| Rolling contact fatigue behavior of carburized C69-1 steel was measured and analyzed using an NTN rolling contact fatigue tester. Core precipitation of nanoscale 6 phase in C69-2 steel was measured with 1DAP microanalysis. Precipitation behavior in M50NiL-0.38C was examined using small angle neutron scattering, transmission electron microscopy, one-dimensional atom probe microanalysis, three-dimensional atom probe microanalysis, Vickers microhardness, and ThermoCalc thermodynamic modeling software. Five different carbide phases were tentatively identified as Fe3C, M2C, MC, M6C, and M 23C6. The hardness evolution was modeled with the measured microstructural data and scaled to measured microhardness. A multiphase precipitation model was developed to predict the volume fraction of each phase during tempering. Stress relaxation during tempering of M50NiL-0.38C was shown to be controlled by carbide precipitation kinetics using tensile and split-ring methods.; From these experiments design principles for advanced carburized steels were deduced. Because of their role in fatigue nucleation, no primary carbides should be present after solution treatment. A single phase M2C precipitate dispersion should be over-aged to be slightly larger than its peak strength state to avoid cyclic shearing and improve rolling contact fatigue resistance. Other carbide phases can be avoided because they are less efficient strengtheners than the M2C phase. The embrittling sigma phase should be avoided in the low carbon core by reducing the driving force for precipitation. The steel should have some residual austenite in the carburized case after quenching from the solution treatment; this retained austenite should be completely transformed upon a cryogenic treatment after tempering to restore favorable, residual compressive stress in the case. |
