Contact

Franzen, Daniel M.Sc.
Head of Investment Casting
Chair for comprehensive Foundry Science and Foundry Institute

Tel.: +49 (0) 241 80 - 98250
d.franzen@gi.rwth-aachen.de

Cast Iron Research

The work at the Foundry Institute of RWTH Aachen University focuses on the correlation of process - microstructure - properties. By means of statistical and numerical experimental design, the processes are specifically designed to produce defined microstructures. The microstructures are first characterized comprehensively and automatically with regard to graphite, pearlite and carbide content using various etching methods, and further analysis methods such as EDX, EBSD, FIB or even TEM are used as required. Due to the availability of a 250 kg and a 70 kg melting unit, the metallurgical process control can be varied with different melt cleaning, Mg treatment and inoculation methods. Process variations are currently being carried out to investigate the influence of alloying, accompanying and interfering elements, as well as the influence of different cooling conditions, different inoculation technologies and with regard to mold-melt reactions. The defined microstructures are correlated with static, cyclic and dynamic properties in cooperation with the research institutes IEHK & IWM of RWTH Aachen University and Fraunhofer LBF, among others. Under the following points, completed and ongoing projects since 2015 are briefly outlined.

Optimization of solid solution strengthening

Metallurgical optimization of high silicon solid solution strengthened spheroidal graphite cast iron with respect to ensuring toughness and processability (AiF)

Duration: 01.01.2015 - 30.06.2017

Research Institutions: Foundry Institute

Within the scope of the project "Metallurgical optimization of high-silicon solid-solution cast iron with nodular graphite with regard to ensuring toughness and processability", the grades included in DIN 1563 since 2012 are to be metallurgically optimized. The project is primarily aimed at improving the toughness and process reliability of these extremely attractive engineering materials. To this end, the scientific approach envisages targeted alloying with further solid solution strengthening elements which support the effect of silicon and partially substitute it. While comprehensive alloying tests are being carried out to identify the individual interactions of these elements, the focus of further work packages is on ensuring transferability to industrial practice at an early stage together with the partners of the project committee, on an inoculation technique optimized for the alloys developed, and on the application of new analytical methods for evaluating the resulting 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)

Quantitative carbide prediction

Quantitative prediction and experimental analysis of carbides in high silicon spheroidal graphite cast iron (AiF)

Duration: 01.04.2015 - 31.12.2017

Research facilities: Foundry Institute, Institute for Material Applications in Mechanical Engineering.

Within the framework of the project "Quantitative carbide prediction", possibilities for the quantitative prediction of carbides in high-silicon GJS were successfully developed experimentally by means of a factorial experimental design including the elements Cr, Mn ,Mo, Nb and V as well as simulation-based with the aid of a specially developed micro-segregation model. This enables the use of low-cost input materials such as steel scrap for the production of this material. Empirical microstructure-property relationships for predicting the microstructure and mechanical properties from the chemical composition were determined experimentally. The microsegregation model can be used to simulate the amount of pearlite and carbides for arbitrary cooling conditions. The model was validated using numerous experimental findings. To apply the results in practice, a carbide database with Excel interface was created, which allows the user to predict the microstructure and mechanical properties for a given chemical composition.

Failure potential

Simulation-based determination of the failure potential of silicon alloyed cast iron with spheroidal graphite under oscillating loading - Failure potential (AiF)

Duration: 01.04.2015 - 30.06.2018

Research institutes:  GI, IWM

The aim of the project "Simulation-based determination of the failure potential of silicon alloyed cast irons with nodular graphite under oscillating stress" is to provide cyclic material parameters for designers as a design basis for silicon alloyed cast iron materials.

Two materials, GJS 500-14 and GJS 600-10, are tested under both axial and torsional loading at varying stress ratios and notch factors. In addition to the spherical nature of the graphite (shapes VI and V), the influence of graphite degenerations on the cyclic mechanical properties is also investigated. For this purpose, defined material states with reduced roundness of the graphite particles as well as chunky graphite, which leads to a strong reduction of the static properties in components with thick walls, are set and tested. Based on the correlation of microstructural parameters such as size, shape and distribution of graphite particles with fatigue strengths, a limit pattern catalog for graphite degenerations is established. In addition, metallographic image analyses serve as the basis for developing a model that can be used to simulate the cyclic mechanical properties of real microstructures using representative volume elements.

Publications within the project:

J. Frieß*, B. Pustal, P. Weiß, M. Riebisch, A. Kutz, A. Bührig-Polaczek: Enhanced Applications and Improved Properties of High Silicon Ductile Iron, Proceeding of CastTec 2016

Casting skin

Characterization of varied edge zone conditions on cast iron components and evaluation of the influence on fatigue strength - cast skin (AiF)

Duration: 01.01.2016 - 30.06.2019

Research institutes:     GI, LBF, SAM

The aim of the project is to enable a resilient and efficient design and dimensioning of spheroidal graphite cast iron (GJS) components, taking into account the cast skin and its influence on the cyclic material properties.

The cast skin edge zone is understood to be a limited area on the surface of the casting which, depending on the material, can have different imperfections and deviations from the basic structure inside the casting. The precise influence of the various imperfections on the cyclic properties has been insufficiently investigated to date, which means that it has not been possible to take these process-related defects into account in the design of castings. Within the scope of the project, the industrially relevant casting skin conditions will be specifically adjusted via the molding material system, the coating and the melting metallurgy on two conventional GJS materials and a high-silicon grade, and their influence on the fatigue strength will be investigated. This opens up opportunities for foundries to improve the casting skin edge zone properties and to design their castings more specifically.

Diagraph

Investigation of graphite nodularity in spheroidal graphite cast iron by means of digital image processing - DIAgraph (AiF)

Duration: 01.03.2017 to 28.02.2019

Research Institutions:  GI, GFaI

The DIAgraph project aims at the systematic investigation of the influencing variables in the determination of the microstructure parameter 'nodularity' of GJS and the standardization of graphite analysis.

The quantitative determination of the microstructural constituents of nodular graphite cast iron (GJS) is an essential component of material development and quality assurance, since on the one hand the material and component properties depend on it, and on the other hand the requirements of the casting customers have to be met. At present, graphite shape determination is performed by visual inspection as well as by insufficiently defined image analysis, both of which, however, do not allow reliable and reproducible evaluation due to insufficient objectivity. The implementation of a teachable classifier based on object-based parameters should enable the identification of graphite particles, especially spherulites, and the detection of graphite degenerations. The influences during image acquisition and evaluation of spheroidal graphite cast iron are investigated and the different approaches to determine nodularity are compared and evaluated. The developed algorithms will be tested and evaluated by image analysis providers and users in consecutive round robin tests. The results will be incorporated into standardization and enable foundries to implement procedural instructions for rapid and reproducible image analysis of GJS.

DIAgraph II

DIAgraph II (AIF)

Area-related classification of graphite formations in cast iron with lamellar or vermicular graphite as well as of graphite structures that cannot be assigned to the basic shapes by means of digital image processing - DIAgraph II (AiF)

Duration: 01.06.2019 until 31.05.2021

Research institutes:  Foundry Institute, GFaI

The project DIAgraph II aims at the development of an area-based classification of graphite morphologies in cast iron.

Quantity, size and arrangement of graphite particles have an influence on the mechanical properties of cast irons, so that a reproducible, objective evaluation must be ensured in order to enable an essential description of the microstructures on these properties. Currently, microstructure evaluation is performed visually or, in some cases, by digital image analysis, which, however, is not uniformly defined for GJL and GJV. The objective graphite analysis to be developed should enable foundries to reach quality agreements with customers and to quantify microstructural constituents and deviations so that, for example, steel scrap containing impurities but at low cost can be used selectively. The reproducible microstructure analysis to be developed will also provide small and medium-sized iron foundries with a tool for targeted material development.

Smartscrap

Perspective development and influence of accompanying elements in steel scrap on the production and properties of cast iron - "Smartscrap" (CORNET)

Duration: 01.03.2018 - 31.02.2020

Research institutions: CRM (Ghent, Belgium), Sirris (Ghent, Belgium), GI (RWTH Aachen University).

In the CORNET project "Smartscrap", the future development of steel scrap as well as the influence of the contained accompanying elements on cast iron production will be analyzed and measures to avoid negative property changes will be developed. To this end, the influence of various carbide-forming elements such as manganese, boron or molybdenum on the properties of GJL-250, GJS-400-15 and GJS-600-3 on the microstructure and the static and cyclic mechanical properties will be investigated experimentally in order to determine tolerance limits with regard to the material properties. In addition, solidification simulations using the phase field method are carried out to determine the influence of accompanying elements on graphite formation in spheroidal graphite cast irons. Secondary metallurgical measures to remove certain elements from the cast iron melt are also being investigated. The results are incorporated into a carbide database, which was created in the completed project "Quantitative carbide prediction".

Damage Mechanics

Damage Mechanics Approaches for the Targeted Adjustment and Efficient Utilization of the Toughness Properties of High-Silicon Cast Iron with Spheroidal Graphite - Damage Mechanics of MK-Strengthened GJS (AiF)

Duration: 01.10.2018 - 31.03.2021

Research institutes: GI, IEHK

Despite excellent static mechanical property combinations, widespread application of high-silicon, solid solution strengthened grades of GJS (MK-GJS) is currently hindered due to their extremely low notched impact strength values - use is only possible after extensive individual qualification. However, the toughness properties of the material are significantly underestimated in the notched bar impact test, so that hardly any statements can be made about the behavior in actual component use.

The aim of the project is to investigate the toughness behavior of MK-GJS under specific loading and under the influence of the material design. By directly transferring the findings to practical testing and design techniques, it will thus be possible to design components made of MK-GJS without doubt and adapted to the load case.

For this purpose, damage-mechanical, numerical as well as experimental approaches are pursued, which allow the characterization of toughness under the influence of temperature, strain rate, stress state as well as material design. In order to validate the investigations and to transfer them to practical testing techniques, an alternative test method is developed which is based on the impact tensile test and allows the toughness to be measured in a specific loading case.

Optiguss

Development of a holistic evaluation concept and a lifetime-based component design by means of a microstructure-dependent fatigue assessment of cast iron with lamellar graphite (AiF)

Duration: 01.04.2019 - 30.09.2021

Research institutes: Foundry Institute, Fraunhofer LBF, ACCESS e.V.

Due to the heterogeneous microstructure of lamellar graphite cast iron (GJL), uncertainties exist with respect to the assessment of local cyclic component stressability, which poses challenges to designers in the correct component design. While this problem can be circumvented by series foundries by means of elaborate investigations, SME foundries do not have this possibility due to the smaller batch size.

The aim of the project is to correctly describe the cyclic material behavior of GJL taking into account the size influence. In this context, the technological size influence of graphite is crucial, since the wall thickness dependence of its complex morphology significantly affects the properties.

For this purpose, microstructural parameters are specifically varied and characterized with respect to the fatigue strength. The development of a microstructure classification ensures the generation of robust correlations between microstructure and the corresponding cyclic properties. In this way, a methodology is developed with which components made of GJL can be designed to withstand stresses.

Development of the lightweight potential

Development of the lightweight potential of high-silicon cast iron with nodular graphite (AiF)

Duration: 01.01.2018 - 30.11.2020

Research institutes: GI

The project "Exploiting the lightweight potential of high-silicon ductile cast iron" will advance the application of high-silicon GJS materials in the lightweight sector by conducting a systematic investigation of casting and solidification properties in thin-walled geometries. A critical wall thickness with respect to mold filling as well as microstructure properties will be determined. In addition, optimization of feeder use in casting these materials will be achieved by exploring the expansion and contraction effects during solidification experimentally and by simulation, and by quantitatively analyzing the porosity volume occurring in thick-walled castings. The influence of silicon content, molding material strength, casting temperature and inoculation on casting properties will be investigated. This will provide a holistic process window for thin-wall casting of these materials and a quantitative correlation of the porosity volume as a function of the process parameters.

This will increase the competitiveness of the iron foundry industry by minimizing the use of feeders and by opening up new sales opportunities for its castings in the lightweight construction sector, where thin-walled castings made of GJS can substitute other materials such as steel and provide for weight reduction.