Specimen: 1.0-in. diameter x 2.75-in. square head steel pins from agricultural equipment.
Material: Specified as AISI/SAE 4140 low allow carbon steel
Background: The pins were specified to be carbonitrided to create a 0.003-in. case hardened layer for increased wear resistance. The pins were fracturing at a point of impact loading approximately 1.5 – 2.0-in. from the head of the pin.
Findings: Examination of the pin fracture surface (shown above) revealed the presence of beach marks across approximately 90% 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 impact strength. The hardened case did not have sufficient ductility to withstand the repeated impact/bending loads imposed upon the pins.