Reclassification of Spheroidal Graphite Ductile Cast Irons Grades According to Design Needs

摘要

The classification of grades inside a material family should be based on the properties required by design procedures. This paper proposes a reclassification of spheroidal graphite ferritic pearlitic and ausferritic (ADI) ductile cast irons grades based on yield strength (YS), strength ratio (SR) UTS/YS and elongation at fracture (EF). In fact, these parameters are fundamental for the static assessment according to the procedures FKM Guideline and BS 7910:2005. Static assessment at room temperature, involving plastic deformation and depending on the wall thickness and stress state triaxiality, is here proposed as the most significant for the material classification. All other properties (e.g., fatigue under cyclic loads, high strain rates and temperature effect, etc.) should be reported with reference to the classification mentioned above. SR and EF control the plastic deformation at the notch tip, where maximum calculated elastic stress is redistributed. Minimum YS is usually assumed as the basic parameter for static and cyclic loading design. Because of the inverse relationship that exists between strength and ductility, Brinell hardness control and material quality index should be adopted as Material Quality Control tools, preventing from a too low EF. Fracture Toughness and its ratio with YS must be considered for preventing brittle fracture due to the presence of flaws. Fracture toughness definitions and available data are not sufficiently consistent for a correct comparison between different material grades. A surrogate Charpy energy measurement is indicated for an indirect estimate of toughness.