High Stress Nodular Iron Gears and Method of Making Same

Author:  Robert B. Grindahl
Source:  US Patent 4,222,793
Doc ID:  1979047
Year of Publication:  1979
Abstract:  
High stress, acicular bainitic, nodular iron gears having residual compressive stresses at the surface of the teeth's roots of at least about 40,000 psi and a subsurface residual compressive stress of about 10,000 psi at a depth between about .006 inch and .015 inch beneath the surface. Method for making gears including: casting nodular iron blank; heating blank to ferritize its microstructure prior to cutting teeth into the blank; heating it in a nonoxidizing environment to an austenitic phase dissolved-carbon-content of about 0.7% to aboutn 1.1%; rapidly quenching the austenitized casting to an acicular-bainite-forming isothermal transformation temperature; isothermally transforming the austenite at that temperature to at least 50% acicular bainite before cooling; and shot peening at least the roots of the teeth to impart the residual compressive stresses thereto.