Casting process design of lower half casting of rear cylinder of marine steam turbine

  1. Analysis of casting structure and casting process

The main body of the lower half casting of the rear cylinder is composed of a 30 mm thick exhaust volute, which is an irregular thin-walled shell with complex line shape. In the process of pouring, the molten steel flow distance is long and the temperature drop is large, so it is easy to produce defects such as insufficient pouring and cold shut. During the cooling process, the shrinkage of the casting is uneven, and it is easy to deform and produce casting defects such as size deviation. The horizontal split flange and exhaust flange of casting are UT1 level requirement areas. The riser is easy to be placed on the horizontal split flange, and the quality is easy to be guaranteed. The exhaust flange is located at the bottom of the casting and is irregularly connected with the exhaust volute. There is a thick boss on the outside and a cross stiffened plate on the inside. Because the exhaust flange is not easy to place riser feeding, it is a difficult point of casting feeding. There is a narrow cavity inside the bearing seat, which causes hot spot at the joint with the cylinder, makes it difficult to feed, and is easy to produce shrinkage, cracks and other casting defects. The bottom plate of the support seat on both sides of the cylinder is 70 mm thick, which is connected with the cylinder block by three stiffeners. It is difficult to place the riser for feeding, which is also one of the difficulties in feeding.

It is difficult to judge the solidification sequence of complex castings manually, so it is necessary to calculate the cooling process of castings by casting simulation software. Figure 1 is the simulation of the natural solidification process of the casting (only simulating the casting body, without riser, subsidy, chill, gating system and other casting processes), which can determine the solidification sequence and the final solidification position of the casting, and provide the basis for the placement of riser, subsidy and chill.

  1. Determination of classification scheme

In order to facilitate the feeding of horizontal split flange of cast steel cylinder, the horizontal split is used as the parting surface, and the whole casting is located in the lower box. From the simulation of natural solidification process, it can be seen that most of the final solidification parts of the casting are located near the horizontal split flange, and the riser can be conveniently set up with the horizontal split facing up to feed the casting. At the same time, because the whole casting is located in the lower box, the molding operation and box size control are more convenient. Therefore, it is reasonable to use the horizontal split flange upward.

  1. Setting of gating system

Cylinder castings bear a lot of pressure in the work, which requires high density of castings. In order to smooth mold filling and reduce the inclusions in the pouring process, the bottom pouring system should be adopted as far as possible, and the flow velocity at the exit of the ingate should be controlled to be less than 0.5 m / s. The exhaust flange of the lower half of the rear cylinder is located at the bottom of the casting, and the side riser is needed for feeding. In order to avoid the shrinkage defects of the hot spot at the junction of the ingate and the casting, the ingate is usually set at the bottom of the side riser, which can not only avoid the shrinkage defects of the ingate, but also increase the temperature of the side riser, which is more conducive to the feeding of the riser. Whether the rising speed of liquid steel is suitable or not is one of the important factors to obtain high quality steel castings. According to the rear steam

The main wall thickness of cylinder volute is 30 mm. According to the process manual, the reasonable liquid level rising speed is not less than 30 ~ 36 mm / s. The simulation of casting pouring process is shown in Fig. 2. It can be seen that the liquid level in the whole pouring process is stable, and there are no cold shut, insufficient pouring and other defects. The pouring process took 28 s, and the average rising speed of casting liquid level was 42.5 mm / s.

  1. Design of feeding system

Due to the high flaw detection requirements of the whole cylinder casting, the whole casting must have sufficient feeding to ensure the internal density, especially the density of level split flange and exhaust flange at UT1 level. There are many intersection points in the casting, which form local hot spots. Due to the casting structure, these hot spots can not be fed by riser. Because of the thin wall of the cylinder volute, these hot spots can be quenched with round steel as chill iron to eliminate the local hot spots at the intersection. Because the riser calculated by modulus method is more accurate, the riser size of steel castings is usually calculated by modulus method. However, for castings with complex structure, it is difficult to calculate by hand, and it is usually difficult to calculate accurately. It is convenient and fast to calculate the modulus distribution nephogram of casting by using casting simulation software. The modulus of any part can be obtained from the nephogram, and the riser size can be calculated very conveniently. The modulus distribution of the lower half casting of the rear cylinder is shown in Figure 3. According to the casting modulus obtained by simulation calculation, the riser size and placement position can be preliminarily determined. The final riser, allowance and chill scheme can ensure that the whole casting feeding process can realize sequential solidification and make the casting meet the requirements of flaw detection.

The optimized riser, chiller and subsidy arrangement are shown in Figure 4. Figure 5 is the solidification process simulation of the final feeding process. From the simulation of solidification process, it can be seen that the whole casting has achieved sequential solidification, the final solidification parts are concentrated in the riser, and there is no isolated liquid region in the casting.

  1. Dimension control of lower half casting of rear cylinder

The linear shrinkage rate of cast steel is large. For the steel castings with complex structure and large wall thickness difference such as the lower half of rear cylinder, the cooling speed of different thickness parts is different, which leads to different shrinkage amount and different shrinkage rate in all directions.

The cylinder has very high requirements for dimension, the dimension tolerance is – 3 ~ + 2 mm, the flange thickness tolerance is 0 ~ + 2 mm. Therefore, it is necessary to select the appropriate scale and take appropriate anti deformation measures. According to the production experience of similar thin-walled cylinder castings, the overall casting scale is 1.5%, and the thicker flange, boss and other parts of the cylinder are additionally thickened by 5 ~ 7 mm, so as to prevent excessive size shortage due to local shrinkage. In order to prevent the size deformation of the opening of the horizontal split of the cylinder, three stiffeners are added at the opening of the horizontal split.

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