Predicting Distortion and Residual Stress In a Vacuum Carburized and Gas Quenched Steel Coupon

Aerospace transmission components are typically manufactured from high strength, case carburized alloy steels such as AMS 6308 (Pyrowear53). The combination of carburization and quench hardening of these steels produces residual compressive surface stresses and high surface hardness, thus enhancing both surface durability and fatigue resistance. The hard case, coupled with a tough non-carburized core, provides the foundation upon which additional processing can further improve surface fatigue response. An internal state variable (ISV) material model for carburized and heat treated gear steels has been implemented into the DANTE heat treatment simulation software for the purpose of engineering microstructural, residual stress and distortion response to meet specific steel component application requirements. This paper describes the use of heat treatment simulation to engineer residual stress and distortion response in an AMS 6308 alloy steel coupon to subsequently be used for fatigue testing. The criticality for accurate use of process-descriptive boundary conditions is presented in the context of vacuum carburizing and gas quenching. Model predicted residual stress and distortion response for a tapered, notched coupon are validated against x-ray diffraction and dimensional physical testing. Introduction Precision engineered gears are a critical component in rotorcraft transmission systems. These gears provide transfer of power from the horizontal drivetrain to vertically mounted rotor shafts, enabling high speed and torque with non-parallel input and output. The increasing performance requirements in both military and commercial rotorcraft necessitate improvement in transmission power density (horsepower/lb) capabilities. This provided the incentive for steel manufacturer’s to shift aerospace steel production and processing towards use of specialized high strength alloy steels. These steels are characterized by a combination of high strength and high toughness, and are noted for their combined use of a carburized case and fine alloy carbide dispersions to achieve these properties. [1-3] Carburizing steels with high alloy content are both an attractive and economically affordable alternative to addressing challenges related to the durability and power density of components used in transmission gears. The carburized case provides high strength and wear