High temperature hydrogen attack (HTHA) can be a threat to the integrity of non-post-weld heat treated (PWHT’d) carbon steel piping exposed to hydrogen at elevated temperatures. Historically, this damage mechanism has been addressed by operating within an envelope of temperature and hydrogen pressure defined by API(1) 941. Over recent years, HTHA has been identified in non-PWHT’d carbon steel piping, operating in regions that were previously defined by API 941 as not being susceptible to this mechanism. UNS K03006 (ASTM(2) A-106 Grade B) is known to undergo HTHA depending on service conditions. The fracture properties of a vessel with HTHA need to be better understood.

A testing program was conducted to evaluate the environmental fracture properties of ex-service material at typical operating temperatures. As operation is well on the upper shelf, the program focused on the tearing characteristics of carbon steel exposed to conditions where hydrogen attack was likely to occur. Testing was conducted at temperatures of 600°F (316°C) and 32°F (0°C) with and without hydrogen. After testing, the fracture morphology and extent of hydrogen damage was documented using a scanning electron microscope (SEM). In addition to the environmental fracture properties testing, a hydrogen damaging study was performed by exposing small coupons to 800 psig (5.52 MPa) hydrogen at a temperature of 800°F (427°C) for different time durations. After exposure, the extent of damage was documented using a light microscope and an SEM. The test methodology and results are discussed in this paper.

Subject
Scanning electron microscopy, High temperature hydrogen attack, Test methods, Materials, Hydrogen damage, Autoclaves, Stress cracking, Intergranular cracking, Nitrogen, Carbon steel, Fracture, Damage, Hydrogen
Keywords:
hydrogen, high temperature hydrogen attack, HTHA, carbon steel, fracture toughness, API 941, Nelson Curves
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2019
Association for Materials Protection and Performance (AMPP)
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