AGMA 17FTM18 PDF

The fatigue performance of three sets of quench and tempered 4140 steel samples, representing three distinctly different inclusion populations—low oxygen / low sulfur, high oxygen / low sulfur and low oxygen / high sulfur—were evaluated through a series of various bending fatigue tests. Three different bending fatigue tests with differing stress ratios were employed—rotating bending (−1), single tooth bending (0.1), and modified Brugger (0.1). The inclusion populations for each of the three steel sample sets were characterized using both an SEM-based image analysis system, primarily for the micro-inclusions, and a high-resolution UT system for the macro-inclusions. All three sample sets were evaluated using both longitudinal and transverse specimens in all the bending fatigue tests. The transverse samples displayed significantly lower fatigue performance (typically ~50% lower fatigue strength values) than the longitudinal samples. Furthermore, the high sulfur sample set clearly had the lowest performance in the transverse orientation. While there was more scatter with the data on the longitudinal samples, the high oxygen sample set had a lower fatigue strength and a higher percentage of the failures initiating at subsurface oxides than the other two sample sets.