Distribution of isolated hot spot and shrinkage porosity in heavy section position of ductile iron intake cylinder

Shrinkage cavity and shrinkage porosity are common internal casting defects in heavy position of ductile iron castings. The shrinkage cavity and porosity of the intake cylinder casting are mainly distributed in the cross section of the upper 400mm stator blade flange and the wall thickness intersection area with complex hot spot distribution. From the observation of the cross-section morphology revealed after processing, the hole wall of shrinkage cavity and porosity defect is rough with dendrite, and the contour is very irregular, which is centralized distribution (usually called shrinkage cavity) and dispersed distribution (commonly called shrinkage porosity). Some defects are exposed after machining (as shown in Fig. 1), and some defects are distributed in the casting, which can be detected by ultrasonic testing.

Shrinkage cavity and porosity will reduce the physical and mechanical properties of the gas turbine inlet cylinder casting, especially the bearing area of the bearing part, and even cause serious accidents in the process of use and service due to the stress concentration. If there is large area of shrinkage cavity and porosity defects in the heavy load-bearing castings similar to the gas turbine inlet cylinder, the castings will be scrapped directly. Nodular cast iron casting has poor repair weldability, so it is not allowed to repair defects by welding. If some defects are exposed when the subsequent finishing is about to be completed and belong to the key bearing position, it will lead to the scrapping of all parts and components matched with the inlet cylinder, and the economic loss to the company is estimated to be more than 2.3 million yuan.

Therefore, shrinkage cavity and porosity are not allowed in the important inspection position required in the acceptance standard of intake cylinder casting. Shrinkage cavity and porosity defect with large section is one of the key points in the research of intake cylinder casting technology.