The upper hemisphere of the low pressure inner cylinder is taken as the research object, and the overall structure is shown in the figure. The overall dimension of the product is 4520 mm × 2 750 mm × 3 020 mm, weight 25 t, body wall thickness 40 mm, split flange thickness 210 mm. The structure of the product is complex, and many thick and large sections are connected with the thin-walled main body to form an independent hot joint. The solidification analysis needs to be carried out in different regions, and reasonable technological measures should be adopted to prevent internal shrinkage defects. There is a volute-type steam passage design in the middle of the product, and the inner cavity space is divided into several semi-closed narrow and long areas by multi-layer partitions. There are great difficulties in the core extraction scheme design, core beating, box closing control and other processes.
The material of ductile cast iron is QT400-18A, which requires metallographic analysis and mechanical property test. The dimensional tolerance of the blank of ductile iron castings shall be subject to the acceptance of level CT12 in GB/T 6414-2017. Ductile iron castings shall be subject to 100% NDT inspection as a whole, and the acceptance level of magnetic particle and ultrasonic inspection shall be 2~3.