Temper Embrittlement Failure of Case Hardened Drill Rod

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