Abstract
Some results of materials characterization activities, dedicated to classical and notch mechanics fatigue and elastoplastic properties, have already been published for some Ferritic–Pearlitic Ductile Iron, including the patented heat treated Isothermed (IDI) and Austempered Ductile Iron (ADI) grades. Others have not yet been published. The possible use of all of these results in new standards is discussed in this paper. It is proposed that new standards should provide a criterion that is able to measure the process quality that represents more accurately the actual market needs and manufacturing capabilities. Classification of grades, considered by existing standards, is based on minimum properties for strength and ductility parameters that are separately evaluated. A different approach that is based on a quality index, which considers strength and ductility all in one, is proposed. However, this new proposed approach may not be sufficient to provide a satisfactory classification for the ADIs. This is because their fracture mechanical behavior and machinability can be correlated with their austenite stability. It could also be insufficient for the classification of the recent High Silicon Solid Solution Strengthened Ductile Irons that exhibit a decreasing ultimate tensile strength/proof stress ratio with increasing Si. For construction steels, fracture mechanics properties are sometimes believed to be related to the Charpy impact energy. This paper introduces an innovative practical and inexpensive data analysis, performed on the tensile test curve, which appears to be a potential estimator of fracture mechanical properties, at least for ADIs, where said properties could be correlated with the austenite stability.
Highlights
Ductile iron, referred to as nodular iron or spheroidal graphite iron, was patented in 1948
The different behavior of Austempered Ductile Iron (ADI) obtained by robust processes, compared with non-conventional ferritic Solid Solution Strengthened’’ Ferritic Ductile Irons (SSSFDIs) and/or ferritic–pearlitic NCFPDIs and including the patented heat treated Isothermed (IDI), is another reason for other superior properties of this material family: ductility, Charpy impact at room and very low temperatures and fracture mechanical properties, together with a good machinability
In ‘‘Normative’’ section, material classification should be mainly based upon measurements coming from the uniaxial tensile test on test probes taken from Lynchburg 25 mm in diameter (L25) and/or 25 mm in thickness (Y25) test samples, and considering the conventional values Rm, Rp0.2, A5, HBW, and a combination of these and, possibly in the future, a measurement of the strain hardening profile of the tensile test curve
Summary
Referred to as nodular iron or spheroidal graphite iron, was patented in 1948. The different behavior of ADIs obtained by robust processes (stable austenite), compared with non-conventional ferritic SSSFDIs and/or ferritic–pearlitic NCFPDIs and IDIs, is another reason for other superior properties of this material family: ductility, Charpy impact at room and very low temperatures and fracture mechanical properties, together with a good machinability. This approach could be of interest for the designer asking the foundry for a given performance at a critical location in the casting, without regarding if unsatisfactory results are coming from poor material or from a defect This approach could, be accepted by foundries because what is measured is what is needed, fatigue strength, and not a non-correlated variable, as is elongation at fracture in the conventional tensile test. Paris law parameters at room temperature; 8. fracture toughness values at room and low temperatures; 9. stress strain curves in bending, torsion, compression 10. other properties (e.g., weldability,...)
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