Casting process and requirements of lower half casting of rear cylinder of marine steam turbine

With the development of information technology, modern ships need to be equipped with a large number of information equipment and electronic equipment, increasing the demand for power, and need to be equipped with more power generating units to cope with the surge of power demand. As an important power source of ships, steam turbine has the characteristics of high power density and high reliability. It is the core equipment of ship large capacity power supply system. Marine steam turbine cylinder structure is complex, compact, high quality requirements, with high production difficulty. In this paper, the casting process of the lower half of the rear cylinder was analyzed in detail, and the casting simulation software ProCAST was used to design and optimize the casting process. Finally, the lower half of the rear cylinder was successfully produced to meet the technical requirements.

The structure of the lower half of the rear cylinder is shown in Figure 1. The cylinder is mainly composed of exhaust volute, horizontal split flange, exhaust flange, bearing seat and support seat. The net weight of the casting is 2 060 kg, the gross weight is 2 680 kg, and the material is ZG20CrMo. The casting outline size is 2 160 mm × 1 527 mm × 790 mm, the maximum wall thickness is 123 mm, the minimum wall thickness is 20 mm, and the main wall thickness of cylinder exhaust volute is 30 mm. The dimension tolerance of cylinder is – 3 ~ + 2 mm, and the thickness tolerance of flange is 0 ~ + 2 mm.

NDT requirements of cylinder: 100% UT, grade 1 for weld groove area, horizontal and vertical flange processing surface, grade 2 for other processing surface and casting surface. 100% magnetic particle inspection (MT), machining surface 1, the rest 2.

(1) In the process of casting process design for the lower half of the rear cylinder, the casting modulus is calculated by ProCAST casting simulation software, the calculation results are accurate and the riser calculation process is simplified.

(2) The flow field of the filling process is simulated, and the smooth filling is achieved.

(3) The solidification process was simulated and optimized in detail. The whole casting was solidified in sequence and the shrinkage defects were eliminated.

(4) Reasonable size control measures are taken to ensure that the casting size meets the requirements.

Several lower half castings of rear cylinder were produced according to the above process. The surface of the castings was smooth and the outline was clear (Fig. 2). After marking processing, grinding and flaw detection, it is confirmed that the dimensional deformation of the casting is in line with the expectation, only the partial dimension is slightly out of tolerance, and it meets the requirements of the drawing after repair. No over standard defects were found in ut of castings, and the castings completely meet the requirements of drawings after finishing.

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