High-temperature hydrogen attack (HTHA) is defined as a reduction of strength and ductility of steel by high temperature reaction of absorbed hydrogen with carbides in the steel resulting in decarburisation and internal fissuring. HTHA can occur when steel is exposed to environments containing dissociated (atomic) hydrogen at elevated temperatures and pressures. At elevated temperatures, some of the molecular hydrogen present in a process environment dissociates into atomic hydrogen. Whereas molecules of hydrogen are too large to enter the steel, the atomic hydrogen readily dissolves in and diffuses through the steel. Two types of deterioration of steel occur in elevated temperature hydrogen service – surface decarburisation and internal decarburisation/fissuring.
Surface decarburisation is characterised by a decrease in carbon content of a shallow layer on the process-exposed surface of the steel. The usual effects of surface decarburisation are a slight reduction in strength and hardness and an increase in ductility. Surface decarburisation does not produce fissures and therefore it is not of significant concern. However, its presence signals borderline conditions for potential internal decarburisation and fissuring. Internal decarburisation/fissuring is the predominant mode of deterioration when operating near or above the Nelson curve. Under these conditions, the atomic hydrogen reacts with the iron carbides, forming methane. Molecules of the methane gas formed are too large to diffuse out of the steel and accumulate at grain boundaries or other imperfections.
HTHA is generally preceded by an incubation period during which sub-microscopic damage (size far less than 10 micrometer, density increasing in time) is occurring but no noticeable change in steel properties can be detected. After the incubation period, the deterioration of carbides and the loss of carbon (decarburisation) results in an irreversible reduction of the strength of the steel, and sufficient internal stresses eventually build up to produce microfissures (size 10 to 50 and over micrometer, density many per cubic mm). As the damage progresses, microfissures interconnect to form macroscopic fissures. In the advanced stage of attack, the presence of numerous fissures produces a substantial deterioration of mechanical properties such as tensile strength, hardness, and ductility. This condition is of significant concern with respect to pressure equipment integrity and it may ultimately lead to in-service failure by brittle fracture.
The permeability of hydrogen in steel is equal to the product of its solubility and diffusivity. The solubility of hydrogen in austenitic stainless steels is about an order of magnitude greater than that for ferritic steels, but the diffusivity of hydrogen through austenitic stainless steels is about two orders of magnitude lower than that for ferritic steels. Therefore, an austenitic stainless steel cladding/weld overlay would be expected to reduce the effective hydrogen pressure acting on the underlying base metal. Ferritic or martensitic stainless steel cladding would not be expected to provide a similar benefit. Several cases of HTHA of base metal beneath austenitic stainless steel cladding have been documented.
Inspection options: The inspection options are dependent on the objectives (“type of damage”) and access possibilities. If the objective of the inspection is the detection of internal surface decarburisation or internal surface breaking macroscopic fissures (or “blistering”) caused by internal decarburisation, then internal access is required/ recommended. If the objective is to detect all stages of internal decarburisation, including internal (and internal surface breaking) blistering, then internal access is not required.
However establishing reliably whether internal cracking of the blistering is present in the weld area (including HAZ) and thus to the internal surface weld, opening is a pre requisite. Another important boundary condition that has to be considered is the presence of an internal clad layer.
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