HIGH TEMPERATURE HYDROGEN CRACK
HIGH TEMPERATURE HYDROGEN CRACK
High 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.
High 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 (HTHC).
Surface 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.
The 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).
In 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).
There 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 not visible.
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.
High temperature hydrogen crack (HTHC) 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.
Later-stage damage 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.
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