Effect of Carbide Segregation on Mode I Fatigue Resistance Properties of the Bearing Tool Steel Roll Blade Die

Abstract

Roll blade die (RBD) is a carburized tool steel containing 10 weight percent (wt%) tungsten that was initially designed for forging gas turbine engine blades. It therefore has very high hardness at room and elevated temperatures, resistance to wear, and low distortion. Due to high tungsten content, RBD can exhibit segregation of tungsten carbide in the form of randomly distributed coarse carbide stringers (greater than 600 µm in length) instead of the usual more uniform distribution of finer (less than or equal to 4.3 µm in equivalent circle diameter) carbides within the martensitic structure. Individual tungsten carbides within these coarse stringers can have sizes up to 14.7 µm in equivalent circle diameter (ECD). As a result of such segregation, these carbide stringers have been shown to be visible as an indication on the working surface of a bearing track when subjected to magnetic particle inspection (MPI) at high current flow. This paper presents the use of Mode I fatigue testing to test such indications and compare the results with identical indication free samples. Mode I was chosen as it had the advantage of maintaining the MPI indications in their original condition and it was also considered pessimistic with regard to any potential fatigue deficit observed compared to the rolling contact fatigue. The Mode I fatigue test results demonstrated no reduction in fatigue resistance in the samples with MPI indications compared to the identical indication free samples. Fractography revealed that non-metallic inclusions were located at the fatigue crack initiation sites. These findings suggest that the presence of MPI indications, which are associated with the coarse tungsten carbide stringers, had minimal effect on the fatigue resistance properties of the RBD tool steel under Mode I conditions.