Abstract
An investigation of aging phenomena during creep exposure has been conducted for HP-Nb cast reformer tubes for several exposure conditions. Aging was manifested by carbide precipitation, carbide coarsening, and carbide transformation. The transformation of primary M7C3 to the more stable M23C6 carbide takes place at high exposure temperature (910 °C and above). The primary MC carbides transform to the Ni-Nb silicide or G-phase during creep exposure. The presence of Ti in the steel prevented the transformation of MC carbides to the G-phase. Morphological changes like needle to globular transitions, rounding of carbide edges, and carbide coarsening take place during creep exposure. The room-temperature tensile elongation and ultimate tensile strength are significantly reduced during creep exposure. The above aging phenomena are precursors to creep damage.
Highlights
Catalytic reforming is routinely employed in oil refineries for the production of petroleum byproducts
Significant coarsening of the carbides takes place at high while at lower temperatures the changes are morphological, i.e., the rounding of temperatures while at lower temperatures the changes are morphological, i.e., edges or the change from needle-shaped carbides to a globular morphology. Another the rounding of edges or the change from needle-shaped carbides to a globular morphology interesting aging phenomenon during creep exposure is the transformation of the primary MC carbides. Another interesting aging phenomenon during creep exposure is the transformation of the primary MC carbides to the Ni-Nb silicide of G-phase manifested at intermediate temperatures
An investigation of aging phenomena during creep has been conducted for HP-Nb cast reformer tubes
Summary
Catalytic reforming is routinely employed in oil refineries for the production of petroleum byproducts. The process takes place in tubes suspended in a furnace at a high temperature. The tubes are fed with hydrocarbon, mostly methane, and steam mixture in the presence of a catalyst according to the reaction network: m. The first is the steam reforming reaction, which, for the case of methane, is strongly endothermic (∆Hr = 206 KJ/mol) and takes place at a high temperature of the order of 900 ◦ C. The second is the water-gas shift reaction and is mildly exothermic (∆Hr = −41 KJ/mol). The internal pressure in the tubes generates stresses, causing creep of the tube material. Prior to creep damage, several aging phenomena take place, which gradually changes the microstructure of the material
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