A reclassification of Spheroidal Graphite Ferritic Pearlitic and Ausferritic (ADI) Ductile Cast Irons Grades based on Yield Strength and Strength Ratio (Ultimate Tensile Strength / Yield Strength)
The International Journal of Metalcasting (IJMC) paper “Reclassification of Spheroidal Graphite Ductile Cast Irons Grades according to Design Needs” by Franco Zanardi, Carlo Mapelli and Silvia Barella- available as an Open Access download at the link https://doi.org/10.1007/s40962-020-00454-x – proposes a reclassification of Spheroidal Graphite Ferritic Pearlitic and Ausferritic (ADI) Ductile Cast Irons Grades based on Yield Strength and Strength Ratio (Ultimate Tensile Strength / Yield Strength).
This paper was an invited work from the 2nd Carl Loper Cast Iron Symposium held last fall (2019) in Bilbao, Spain. It will be included as part of the IJMC Carl Loper Symposium Issue, Vol. 14 Issue 3 Summer 2020, along with 27 other invited papers from global experts in cast iron in what will be a truly remarkable IJMC issue. This 300 page issue will go online in the digital format in late June and then the print version in early July, 2020.
Contents of the paper
The Strength Ratio is an immediate rough but reliable indicator of the material strain hardening behavior, useful for distinguish the design properties of materials having the same Yield Strength but different matrix (e.g. High Silicon Ferritic and Ferritic Pearlitic Ductile Irons).
Reclassification criteria include also the Elongation at Fracture, as measured in the quasi-static tensile test, as determinant parameter of the Critical Strain versus the Triaxiality Factor.
It is proposed to consider the Elongation at Fracture in combination with the Ultimate Tensile Strength through a “Material Quality Index MQI”, having shown that said index is effective in controlling the Critical Strain considering the process hardness scatter. It is reasonable to assume that the MQI could also control other material properties as, for example, the Fatigue Strength.
Design procedures based on the Yield and Fatigue Strength are typically applied after having selected the material grade on the base of empirical criteria taking in consideration the embrittlement factors to be considered in design : Temperature, Strain Rate, Triaxiality of the State of Stress, Casting Wall Thickness. The influence of this factors should be described with reference to design procedures considering static loadings.
The present work provides a rational framework to be adopted as identifier to associate with experimental material characterization data.
It contains also a description of the different Spheroidal Graphite Iron processes as they have evolved to the present days.
It provides also a review of two significant approaches in static design: “Analytical strength assessment of components technically without fault” (FKM guidelines) and “Acceptability of flaws” (BS 7910).
The coexistence of two different approaches, descriptive and technical, in the same work, invites material strategists and design technicians to confront themselves with a common language. It also offers researchers a basic scheme to associate the results of their experiments with, thus contributing to the consolidation of the material database.
Honorary President of Zanardi Fonderie Spa