Through-Bolt Nut Fracture, Teledyne Continental Motors TSIO-520 M

During the assembly of Teledyne Continental Motors TSIO-520 M
engine s/n 291531R, a number of twelve point nuts (p/n 652541) were
found fractured 14 hours after they had been tightened by applying
the specified torque (65.8 67.5 ft lbs). The engine assembly
process had not been completed.

It was reported that the nuts had been replated with cadmium in
accordance with MAF Aviation Services Engineering Order 0071-00 E1.
One failed nut and a serviceable nut from the same batch were made
available to the ATSB for examination.

Fracture of the failed nut occurred on several longitudinal
planes. Fracture initiated from regions of intergranular cracking
in the increased section created by the nut flange. Final fracture
occurred in a ductile manner.

These features are typical of hydrogen-induced delayed cracking
(hydrogen embrittlement), a condition resulting from the presence
of hydrogen in the steel and a sustained tensile stress. The effect
of thread shape creates a sustained tensile hoop (circumferential)
stress in a tightened nut. A particular hazard of cadmium plating
high strength steels is the absorption of hydrogen into the base
metal. This hazard is countered by baking the parts after
plating.

In order to prevent recurrence it is necessary to establish:

• If the hydrogen-removal baking heat treatment had been applied
  to the nuts.
• If the baking process was controlled in accordance with process
  specifications. For example was the temperature distribution
  throughout the batch uniform, and was the time of exposure to the
  specified temperature the same for all nuts in the batch.
• If the engineering work order had taken into consideration that
  the twelve-point nuts are heat treated to a higher strength than
  the six-point nuts they replace. Longer baking periods may be
  required to eliminate the hazard of hydrogen embrittlement in
  higher strength steels.