Failure Analysis Case History No. 012

Industry: Electric power generation
Specimen Description: Turbine gearbox pinion gear
Material: 17CrNiMo6 low alloy steel, case hardened by gas carburizing
Environment: Lubricating oil
Background:
A double-helical pinion gear in a gas turbine gearbox developed pits on the loaded teeth. A laboratory failure analysis was conducted to determine the cause for the gear teeth pitting.
Time in Service: several years
Findings:
The pinion gear suffered random, open progressive pitting on the loaded flanks of the teeth. Other areas of the loaded gear teeth exhibited visual images of subsurface fatigue cracks that had not developed into open pits yet. The companion bull gear had not pitted. Microstructural investigation of a representative subsurface pit showed that it propagated parallel to the case hardened surface and contained white etching areas (WEA's). Other subsurface, intergranular cracks were also present that had initiated at inclusions.

Results of the failure analysis showed that the gear tooth pitting was due to surface-contact fatigue cracking. The WEA's have been described as "butterfly wings" and white bands of altered martensite. They reportedly occur in gear teeth that have experienced heavy shear or impact loads.

 

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