Temper Embrittlement Failure of Case Hardened Drill Rod
4/22/2019

Side view of a hex coupling fractured immediately upon use.










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.

Material
ANSI 8620

Chemistry
Carbon (wt%): .22
Manganese (wt%): .81
Phosphorus (wt%): .011
Sulfur (wt%): .015
Silicon (wt%): .28
Nickel (wt%): .65
Chromium (wt%): .56
Copper (wt%): .045
Molybdenum (wt%): .195
Vanadium (wt%): .009

Application
Drilling of marine piling moorings

Product
Hex Drive Drill Rod

Properties
Tensile Strength (ksi, min): 101-135
Yield Strength (ksi, min): 71
Elongation 2″ (% min): 10
Reduction Area (% min): 9
Hardness (HB, max): 32

Microstructures
Martensite, bainite and carbide precipitates