Abstract:
The in-service degradation of the mechanical properties of steel components through the damage
mechanism of high temperature hydrogen attack (HTHA), is a topic of concern in the refining
and green hydrogen industry. This damage mechanism occurs in susceptible steels operating in
environments at high temperatures and hydrogen pressures. The current investigation deals with
the indirect monitoring of mechanical degradation via tracking of the swelling strain in affected
structures. An autoclave with an AISI 316 shell was utilized to simulate accelerated HTHA damage
at 550 °C and 46 bar for exposure times ranging from zero to 700 hours. The progress of the HTHA
damage was tracked using encapsulated high-temperature strain gauges. The correlation between
the swelling strain and mechanical property degradation was studied to develop a methodology
for the continuous monitoring of embrittlement. The tensile sample orientation of the carbon steel
plate was included as a variable, i.e., samples parallel, transverse, or perpendicular to the plate
rolling direction were included, since it has been shown that the sample orientation influences the
HTHA damage features.
For the through-thickness orientation, and upon exceeding a threshold value of exposure time,
all tensile properties were severely degraded, with values in the order of a 90 per cent reduction
being observed.
It was found that the degradation of carbon steel mechanical properties can be correlated to the
swelling strain measured during exposure. The critical point for mechanical property degradation
in the plate through-thickness orientation, whereafter a severe decrease in the ductility of the
material occurs, was found to be in the order of 1% of the total swelling strain measured during
exposure, equivalent to 65 microstrain. This threshold was found to be significantly lower than that
expected from the literature, where limits of 400 to 1000 microstrain were postulated.
