Silicon acts as ferrite forming agent. It increas- es the strength and hardness through a solution strengthening effect, generating a ferritic matrix with tensile strengths comparable with those of ferritic/pearlitic grades. Once the Si content ex- ceeds 4.3%, the tensile and yield strength values decrease markedly.


Material grades solution strengthened through the addition of Si provide the potential of using lower cost, low-alloyed scraps in the metallic charge, without the risk of pearlite or carbide for- mation in the microstructure. For example, Mn contents up to approximately 1% and Cr contents of a few tenths of a percent are tolerable. In a particular case of application, a foundry reported material cost savings by 5% through the use of the new material.


Optimal graphite shapes and hence optimal me- chanical properties can only be achieved with inoculation techniques properly adapted to the higher Si content and the maximum solidification time. Especially the Bi-containing inoculants have proved successful here. For solution strengthened ductile cast iron grades it is more important that an appropriate inoculation technique is chosen, because the high Si content may cause deviating graphite shapes.


The production of castings can be optimized in terms of material properties as well as production and machining costs. Optimization means making use of the more favourable mechanical properties either in the form of small wall thicknesses (light weight con- struction) or higher loadability of the castings without the need to change the geometry. The up to 20% higher yield strength allows the wall thicknesses to be designed accordingly smaller. Moreover, machining the solution strengthened materials is less costly because tool wear is lower.


Casting properties, especially the susceptibility to porosity, do not change with increasing Si content. Existing pattern and feeding systems used for conventional casting alloys may not need to be modified for the new materials.


From the casting technological tests it can be concluded that the new grades show low risk of increased dross formation. Especially at room temperature and slightly elevated tempera- tures, the investigated new ductile iron grades exhibit good cyclic properties as a result of the completely ferritic matrix. All these aspects give these new materials a very high poten- tial for future application in automotive engineering.


Figure 12. Stress-cycle-diagramm of the ferritic material GJS-500-14; fatigue strength: 40624 psi (P=50%).


Figure 11. Stress-cycle-diagram of the ferritic/pearlitic material GJS- 500-7; fatigue strength: 37235 psi (P=50%).


REFERENCES


1. DIN EN 1563: 2012-03: Gusseisen mit Kugelgraphit (2012). (German Institute for Standardization— Deutsches Institut fΰr Normung)


2. Petzschmann, U.; Stets, W.; Wolf, G.: International Foundry Research 63 (2011), No. 4, S. 8-19.


3. The British Foundryman (1964), S. 437. 4. Björkegren, L.-E., Hamberg, K., Johannesson, B.: Gießerei-Praxis Nr. (1/1999), p. 11-17


5. Hummer, R.: Praktische Anwendung der Sauerstoffaktivitätsmessung zur Qualitätssicherung von Schmelzen für Gusseisen mit Kugelgraphit. Giesserei 78 (1991), No. 24, p. 884-88.


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International Journal of Metalcasting/Volume 8, Issue 2, 2014


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