Case Histories » Agriculture

Fatigue Failure

Fatigue fracture of steel pin
Fatigue fracture of an AISI/SAE 4140 low alloy carbon steel pin

Specimen

1.0-inch diameter x 2.75-inch square head steel pins from agricultural equipment.

Material

Specified as AISI/SAE 4140 low alloy carbon steel.

Background

The pins were specified to be carbonitrided to create a 0.003-inch thick, case-hardened layer for increased wear resistance. The pins were fracturing at a point of impact loading sites located approximately 1.5 – 2.0-inch from the head of the pin.

Findings

Examination of the pin fracture surface (shown above) revealed the presence of beach marks across approximately 90 percent of the pin cross-section. Beach marks are fracture surface features indicating progressive crack growth or fatigue crack propagation. The bolts were failing due to fatigue with the cracks initiating at a point of impact loading. Micro-hardness testing of the case hardened layer revealed a case hardness in the range of Rockwell C 40 – 50. Carbonitriding of the surface had increased the wear resistance but at the cost of reducing the impact strength. The hardened case did not have sufficient ductility to withstand the repeated impact/bending loads imposed upon the pins.