HTHA occurs when gaseous hydrogen (H2) diffuses through the steel and reacts with carbon (C) within the metal matrix to form methane gas (CH4). Once the methane gas is formed, the CH4 molecules are too large to diffuse back out of the steel and begin to form micro voiding at the grain boundaries. Given enough time, the micro-voids will grow into micro-fissures, then into micro-fissures to macro-fissures, and eventually coalesce into crack-like flaws. Additionally, as the carbon reacts to form methane, the steel will also decarburise, or deplete the carbon within the steel matrix, which would further weaken the material properties. Again, HTHA damage also reduces fracture toughness, which renders equipment more susceptible to brittle fracture.
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