Effect of Sour Environment pH on Crack Morphology in Ultra-High Strength Drilling Steel Under Cyclic Stress

Abstract

Catastrophic failure of a pre-cracked compact tension UD-165 sample after a cyclic stress versus frequency experiment in a 5% NaCl solution buffered with CO2/NaHCO3 and in contact with 0.83 MPa H2S at 20°C, pH=7, occurred in air. Only fatigue crack propagation was observed in a similar sample after a similar test in the pH=12 solution. The log fatigue crack rate measured as a function of log frequency for UD-165 in the pH=7 solution was linear with a gradient of -0.492 while in the pH=12 solution exhibited a plateau indicating different mechanisms based on fracture mechanics theory. The microscopic investigations of the sample after testing at pH=7 revealed the presence of fatigue cracks, initiated by sample pre-cracking. Also, on the surface of the catastrophically fractured base metal, pits and other secondary cracks were observed. The latter propagated along the beach marks. The surface analysis of UD-165 after testing in pH=12 solutions showed corrosion products covering the crack but not crack tip. Intergranular cracks were found on the surface of the fractured base metal in liquid nitrogen. The microscopic surface investigations corroborate the fracture mechanics results.