Hydrogen attack is one of the most important problems with materials used in ammonia synthesis, oil refining, and coal gasification equipment. This is Piping & Fabrication and will continue the post, The first major failure of an ammonia convertor attributable to hydrogen damage was in 1933. Since then more failures and untold damage to materials have accumulated, with the majority of damage occurring in the welds or weld heat-affected zones of these components.
When carbon and low-alloy steels are held in hydrogen at high temperature and pressure for an extended period of time, these materials can suffer degrading effects to their tensile and creep rupture properties. This is accompanied by the formation of intergranular fissures, blisters on the surface, and loss of carbon content (decarburization).
The phenomenon is called hydrogen attack and is generally attributed to the formation of methane (CH4) within the steel. The microstructural damage occurs when methane bubbles form and grow around precipitates at the grain boundaries within the material. The continued growth of the bubbles causes grains to separate along their boundaries and the bubbles, or voids, to coalesce. The rate of growth of the bubbles is a function of the ease by which the steel carbides give up carbon atoms to the intruding hydrogen atoms to form the methane. The more stable the carbide, the slower this reaction
will take place. Thus, it has been long recognized that additions of chromium and molybdenum, both strong carbide stablizers, improves hydrogen-attack resistance of steels. Addition of other carbide-stabilizing elements such as titanium and tungsten has also assisted in reducing susceptibility.
Weld regions are more susceptible to hydrogen damage because they possess less stable carbides. It is also readily apparent that carbon content of the base material is an important variable in determining the susceptibility of a steel to hydrogen damage. In general, steels used in this service are kept below 0.20 weight percent carbon content. Certain other elements, such as nickel and copper, are known to also have a detrimental effect.
Nelson curves have proven indispensable in the selection of materials in hydrogen service. These curves (see Fig. A3.20. for an example) were originally based on experience gathered over several decades, and have been revised as new experience has been gained. These curves identify a ‘‘safe’’ regime in which an alloy will perform acceptably at various temperatures and hydrogen partial pressures. Where these curves have proven unconservative have been associated with weld heat affected zones that had been inadequately postweld heat-treated (PWHT). The high residual stresses and high hardness left in the weld region contribute to accelerated damage. For this reason, most specifications for hydrogen service equipment stipulate a maximum hardness in weld regions that will assure adequacy of the PWHT.
The limit is usually placed at 210 Brinell hardness, corresponding approximately to a 100,000 psi (690 Ma) ultimate tensile strength. Austenitic stainless steels are essentially immune to hydrogen damage. The numerous sites within the FCC lattice in which the hydrogen atoms can be safely accommodated, and the inherent ductility of the lattice, gives austenitic materials this freedom from hydrogen damage. However, when stainless overlay weld metal has been used over carbon or low-alloy vessel steels, hydrogen-induced cracking can occur at the weld fusion line just inside the ferritic material.
When carbon and low-alloy steels are held in hydrogen at high temperature and pressure for an extended period of time, these materials can suffer degrading effects to their tensile and creep rupture properties. This is accompanied by the formation of intergranular fissures, blisters on the surface, and loss of carbon content (decarburization).
The phenomenon is called hydrogen attack and is generally attributed to the formation of methane (CH4) within the steel. The microstructural damage occurs when methane bubbles form and grow around precipitates at the grain boundaries within the material. The continued growth of the bubbles causes grains to separate along their boundaries and the bubbles, or voids, to coalesce. The rate of growth of the bubbles is a function of the ease by which the steel carbides give up carbon atoms to the intruding hydrogen atoms to form the methane. The more stable the carbide, the slower this reaction
will take place. Thus, it has been long recognized that additions of chromium and molybdenum, both strong carbide stablizers, improves hydrogen-attack resistance of steels. Addition of other carbide-stabilizing elements such as titanium and tungsten has also assisted in reducing susceptibility.
Weld regions are more susceptible to hydrogen damage because they possess less stable carbides. It is also readily apparent that carbon content of the base material is an important variable in determining the susceptibility of a steel to hydrogen damage. In general, steels used in this service are kept below 0.20 weight percent carbon content. Certain other elements, such as nickel and copper, are known to also have a detrimental effect.
Nelson curves have proven indispensable in the selection of materials in hydrogen service. These curves (see Fig. A3.20. for an example) were originally based on experience gathered over several decades, and have been revised as new experience has been gained. These curves identify a ‘‘safe’’ regime in which an alloy will perform acceptably at various temperatures and hydrogen partial pressures. Where these curves have proven unconservative have been associated with weld heat affected zones that had been inadequately postweld heat-treated (PWHT). The high residual stresses and high hardness left in the weld region contribute to accelerated damage. For this reason, most specifications for hydrogen service equipment stipulate a maximum hardness in weld regions that will assure adequacy of the PWHT.
The limit is usually placed at 210 Brinell hardness, corresponding approximately to a 100,000 psi (690 Ma) ultimate tensile strength. Austenitic stainless steels are essentially immune to hydrogen damage. The numerous sites within the FCC lattice in which the hydrogen atoms can be safely accommodated, and the inherent ductility of the lattice, gives austenitic materials this freedom from hydrogen damage. However, when stainless overlay weld metal has been used over carbon or low-alloy vessel steels, hydrogen-induced cracking can occur at the weld fusion line just inside the ferritic material.
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