Austenitic stainless steels for non-magnetic drill collars (NMDC) may be susceptible to precipitation of Cr-rich phases at grain-boundaries during the necessary thermo-mechanical processing. This condition, known as sensitization, is detrimental to resistance to localized corrosion, stress-corrosion cracking and corrosion fatigue. API Specification 7-1 requires testing these materials for sensitization according to ASTM A262 Practice E. However, Practice E is considered incapable of discriminating sensitized microstructures in some NMDC grades. This led some users to rely on Practice A for rejection of material, an application that is outside its original scope. To find an alternative sensitization test method, results from ASTM A262 practices A, B, and E, ASTM G48 Method A, Charpy impact tests, critical pitting temperature, and cyclic potentiodynamic polarization tests were compared on two NMDC grades in the warm-worked, annealed and deliberately sensitized conditions. The unreliability of Practice E has been confirmed, while ASTM G48 Method A has shown promise as an effective test method. The latter also has the advantages of being quantitative and relevant to service conditions in oil and gas drilling.

Subject
Pits, Test methods, Materials, Microstructure, Sodium chloride, Corrosion detection, Impact toughness, Sensitization, Pitting, Corrosion resistance, Localized corrosion, Stainless steel, Cracks
Keywords:
non-magnetic drill collars (NMDC), austenitic stainless steel, sensitization, intergranular corrosion, localized corrosion
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2020
Association for Materials Protection and Performance (AMPP)
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