Systems design of advanced gear steels

A new generation of Ni-Co secondary hardening gear steels has been developed using a systems approach. These high toughness ultrahigh-strength martensitic steels show great promise for demanding gear applications. Quantitative science-based modeling was used to create prototype alloys of superior strength and fatigue resistance over conventional steels.; Carburizing and strengthening models were developed to relate processing parameters to microstructure and microstructure to strength. The failure of the DICTRA software to accurately predict the carburizing behavior of Ni-Co steels led to a series of experiments to refine its kinetic database. New carbon diffusivities were calculated from the concentration gradients of carburized model alloys, resulting in a significant improvement of simulation accuracy. A structure/property model was created to equate the strength of a secondary hardening steel to the sum of the effects of solid solution, precipitates, dislocation density, and the substructure of the lath martensite matrix. The strengthening model was subsequently combined with the carburizing simulations to predict the hardness gradient in a case-hardened alloy based upon initial carburizing conditions. In addition, existing precipitation theory was used in conjunction with the microstructure/strength relationship to simulate the evolution of material hardness during secondary hardening.; The creation of three prototype gear steels began with the use of the strengthening model to establish the carbon and alloying element contents required to reach the core and case hardness objectives of 50 and 70 HRC respectively. The design approach also included the establishment of proper transformation and solution temperatures and the maximization of the efficiency of the M{dollar}sb2{dollar}C carbide strengthening dispersion. The core hardnesses of the C3-A and B prototypes significantly exceeded the design goal. A reduction in core carbon content from 0.16 to 0.12 weight percent was identified to reduce strength to attain the desired fracture toughness. The C3 alloys achieved a case hardness of 69 HRC, a 30 percent increase in strength over conventional steels. The prototypes' L{dollar}sb{lcub}10{rcub}{dollar} rolling contact fatigue lives more than double those of the M-50 and 440C bearing alloys.