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Forging simulations with a 1522 steel microalloyed by additions of 0.25% Mo, 0.13% V and 0.01% Ti were performed on a laboratory thermomechanical processing simulator. The forging conditions included a strain rate of 22s-1, 50% strain, and temperatures in the range from 1200°C to 950°C. The true stress was found to increase with decreasing deformation temperature for all values of instantaneous true strain. The maximum flow stress increased two-fold as deformation temperature decreased from 1200°C to 950°C, and the recrystallized austenite grain size decreased by a factor of two for this same decrease in temperature. Microstructures evolve from bainitic/ferritic at a cooling rate of 1.4°C/s, to fully martensitic at 16.8°C/s, independent of deformation temperature. Room temperature hardnesses depended primarily on cooling rate and were essentially independent of deformation temperature.

This study evaluated the bending fatigue performance of a modified SAE 4320 steel as a function of carburizing technique. S-N curves and endurance limits were established by fatigue testing modified Brugger-type specimens that are designed to simulate a single gear tooth. Fractured specimens were examined by light and electron microscopy to determine crack initiation sites, establish the extent of stable crack propagation, and analyze surface oxide types and distributions. Test results show that plasma-carburizing boosted the endurance limit of an oxidation-susceptible gear steel from 1100 MPa to 1375 MPa. Fatigue endurance limits in excess of 1400 MPa had previously been achieved in gas-carburized SAE 4320 steels by reheat treatments and reductions in high-oxidation potential elements. The level of improvement observed in this study suggests that any of these advanced processing techniques can allow significant size reductions and weight savings in automotive transmission gears.