Effects of Inclusion Properties on Fatigue Behavior of High Strength Bar Steel

Non‑metallic inclusions (NMI) within high-strength steels, specifically oxide inclusions, can be detrimental and significantly decrease fatigue life. Understanding the relationship between the morphology of inclusions within the material and the fatigue performance is critical from both a design and material acceptance perspective. To address this, the study aims to compare three cleanliness levels of 4140 Q&T martensitic steel. Each material group’s inclusion populations were characterized by utilizing two different methodologies, ultrasonic testing and an automated SEM analysis. Specimens were tested in rotating bending fatigue above their experimentally found fatigue limit, and the results were compared utilizing a Weibull analysis. Fractography and inclusion characterization were performed on all fatigue failures with results being integrated into previously developed mathematical models. These models determine the local stress at the NMI and predict the fatigue limit of the material given the size of the inclusion and properties of the matrix. A comparison of models to predict the fatigue limit was conducted for each material group with a focus on the assumed failure mechanisms for each model. With a decrease in cleanliness, there was an increase in average inclusion size and a decrease in the cycles to failure and fatigue limit.