Pin Fatigue Failure Analysis

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.


Optical micrograph of concave fracture surface_0

  1. Optical micrograph of the fracture surface. Beach marks (examples indicated by red arrows) are characteristic of fatigue.



SEM view of fracture surface. Note what appears to be a deep machining mark along the edge. _0

  1. SEM view of the fracture surface with deep machining mark at the edge.




Wear marks on pin indicating rotational forces were applied to the OD_0


Wear marks on pin surface near the fracture.


Microstructure in the etched (2% Nital) condition consists of primarily martensite with sulfide inclusions_0
Microstructure in the etched condition (2% Nital) consists of primarily martensite with sulfide inclusions.