A Good Control of the Microstructure as a Potential Tool to Improve Wear Behavior of Hypereutectic Grey Cast Iron

Strict control of polluting emissions from braking systems, (regulated by EURO 7, mandatory from 2027) is a matter of vital importance. The need to mitigate the effects of climate change as well as the emission of particles into the atmosphere, which are linked to respiratory diseases, cardiovascular problems and premature deaths, requires the development of new materials that are more environmentally friendly. For brake rotors in particular, elevated wear resistance of gray cast iron could be an essential way to extend product service lives, reducing the emissions and enhancing operating performance. An exhaustive control of the morphology of graphite and microstructure is also a potential and economical alternative to increase casting quality. Customized contents of certain alloying elements such as chromium and tin, the application of heat treatments like normalized or different austempered gray iron, and the special addition of nanoparticles of SiC, WC and TiC, are proposed as possible solutions to improve the wear behavior. The goal of this research is to analyze the effect of all these techniques on the microstructure and therefore, on the mechanical properties, wear resistance and particle emissions. A total of twelve heats were melted. Two rectangular plates with dimensions 150x75x20 mm were produced for each heat. Samples were inoculated with 0.12 % with a ZrMn inoculant. Metallographic analysis and SEM studies were conducted. Wear resistance was evaluated as mass loss by means of pin-on-disc (PoD) testing. The concentration of airborne wear particles (PM10) was measured continuously during testing.

The morphology of the graphite was slightly modified, but type A was predominant in all cases. Some type B was discovered in the sample alloyed with 0.52% Cr. The microstructure was fully pearlitic in most of cases, except in the austempered samples, where a matrix formed with ferritic needles combined with austenite enriched was observed. A significant number of carbides type MC (with a face-centered cubic crystal structure) and M7C3 (hexagonal crystal structure) were noticed in alloyed samples with Cr. Remarkable amounts of ledeburite (35%) were detected for the highest contents of Cr. Additions of nanoparticles promote considerably the number of non-metallic inclusions in the matrix, thus enhancing the nucleation potential of the material. Mechanical properties were increased for all additions. The highest values were obtained for high contents of Cr (277 MPa and 352 HBW) because of the presence of complex Cr-Nb carbides. The formation of an ausferritic matrix also promotes improvement of hardness and tensile strength (199 MPa and 202 HBW). Superior wear behavior was discovered for the majority of microstructures, improving by almost 30% and 50% for 0.52% Cr and 0.10% Sn respectively.

The main limitation of the research lies in the fact that the conclusions obtained are directly related to the final morphology of the graphite. In this case, the analysis has been done for castings with a low thermal modulus and a hypereutectic composition, and the extrapolation to a large section and/or to hypoeutectic compositions would require additional tests, because the graphite formed would be completely different. Differences in test conditions on a tribometer or on a dynamic test bench are also relevant. This study complements the research presented last year at EUROBRAKE 2024, considering the presence of different alloy elements (Cr and Sn), as well as new alternatives such as heat treatments and the addition of nanoparticles, which try to improve the wear resistance in a more economical approach, reducing the need for finishing operations such as laser cladding or grinding. The promising results obtained offer the possibility to establish a good knowledge of final microstructure and of the morphology of graphite as a powerful tool to control tribological behavior. EURO 7 compliance, without the need for additional expensive industrial processes and highly specific equipment, only playing with the metallurgical quality, seems ever closer.