De-coupling Case-Core Flow Stress Behavior in Carburized AISI 9310 Steel

Determination of flow stress behavior of materials is a critical aspect of understanding and predicting behavior of materials during manufacturing and use. However, accurately capturing the flow stress behavior of a material at different strain rates and temperatures can be challenging. Further, carburized steel poses additional challenges due to the extreme mechanical properties differences between the carburized case and higher toughness core. Non-uniform deformation and thermal gradients during testing make it difficult to match test results directly to constitutive equations that describe the material behavior. Typically, the manufacturing sequence of AISI 9310 steel incorporates normalizing, rough machining, carburizing, austenitizing, quenching, cryo-treatment, tempering, and concludes with finish machining. The process induces thermal and mechanical stresses to the material that can have an impact on final part properties. In this study, AISI 9310 steel rods were heat treated prior to thermomechanical testing as follows: (I) normalized in accordance with AMS 6265, (II) carburized (case depth of 0.014” – 0.026”) with partial thermal processing or (III) carburized (case depth of 0.014” – 0.026”) with the full thermal processing sequence. A Gleeble 3500 physical simulator was used in conjunction with a Correlated Solutions Digital Image Correlation system to capture flow stress behavior at room temperature and elevated temperatures (300oC – 600oC). Additionally, strain rate was controlled (0.01 s-1 to 0.1 s-1) using various strain control methods. The differences between varying test modes and the impact of the resulting data that describe the flow stress behavior are assessed through fractographic and metallographic characterization for a comprehensive analysis.