Temper Embrittlement Failure of Case Hardened Drill Rod

Marine Construction

4/22/2019

Side view of a hex coupling fractured immediately upon use.

Bottom view of a hex coupling. Cracks propagating across the face shown.

Additional piece of fractured hex coupling.

The darker etched area highlights the case hardened cross sectional area of the part.

Carbide precipitates and cracks filled with mounting epoxy.

Cross section of cracked coupling. The black fill is mounting epoxy. Measured case hardness depth was 0.055''.

Martensite-bainite crystalline structures seen in the base material cross section. These phases occur from rapid cooling after the case hardening process.

Carbide precipitates were found through out the part cross section along grain boundaries. Observed cracks propagated along precipitates.

Another larger crack in the part. Note the small grain size immediately surrounding the crack.

Observations

The drill rod knuckle assembly failed after less then 2 hours of use. Multi directional cracking occurred along grain boundary precipitates leading to catastrophic failure. After the case hardening, tempering was performed at 550 F leading to extensive carbide precipitation commonly known as blue brittleness. Recommendations were made to lower tempering temperatures while extending hold times.