HIGH TEMPERATURE HYDROGEN CRACK
TEMPERATURE HYDROGEN CRACK
temperature hydrogen crack (HTHC), also called hot hydrogen crack, is a problem
which 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. It is not to be confused
with hydrogen embrittlement or other forms of low temperature hydrogen damage.
temperature hydrogen crack (HTHC) 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. This can result in either surface decarburization, when
the reaction mostly occurs at the surface and draws carbon from the material or
internal decarburization 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 which are typical of High temperature hydrogen crack
decarburization results in a decrease in hardness and increase in ductility of
the material near the surface. This is usually only a minor concern for these
types of application. However, internal decarburization, and in particular the
formation of methane and consequent development of voids, can lead to
substantial deterioration of mechanical properties due to loss of carbides and
formation of voids, and catastrophic failure.
main factors influencing High temperature hydrogen crack (HTHC) are the
hydrogen partial pressure, the temperature of the steel and the duration of the
exposure. Damage usually occurs after an incubation period, which can vary from
a few hours to many years depending on the severity of the environment. High
temperatures and low hydrogen partial pressures favour surface decarburization
while the opposite conditions (lower temperature, high hydrogen partial
pressure) favour fissuring. In addition, the composition of the steel
influences the resistance to High temperature hydrogen crack (HTHC); in
particular elements that tie-up carbon in stable precipitates such as Cr, Mo
and V are very important. Increasing content of such elements increases the
resistance to High temperature hydrogen crack (HTHC), and Cr-Mo steels with
more than 5% Cr, and austenitic stainless steels, are not susceptible to High
temperature hydrogen crack (HTHC).
1949, Nelson gathered and rationalized a number of experimental observations on
different steels. In the Nelson diagram, boundaries are placed in a
temperature/hydrogen partial pressure graph, which delineates the region of
safe use for carbon steels, 1.25Cr-0.5Mo steels, etc. This diagram has been
updated a number of times by the American Petroleum Institute (API) and
published in the API recommended practice 941. More recently, analytical models
have been used to predict the kinetics of High temperature hydrogen crack (HTHC)
with some success (Shih, 1982 and Parthasarathy, 1985).
is increasing concern that the Nelson curves may not be relevant for the newer
steels being used in high temperature hydrogen service, or may be overly
conservative, and there are increasing trends towards risk-based inspection of
items in hot hydrogen service. For information on how this approach could be
applied for your situation.
If steel is exposed
to very hot hydrogen, the high temperature enables the hydrogen molecules to
dissociate and to diffuse into the alloy as
individual diffusible atoms. There are two stages to the damage:
1. First, dissolved carbon in the steel reacts with the
surface hydrogen and escapes into the gas as methane. This leads to superficial
decarburization and a loss of strength in the surface. Initially the damage is
2. Second, the reduction in the concentration of
dissolved carbon creates a driving force which dissolves the carbides in the
steel. This leads to a loss of strength deeper in the steel and is more
serious. At the same time some hydrogen atoms diffuse into the steel and
combine with carbon to
form tiny pockets of methane at internal surfaces such as grain boundaries and
defects. This methane gas cannot diffuse out of the metal, and collects in the
voids at high pressure and initiates cracks in the steel. This selective leaching of carbon is a more
serious loss of strength and ductility.
can be managed by using a different steel alloy, one where the carbides with
other alloying elements, such as chromium and molybdenum, are more stable than
iron carbides. Surface oxide layers are ineffective as a protection as
they are immediately reduced by the hydrogen forming water vapour.
in the steel component can be seen using ultrasonic examination which detects
the large defects created by methane pressure. These large defects in a
stressed component are usually the cause of failure in service: which is
usually catastrophic as hot inflammable hydrogen gas escapes rapidly.
team well expect of manufacturing High Temperature Hydrogen Crack specimen for
Non-Destructive Testing. It is to evaluate the flaw using Non-Destructive Testing
Methods along with we are providing inspection report with photograph.