting grain structure, can realize double gold, belongs to the metallurgical composite, two layers of metal junction from the outer to the inner component transition smooth, crossing area, a wide transition zone can effectively reduce the two metal thermal expansion coefficient of different combined layer stress, thus prevent the composite pipe under the condition of alternating heat stress stratification. The schematic diagram of the centrifugal casting is shown in Figure 5.
1.1 After the centrifugal casting of the pipe blank, the measurement method is: intercept 12 sections at both ends of the 6 pipe blanks, the diameter of each cross section shall be measured twice along the vertical direction, and calculate the ellipticity of each section. Figure 6 shows the ellipticity of 12 sections for the plot analysis. It can be seen that among the 12 sections, the ellipticity maximum reaches 12mm, each section ellipticity error is large, therefore, the lining thickness is very uneven.1.2 Cracking defects of the billet base After centrifugal casting, radial cracks tend to occur at the base level of the billet base. The cracks extend from the base level along the base level to the end of the transition area between the base and lining layer, while there is no crack in the lining layer.
3 Centrifugal casting defect control 3.1 centrifugal casting defect control centrifugal casting too low speed will appear metal liquid rain phenomenon, will also make the pipe shrinkage, loose, slag, pipe billet inner surface uneven defects; the speed is too high, the tube is prone to crack, segregation and other defects, and the uneven thickness of the double metal lining layer is also caused by the high speed, so the prevention of the pipe thickness and the crack defect of the pipe base is mainly focused on the control speed. The uneven tube billet thickness and the crack defect of the tube billet base are caused by the rotation speed, while other factors have little influence. Experience formula of centrifugal casting:
n 299 G R
Where R is the inner radius of the casting, cm; G is the gravity coefficient; n is the centrifugal speed, r / min. The gravity coefficient of cast steel is 40~76, the inner diameter of bimetal composite pipe is 146mm, and the calculation of the centrifugal casting speed is 699 ~ 964 r/min. Therefore, the centrifugal casting speed is corrected according to the actual production situation. After the test, the speed is controlled between 950 and 1050 r / min, the probability of uneven pipe thickness is greatly reduced, and the crack defects are basically eliminated.
The centrifugal casting has many same defects as static casting, but there is less tendency to form defects in centrifugal casting. The common defects in casting production include stoma (pinhole), crack (tear), lap, shrinkage hole, etc., and ribbon bedding and rain defects are unique to centrifugal casting. The production of thick-wall steel pipe in centrifugal casting is the most difficult, because thick-wall steel pipe is easy to produce hot crack, irregular segregation, lap and other defects, and thin-wall steel pipe or cast iron pipe is easy to produce lap and crack defects. In the centrifugal casting, the copper alloy solidification mode is paste solidification, so the copper alloy centrifugal casting usually does not produce defects such as cracks and tears, and the casting produced has almost no defects.2 Main defects in centrifugal casting production 2.1 layered defects centrifugal casting layered defects are also known as band bedding defects, such defects are only produced in centrifugal casting, generally casting wall thickness is more than 80mm, while thin-wall castings rarely appear. Ribbon lamination defects is separated by the low melting point compounds (common crystal phase and oxide or sulfide clamp, debris, ribbon outside the edge contrast strong contrast line, most in centrifugal casting, alloy will be affected by the ribbon bedding defects, the wider the alloy solidification range, more significant, this phenomenon has a great relationship with low centrifuge speed. The molten metal fluid falls off, which can be reduced or eliminated by adjusting the rotation speed, pouring speed and the temperature of the metal and the model. There are many explanations for the various theories of ribbon-like bedding defects. One theory is that it may be due to the change in gravity between the top and bottom of the cast, and that the segregation of the metal liquid composition occurs with every rotation of the cold form during solidification. Another theory is the idea that when the alloy liquid into the rotating cast, the flow of liquid metal is irregular due to the vibration, when the metal liquid into the cast, the formation of tubular cast solidification in a certain way, if the other liquid metal reached a special position too late, the original liquid metal has reached all or part, solidification, this phenomenon will lead to obvious lap, cold or layered, it is called intermittent strip bedding defects.2.2 Rain phenomenon on a horizontal centrifuge, If the cast rotates at too slow speed or the metal flows into the cast too fast, The rain phenomenon will happen, The rain phenomenon can be described as molten metal falling from the top of the cast to the bottom in the rain form, Shshown in Figure 1 is a schematic diagram of rain phenomenon, This phenomenon can be seen by examining the pouring of molten metal fluid into the opposite end of the rotating cast, When the molten metal liquid does not accelerate to the rotation velocity of the cast, Low friction coefficient of the molten metal wall, The molten metal will not reach a rotational speed high enough to overcome gravity, Will fall from the top of the cast, Rain shower phenomenon occurs. Another reason may be to choose the pouring temperature is not reasonable, if the molten metal liquid into the casting temperature is too high, the viscosity is reduced, for molten gold, liquid using high enough speed to overcome the effect of gravity will become very difficult, can, by reducing the pouring speed, especially after reducing the casting half metal liquid speed to remedy. Another reason for the rain phenomenon is that the casting coating is too smooth. In this case, it is difficult to get a better foothold to keep up with the rotation speed of the casting. This phenomenon can increase the rotation speed of the casting, thus increasing the friction between the metal liquid and the casting.
2.3 shrink hole and Cold Separation When the molten metal liquid is quickly introduced into the cast, it will produce overheat inside the cast. Compared with the solidification outside the cast, the solidification of the cast core is slow, and the higher metal temperature also provides conditions for the formation of the shrink hole. When the molten metal liquid is poured slowly down, the metal liquid grows in grains along the cast wall, allowing the directional solidification to proceed across the entire section, and the shrinkage hole near the core will reach a minimum. Cold separation is also called lap, is caused by the use of inappropriate pouring process, such as unstable pouring, injection speed, so the pouring speed should fully ensure that the metal liquid continuously into the whole casting surface and then reach the end of the casting. If the pouring speed is too slow, it will produce the phenomenon of flow off, the phenomenon of cold separation will also appear.
2.4 Degregation and Crack In centrifugal casting, the thick section casting is susceptible to segregation. In the casting, due to the rotation of the centrifuge, the metal liquid at any position will produce the segregation of the outside of the casting of the composition. One of the advantages of centrifugal castings is that they can obtain more compact tissue, but if the casting is too cool and cooled too fast, the impurities and gas will fall into the casting. If the alloy liquid has a fairly high phosphorus and sulfur content, this phenomenon will be more obvious. Main causes of segregation: uncontrolled vibration in unbalanced centrifuge system; rain phenomenon of molten metal fluid caused by slow rotation; excessive pouring temperature; and slow solidification speed.Crack defects cause no specific form for centrifugal casting, and for static casting, the same precautions can be used for centrifugal casting.
2.5 Needle orifice stoma (stoma, eye) in centrifugal casting, stomatal defects are also relatively common, when the liquid metal liquid contains a large amount of gas, there is moisture in the body or cast, or the paint selection is not appropriate, needle orifice or eye defects will occur. In the casting, because the metal casting has no pores for the gas to escape, and the centrifugal casting of the metal liquid is very fast, so the pinhole is produced in the thin-walled casting. Pinhole stoma defects can be avoided by the following measures: use steel cast instead of cast iron cast, type; preheating before casting; do not use coating materials containing crystalline water; do not use too high pouring temperature. When using the paint, if the bentonite is not thoroughly mixed with the water, the crystalline water may exist in the dry bentonite, and the heat of the molten metal liquid will make the crystalline water evaporate, and the stomatal defects will be easy to produce. If the cast coating is breathable, the formed gas spreads between the molten metal liquid and the cast wall, and the gas will be excluded, which is conducive to the elimination of stomatal defects. In bronze and iron castings, pinhole-like pores appear outside the cast like clear and bright spots, like hemp pits inside the cast, and the diameter of the pores is very small. In the metal liquid melting, will also cause the dissolution of gas, dissolved gas has hydrogen, oxygen and nitrogen, etc., through the preheating and drying of raw materials, can effectively reduce gas infiltration, can also use boiling melting method to remove gas.
2.6 Internal loosening and central slag defects To ensure the density of the casting tissue, the casting must achieve sequential solidification from the outside to the inside surface. In this way, the solidification and contraction of each layer requires compensation by the inner layer of liquid metal liquid. If the metal is poured quickly, considerable cooling occurs on the inner surface, and loosening occurs inside the casting, which is caused by solidification occurring from both the inner and outer surfaces. For sand casting, the cooling speed from the outer surface is very low, cooling both from the inner surface and from the outer surface, so the shrinkage occurs in the surface of the solidification front, to avoid these defects, the inner surface temperature should be kept high enough to control the directional solidification, if the internal drying is not evenly, molten metal liquid may erode some impurities during the pouring, produce slag.
