A 4150 steel pin was received for failure analysis; the fracture was determined to be due to rotating bending fatigue.
The sample was consistent with the supplied chemistry specification with the exception of a high sulfur value. This implies that the material was resulfurized for machinability.
The fracture occurred at a machined radius near the center of the pin. The machined radius exhibited rough machining lines which may have acted as crack initiation sites. A finer finish on the part would likely result in improved fatigue performance.
Image Captions (left to right)
- Optical micrograph of the fracture surface. Beach marks (examples indicated by red arrows) are characteristic of fatigue.
- SEM view of the fracture surface with deep machining mark at the edge.
- Wear marks on pin surface near the fracture.
- Microstructure in the etched condition (2% Nital) consists of primarily martensite with sulfide inclusions