Failure investigation of gears on a tyre manufacturing machine

• Chevron fracture surface markings. • Intergranular oxidation. • Carburising case hardening. • Intergranular cracking leading to fatigue. • Gear failure. This failure investigation deals with an apparently straightforward case of gear failure that was undertaken to see whether it contained any cracking that pre-existed the final fracture of a tooth. Some interesting features were observed in this case, as there were several cracks in the gear besides the primary one that led to fracture, showing evidence of at least two different crack initiation mechanisms. These mechanisms were intergranular quench cracking, occasionally involving large inclusions where chevron markings were seen on the ductile fracture surface that pointed back to the local fracture origin, and the observation of very localised intergranular oxidation in the carburised case of the gear that occurred during heat treatment. Both mechanisms of crack initiation then led to fatigue cracking. The final conclusion was that the gears failed prematurely from fatigue cracking initiated by intergranular oxidation combined with an impact load.