Frieß, Jessica M.Sc.
Head of Investment Casting
Chair for comprehensive Foundry Science and Foundry Institute

Tel.: +49 (0) 241 80 - 98152

Solid solution hardened spherical graphite cast irons

At the Foundry-Institute of the RWTH Aachen, a research group of currently four scientists was established at the beginning of 2015 to focus on solid solution hardened spherical graphite cast irons. Additional solid solution hardening elements are being determined for this innovative material, the influence and the limits of carbide forming elements are being investigated, the morphologically dependent potential for failure is being determined and, in a recently approved project, a design concept is to be framed for specifically setting the casting skin’s edge region.

Optimising solid solution hardening

Within the framework of the project “Metallurgical optimisation of high silicon-content solid solution hardened spherical graphite cast irons with respect to ensuring toughness and process capability” the grades specified in DIN 1563 since 2012 are to be metallurgically optimised. Here, the project’s primary objective is to improve the toughness and process reliability of these extremely attractive structural materials. For this purpose, the scientific approach proposes specific alloying using additional solid solution hardening elements which support the silicon’s effect and partially substitutes this element. Whilst individual and reciprocal effects of these elements are ascertained in comprehensive alloying tests, further work packages focus on the early and robust employment into industrial practice which is aided by the partners of the project’s advisory committee, on an inoculation technology optimised for the developed alloys and on employing new analytical methods for evaluating the occurring segregation profiles.

[Löb12] H. Löblich, „AIF Final report – Basic knowledge concerning materials and processing for the production and application of silicon solution strengthened ductile iron“, 2012 (German)

Quantitatively predicting carbide formation

The objective of the project “Quantitatively predicting and experimentally analysing carbides in high silicon-content spherical graphite cast irons” is to enhance the process-window for manufacturing SG iron regarding the use of alloy steel scrap as the starting material. Concerning this, the tolerance limits of subversive trace elements are determined which lead to the formation of carbides. Within the scope of this project, the combined effect of the subversive trace elements Cu, Mn, Cr, Mo, Nb and V are both experimentally investigated using minimum and maximum volume fractions as well as computed by means of thermodynamic-kinetic material simulations of the carbide content profiles. A data bank will be generated from the results which contains data for carbide formation as a function of the chemical composition and the cooling conditions. This data will be transferred to the foundries as a user database and software for planning charge mixtures in order to save costs in selecting raw materials and to simultaneously guarantee the quality of the final product.

Failure potential

The objective of the project “Simulation based determination of the failure potential of silicon alloyed spherical graphite cast irons subject to fatigue loading” is to provide designers with the fatigue parameters that serve as a basis for assessing silicon alloyed cast iron materials.  For this purpose, two materials: GJS 500-14 and GJS 600-10, are tested subjected to both axial as well as torsional loading for varying stress ratios and notch factors. Besides the graphite’s spherical characteristics (forms VI and V), the influence of the graphite’s properties on the cyclic mechanical properties will also be investigated. For this purpose, specific material states are established and tested which possess reduced roundness of the graphite particles as well as chunky-graphite that leads to a severe reduction in the static properties of components having large wall thicknesses. Using the correlation of the microstructural parameters: such as size, form and distribution of the graphite particles, with the fatigue strengths, a catalogue of boundary samples will be compiled for the graphite’s properties. Moreover, metallographic image analyses serve as the basis for developing a model with which the cyclic mechanical properties of real microstructures can be simulated using representative volume elements.