Fracture characterization of new generation ferritic spheroidal cast irons

Nearly all cast parts must undergo a machining process in order to be used in service. This work is exclusively targeted to spheroidal iron casting Grades that are demanded by the automotive industry, due to the intense machining operations that are performed on them. Chip removal processes on ferritic-pearlitic cast irons suffer from frequent incidents, mainly related to the dependence of pearlite development with component thickness. Setting aside the inherent carbide formation risk, it can be stated that, for minimum thickness sections, increases in pearlite concentration promote machining difficulties. This paper deals with part typologies that frequently show small thicknesses and, in consequence, pearlite is significantly present.

Providing solutions to the major machining companies, new spheroidal iron casting Grades have been developed with different mechanical properties, whose key characteristic is their ferritic structure.

These new casting Grades have motivated the FERRICAST R&D Project, which brings together three fields of knowledge that are complementary: casting technology, metallurgic key points and machining optimization for the new ferritic spheroidal iron casting Grades. This paper is specifically focused on the metallurgical characterization of the mentioned ferritic Grades, taking special care of their fracture mechanics behavior. Grades GJS-500-7 and GJS-600-3, both frequently used in automotive applications have been taken as reference in the inter-comparison tests with the new ferritic Grades. 

Besides the conventional mechanical testing, the aforementioned metallurgical characterization has been focused on fatigue and fracture behaviors. Several melt batches have been prepared with variable concentrations of silicon and micro-alloying elements and, in order to extract the testing specimens, different thermal module blocks were chosen. The response to the presence of cracks has been studied according BS7448, calculating the critical CTOD or KIC.

The most relevant conclusions of this work are related to the metallurgical properties of the studied ferritic Grades. Both their strengths and limitations have been assessed together with their feasibility for being employed as materials in safety critical parts.