The operation environment of wind turbine is relatively bad, and the supporting large parts should be used for a long time under wind load, impact, low temperature and other working conditions. Therefore, they need to have the advantages of good mechanical properties, low temperature impact resistance, good shock absorption and so on. Nodular cast iron belongs to high-strength cast iron, which has high strength and fatigue resistance, certain plastic toughness, good wear resistance and shock absorption, low notch sensitivity and good casting technology. Therefore, it has been widely used in wind turbine. The hub, main frame, bearing seat, gearbox box, etc. are made of nodular cast iron.
Shrinkage porosity is the most common casting defect of nodular iron castings, especially thick castings, which has always been a difficult problem for foundry manufacturers. Shrinkage porosity is often hidden inside the casting, in a dispersed and fine shape, which is not easy to be found in the appearance. The main harm of shrinkage porosity significantly reduces the mechanical properties of castings, the tensile strength decreases by about 60%, and the fatigue strength decreases by 40% ~ 50%. Therefore, it is of great significance to analyze the causes of shrinkage porosity of ball milled iron castings and take corresponding measures to improve the quality of castings.
At present, the main frame, wheel hub, bearing seat and other castings are made of qt400-18al-1, which meets the requirements of mechanical properties and metallographic structure. Ultrasonic flaw detection shall meet the requirements of grade 2 in en12680 and magnetic particle flaw detection shall meet the requirements of grade 2 in en1369.
With the increase of megawatt grade of wind turbine, nodular iron castings tend to be large and complex, which poses new challenges to the casting process. We need to continue to deeply study and explore the causes of casting shrinkage defects, put forward solutions, avoid shrinkage defects from the source and improve the qualified rate of castings.