Alloy pipes represent a superior type of seamless steel pipe, renowned for their exceptional performance compared to standard seamless steel pipes. This distinction arises primarily from their composition, which includes a higher concentration of chromium (Cr). As a result, alloy pipes exhibit excellent heat resistance, cold tolerance, and corrosion resistance, making them far superior to regular steel pipes. Their versatility and durability have led to widespread applications across industries such as oil, aerospace, chemical processing, electricity generation, boiler systems, and defense.
The theoretical calculation for the weight per meter of an alloy pipe is as follows:
\[ (\text{outer diameter} - \text{wall thickness}) \times \text{wall thickness} \times 0.02483 = \text{weight per meter}. \]
Common materials used for alloy pipes include grades like 16-50Mn, 27SiMn, 20-40Cr, and 12-42CrMo. Some specific examples of alloy materials are 16Mn, 12Cr1MoV, T91, 27SiMn, 30CrMo, 15CrMo, 20G, Cr9Mo, 10CrMo910, 15Mo3, 15CrMoV, 35CrMoV, and 45CrMo.
Alloy pipes can be further categorized based on their intended use. For instance, palladium alloy pipes play a critical role in separating hydrogen from impurities. These pipes operate on the principle that when hydrogen to be purified is introduced into one side of the palladium tube at temperatures ranging from 300°C to 500°C, hydrogen molecules adsorb onto the tube's walls. Due to the lack of two electrons in palladium's 4d electron layer, it forms unstable chemical bonds with hydrogen (a reversible reaction). Under these conditions, hydrogen dissociates into protons with a radius of approximately \(1.5 \times 10^{-15}\) meters, while the palladium tube’s lattice constant of \(3.88 \times 10^{-10}\) meters allows these protons to pass through. Once inside the palladium tube, protons recombine with electrons to reform hydrogen molecules, escaping from the opposite side. Undissociated gases remain impermeable, enabling the production of high-purity hydrogen.
Despite its unique ability to facilitate hydrogen permeation, pure palladium suffers from several limitations, including poor mechanical properties, susceptibility to oxidation at high temperatures, and a tendency to embrittle and deform easily. To overcome these issues, palladium alloys are created by adding elements from Groups IB and VIII, enhancing the alloy’s mechanical characteristics. In high-quality carbon structural steel and alloy structural steel hot-rolled seamless pipes used for shells and shafts, silver typically constitutes around 20-30% of the alloy, with other components (such as gold) present in quantities below 5%.
Currently, in most palladium alloys, silver makes up approximately 20-30%, with trace amounts of other elements comprising less than 5%. The rate of hydrogen permeation through these alloys depends on factors such as temperature, membrane thickness, and the pressure differential (P) between raw hydrogen and pure hydrogen on either side of the permeation membrane. Higher temperatures, greater pressure differences, and thinner membranes increase the hydrogen permeation rate. However, elevated temperatures reduce the tensile strength of the permeable membrane. Consequently, palladium tube usage is generally restricted to operating temperatures around 450°C. Contaminants such as mercury, arsenic, halides, oil vapors, sulfur-containing substances, ammonia, and dust can poison palladium, impairing its permeability and potentially damaging the membrane.
Palladium alloys can be fabricated into tubular forms (referred to as palladium tubes) or flat membranes (called palladium membranes). ABS alloy pipes combined with specialized cold-melt adhesives are another notable application. These materials find extensive use in residential plumbing systems and central air conditioning setups, particularly for vertical riser pipes and central air conditioning conduits. Additionally, PC/ABS alloys serve as ideal choices for automotive exterior components like wheel covers, mirror housings, taillight covers, and even larger parts such as fenders. PC/ABS demonstrates excellent formability, allowing manufacturers to produce large-scale automotive components efficiently.
As essential constituents of steel products, alloy pipes fall into two main categories based on their production methods and billet shapes: seamless steel pipes (formed from round billets) and welded steel pipes (created using plates or strips). The principle behind hydrogen purification via palladium tubes mirrors the aforementioned mechanism. When hydrogen to be purified enters one end of the palladium tube at temperatures between 300°C and 500°C, it adheres to the tube's inner walls. Given palladium's electron configuration, it establishes unstable chemical bonds with hydrogen molecules (a reversible process). Under these conditions, hydrogen dissociates into protons, which pass through the tube due to its lattice structure, before reforming into hydrogen molecules upon exiting the opposite side. Impurities unable to dissociate remain trapped, yielding highly purified hydrogen gas.
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