Improvement of the Wear Resistance and Particle Emissions of Hypereutectic Grey Cast Iron through Targeted Metallurgical Refinement

The control of automotive brake dust emissions released into the atmosphere has become a significant priority for Europe and in other parts of the world. Its origin is directly related to the wear experienced by the brake discs and pads during their useful service lives. Reducing dust emissions from braking presents a formidable challenge for the industry. Limiting the wear of the tribocouple, whilst ensuring ever greater and more stable friction performance for today’s modern vehicles, requires development work across many different areas. Solutions such as cold, thermal spray or laser cladding can increase the wear resistance of discs, although their commercial application is widely questioned due to substantial costs. A very interesting alternative is based on a metallurgical solution, through the addition of different alloying elements able to improve the wear resistivity of the castings. The goal of this research was to investigate the effect of certain contents of niobium (<0.005%, 0.27% and 0.74%), molybdenum (0.46% and 0.99%) and vanadium (0.26% and 0.54%) on the microstructure, mechanical properties, wear resistance and particle emissions of hypereutectic gray cast irons. A total of seven heats were melted. Two rectangular plates with dimensions 150x75x20 mm were produced for each heat. Samples were inoculated with 0.12 % of 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. Small additions of Nb seem to shorten the length of graphite compared to Mo and V, which elongate it. Some type B graphite was discovered in the sample with 0.54% V. Depending on the additions, various types of complex Nb-Mo-V carbides were observed in the matrix. Some of them are in direct contact with the graphite, restricting its growth. The microstructure was fully pearlitic in most of cases. Amounts of 11% ferrite and 7% ledeburite were detected for the highest contents of Mo and V, respectively. Significant improvements were achieved in the mechanical properties. The highest values were obtained for 0.54% V (224 MPa and 212 HBW) versus lowest values (116 MPa and 143 HBW) in sample without any special addition (<0.005% Nb). Mass loss and particle emissions were reduced significantly for all additions. Wear resistance was enhanced almost by 40% for 0.74% Nb. PM10 was reduced by 34% for 0.46% Mo.

Acknowledgements:

The authors would like to thank MAT Group for their support in this research work focused on product improvement, giving place to create a constructive teamwork where discussion and cooperation has been the driving force.