SUMMARY

High strength secondary hardening martensitic gear steel is a strong candidate for high performance and reliable transmission systems in aircraft and automotives. The fatigue life of this material depends both on intrinsic microstructure attributes, such as secondary precipitates, and extrinsic attributes such as nonmetallic primary inclusions, as well as on processing steps. Case hardening treatments such as carburization and tempering, and surface treatments such as shot peening have been widely utilized to improve the high cycle fatigue (HCF) performance of gear steels. It is of prime importance to understand the relevant damage mechanisms influenced by residual stresses and the property gradients induced through heat treatment and surface treatments, and relate them to the expected fatigue life of the component. At the grain scale the selected martensitic steel is comprised of lath martensite formed within prior austenite grains. Detailed experimental investigations reveals a hierarchy of the lath martensite microstructure comprised of sub micron size laths and carbides (3-100 nm), and blocks and packets of laths (>1μm) within the prior austenite grain. Also, within high strength martensitic steels, primary non-metallic inclusions (>5μm) are common sites at which fatigue cracks form. The primary objective of this research is to develop a computational framework to quantify the influence of both extrinsic (e.g., inclusions) and intrinsic (e.g., laths and carbides) microstructure attributes on fatigue crack formation and early growth phases that dominate HCF in processed martensitic gear steels.