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

Abstract Non-metallic inclusions (NMI) within high-strength steels can be detrimental and significantly decrease fatigue life. Understanding the relationship between morphology and size distribution of NMI within a material and the material’s fatigue performance is critical for both design and material acceptance. To address this, three cleanliness levels of 4140 Q&T martensitic steel were compared through NMI characterization methods as well as rotating bending fatigue (RBF) testing. The inclusions of interest were both endogenous, which were found in the ultra-clean (U) and standard (S) conditions, and exogenous, which were found in the non-standard (N) condition. The study found a direct correlation between the cleanliness condition and fatigue life. It was also found, through a Weibull analysis, that the fatigue life predictability (represented by the Weibull modulus) of the U condition was similar but slightly less than that of the S condition; the N condition, which had the lowest cleanliness, had a significantly lower Weibull modulus than both the S and U conditions. Characterization was performed on all fracture surfaces, with a focus on the NMI that nucleated the fracture. Measured characteristics were then used in previously developed models to compare the model predictions to the experimental results. The endurance limit and fatigue life of specimens were underestimated by the models employed.