[2A6] Advanced ultrasound methods for HTHA inspection
S Couture
Evident Scientific, Canada
High-temperature hydrogen attack (HTHA), also called hot hydrogen attack, is a problem that concerns steels operating at elevated temperatures (typically above 400°C) in hydrogen environments, in refinery, petrochemical and other chemical facilities and, possibly, high-pressure steam boilers.
HTHA is the result of hydrogen dissociating and dissolving in the steel and then reacting with the carbon in solution in the steel to form methane bubbles. This can result in either surface decarburisation, when the reaction mostly occurs at the surface and draws carbon from the material, or internal decarburisation, when atomic hydrogen penetrates the material and reacts with carbon to form methane, which accumulates at grain boundaries and/or precipitate interfaces and cannot diffuse out of the steel. This causes the fissures and cracking that are typical of HTHA. The main challenge is characterisation, to help evaluate fitness for service and avoid false positives: small cracks are difficult to distinguish from inclusions; HTHA-induced cracking is hard to distinguish from other damage mechanisms; and defining cracks and clusters requires very fine resolution.
This presentation reviews some common challenges and key considerations of HTHA inspection, goes over the API standard updates and explores the potential of recent innovations in probe design and signal processing.
HTHA is the result of hydrogen dissociating and dissolving in the steel and then reacting with the carbon in solution in the steel to form methane bubbles. This can result in either surface decarburisation, when the reaction mostly occurs at the surface and draws carbon from the material, or internal decarburisation, when atomic hydrogen penetrates the material and reacts with carbon to form methane, which accumulates at grain boundaries and/or precipitate interfaces and cannot diffuse out of the steel. This causes the fissures and cracking that are typical of HTHA. The main challenge is characterisation, to help evaluate fitness for service and avoid false positives: small cracks are difficult to distinguish from inclusions; HTHA-induced cracking is hard to distinguish from other damage mechanisms; and defining cracks and clusters requires very fine resolution.
This presentation reviews some common challenges and key considerations of HTHA inspection, goes over the API standard updates and explores the potential of recent innovations in probe design and signal processing.
