heat exchange tube

Aluminum infiltration of heat exchange tubes is a process in which aluminum is infiltrated into steel tubes. Such tubes are typically used in air preheaters and serve as a transitional element; there are stainless steel tubes in front of the aluminum-infused tubes. The flue gas passes through a high-temperature section (304 steel), a medium-temperature section (aluminum-infused tubes), and a low-temperature section (carbon steel tubes). Additionally, T-shaped tubes are a type similar to finned tubes, with the tips of the fins being T-shaped. Our company is capable of producing these tubes; you can contact me privately at 13921272663

001、What is aluminum infiltration? Aluminizing is a type of chemical heat treatment for metal surfaces; it involves heating steel or another material to a certain temperature so that aluminum atoms can penetrate and diffuse into the steel matrix. 002、What are the methods of aluminum infiltration? Solid powder embedding diffusion aluminizing ； Liquid hot-dip diffusion aluminizing ； Gas aluminizing ； Aluminizing by brushing and spraying ； Electrostatic spraying aluminizing ； Electrodeposition aluminizing ； Aluminizing by molten salt electrolysis ； Various methods such as rapid induction aluminizing of medium-frequency and high-frequency slurries. 003、Why is aluminum infiltration performed on steel? Depending on the specific application requirements, aluminizing can be carried out on carbon steel, low-alloy steel, and high-chromium-nickel alloy steels. After aluminum infiltration, a ferroaluminum alloy layer with special properties is formed on the surface of these materials, thereby improving their resistance to high-temperature oxidation and corrosion. 004、What are the two most commonly used aluminum infiltration methods? Liquid hot-dip diffusion aluminizing ； Solid powder embedding diffusion aluminizing 005 – What is solid powder embedding diffusion aluminizing? Solid powder-embedded diffusion aluminizing is a process in which the workpiece to be aluminized and the solid powder aluminizing agent are placed inside a specially designed container according to specific requirements, then the container is put into a heating furnace. There, the material is heated and kept at a constant temperature; during this heating process, a series of chemical reactions occur within the aluminizing agent, activating the aluminum atoms present in it, thereby allowing these aluminum atoms to penetrate (diffuse) into the matrix of the steel’s surface layer, resulting in the formation of an iron-aluminum alloy layer in that surface layer. 006、What are the characteristics of solid powder-embedded aluminized steel? The aluminized layer is formed within the base material matrix through atomic diffusion; the coated layer is integral with the matrix, so there is no issue of the coating peeling off ； After aluminizing, the workpiece does not gain weight nor thickness ； The mechanical properties remain largely unchanged ； Good weldability ； It can undergo secondary processing such as stretching, bending, and flaring. 007、What is hot-dip aluminizing? Hot dip aluminizing is a process in which the workpiece is immersed in a molten aluminum bath to coat its surface with a layer of pure aluminum, followed by heating and diffusion to form an aluminized layer on the surface of the workpiece. 008、What are the characteristics of hot-dip aluminized steel? The aluminized layer obtained through hot-dip aluminizing is formed through heating and diffusion on the basis of hot-dip aluminum plating. During the hot-dip and heating processes, a reaction takes place between aluminum and iron, accompanied by bidirectional diffusion: one is the diffusion of pure aluminum toward the surface of the workpiece to form the aluminized layer, and the other is the diffusion of iron from the surface of the workpiece into the pure aluminum layer to form an aluminum-rich layer. There is a distinct interface between the two layers; the aluminum-rich layer is hard and brittle, has a high aluminum content, and tends to flake off easily. Therefore, the workpiece gains weight and thickness after aluminizing ； Large variations in mechanical properties ； Poor weldability ； No further processing is allowed. 009、What are the differences between the aluminized layer obtained by solid powder embedding and that obtained by hot-dip aluminizing? The formation process of the diffusion layer is different ； The microstructure of the diffusion layer differs ； The mechanical properties and weldability of the infiltrated layer vary. 010. What is hot-dip aluminizing? Hot-dip aluminizing is the process of immersing a workpiece in a molten aluminum bath to coat its surface with a layer of pure aluminum. 011、What is the process for aluminum infiltration of solid powder coatings? The solid powder-encased diffusion aluminizing process involves placing the workpiece to be aluminized along with the solid powder aluminizing agent into a specially designed container, sealing it, and then placing it in a heating furnace. It is heated to a specific temperature and maintained at that level; during this heating process, a series of chemical reactions occur within the aluminizing agent, which activates the aluminum atoms present in it. These activated aluminum atoms are then diffused into the matrix of the steel’s surface layer, resulting in the formation of an iron-aluminum alloy layer there. 012、What is the process for hot-dip aluminizing? Surface cleaning (degreasing, derusting) – pre-treatment coating – hot-dip aluminizing – heating and diffusion. 013. What is an infiltration layer? The diffusion layer is an aluminum-iron alloy layer formed during the heating and diffusion process. 014. What types of structures are generally included in the diffusion layer? According to the phase diagram of iron-aluminum alloys, as the aluminum content in the case layer increases, a carbon-rich zone, a carbon-poor zone, an alpha-iron solid solution of aluminum, Fe3Al, FeAl, FeAl2, along with small amounts of Fe2Al5 and FeAl3, appear successively from the inside outward in the case layer. 015、What are the components of the diffusion layer? The case layer generally consists of the following seven components: 1) The main components on the surface of the case layer are FeAl2, along with small amounts of Fe2Al5 and FeAl3. The aluminum content in this layer is around 45% (wt) (this layer is present only in the hot-dip aluminized layer; it is generally not present in the aluminized coatings obtained by solid powder embedding). 2) The main component of the second layer is FeAl; the aluminum content in this layer is about 32% (wt). 3) The main component of the third layer is the Al4C3 phase, appearing as gray rod-like structures. 4) The main component of the fourth layer is Fe3Al, and the aluminum content in this layer is below 14%. 5) The solid solution of Al in α-iron at the fifth layer, with an aluminum content of 10% (wt) or less. 6) The sixth layer is the low-carbon zone. 7) The seventh layer is the carbon-rich zone. The division of the aluminized layer into the seven layers mentioned above is merely a theoretical concept; in practice, some of these layers may not exist, and even when they do exist, there may be no clear boundaries between them. 016、What is the minimum aluminum concentration required to form a continuous protective film? A continuous Al2O3 protective film can be formed in the case of an aluminum concentration of 8% (wt) or higher in the diffusion layer. 017、What are the technical specifications for aluminum infiltration? Case depth: 0.10–0.20 mm; aluminum content in the case: 15%–33% (by weight); surface hardness of the case: HV300–600 (not a controlled parameter). 018. Why must the hardness of the aluminized layer be measured as microhardness? After surface aluminizing treatment of metal materials, there is a significant difference between the hardness of the surface layer and that of the base material; generally, the surface hardness is much higher than that of the base material. It’s similar to a cooked egg – if a conventional high-load hardness testing method is used, the eggshell will be crushed, and what is measured is neither the hardness of the eggshell nor that of the egg white. The microhardness testing method uses a small probe and a low load (usually 0.5 N or 1 N), so that the eggshell is not crushed, and the result obtained is the hardness of the eggshell. 019、What is the corrosion resistance of aluminized steel? Years of research and application both domestically and internationally have shown that aluminized steel possesses excellent resistance to high-temperature oxidation. In particular, aluminizing the inner and outer surfaces of low-alloy and high-chromium-nickel alloy furnace tubes not only enhances their resistance to high-temperature oxidation but also ensures that the aluminized layer has good resistance to carburization and hydrogen penetration. Additionally, it helps to reduce coking within the furnace tubes, thereby **increasing their service life. When the aluminum content in the aluminized layer on the surface of carbon steel exceeds 15% (wt), and under conditions of temperatures above 240°C, sulfur content greater than 0.5%, or a pH value greater than 0.5 mgKOH/g, its corrosion resistance is more than three times that of 18-8 grade stainless steel under the same conditions; its corrosion resistance is comparable to that of 316L stainless steel. Especially under conditions of combined corrosion by H2S and RCOOH (cycloalcanic acid), the corrosion resistance of aluminized steel is incomparable. In various corrosion media dominated by hydrogen sulfide at temperatures below 120°C, such as H2S-NH3-H2O, HCN-H2S-H2O, CO2-H2S-H2O, RNH2-CO2-H2S-H2O, and H2S-H2O, aluminized steel also outperforms chromium steel, chromomolybdenum steel, and austenitic stainless steel. Moreover, aluminized steel does not suffer from problems such as stress corrosion and intergranular corrosion cracking. 020. What are the changes in the mechanical properties of aluminum-impregnated steel with solid powder embedding? Generally speaking, the mechanical properties of aluminized steel depend on those of the base metal. However, aluminizing the surface of the steel has an impact on its mechanical properties; although the aluminized layer is thin, its mechanical properties differ significantly from those of the base metal. Generally, the tensile strength of low-carbon steel heat exchange tubes decreases by about 3%. 021、Can aluminum-impregnated steel with solid powder embedding be further processed (drawing, bending, flaring)? Aluminized steel processed by the solid powder embedding method can undergo secondary processing such as drawing, bending, and flaring. The case hardened layer will not be damaged when the cold drawing elongation is no more than 8%, the bending radius is greater than 4D, and the flaring is no more than 3%. 022、Weldability of aluminum-impregnated steel with solid powder embedding? The weldability of aluminized steel processed by the solid powder embedding method differs little from that of its base material, but their welding methods and techniques are different; welding aluminized steel requires taking both aspects into consideration. First, it is necessary to ensure the overall mechanical properties of the material; second, the surface layer of the weld must possess high-temperature oxidation resistance or corrosion resistance similar to that of the aluminum-infused layer, while also ensuring continuity in the properties at the weld site. Therefore, special treatment is required at the weld seam during welding. 023、What is the welding method for solid powder-embedded aluminum-diffused steel heat exchange tube bundles? For fillet welds, TIG welding, low current, and two-pass welding: Can high-alloy corrosion-resistant materials be used for welding aluminum-doped steel heat exchanger tubes? Why? In terms of strength and welding processes, high-alloy corrosion-resistant electrodes can be used for welding aluminum-doped steel heat exchanger tubes. However, from the perspective of corrosion resistance, the use of high-alloy corrosion-resistant welding electrodes, especially austenitic stainless steel electrodes, does not necessarily guarantee good corrosion resistance against certain media. 025、What issues should be considered when welding aluminum-coated steel heat exchange tube bundles? Other matters to note for welding aluminized pipes: 1) The pipe ends must be cleaned thoroughly, free of oil, water, and other impurities. 2) Strictly control the welding current to prevent overheating and damage to the aluminum infiltration layer inside the tube. 3) Arc striking on the pipe is strictly prohibited; the ground wire should not be connected to the pipe to prevent arc discharge from damaging the coating. 026. Why is anti-corrosion treatment required for the end face of the tube sheet after welding aluminum-impregnated steel heat exchange tube bundles? To ensure the continuity of the corrosion resistance at the end faces of the tube sheet after welding of the heat exchange tube bundle. 027、Can aluminum-diffused steel heat exchange tube bundles be strength-expanded? Why? The basic requirement for strength expansion joining is that the hardness of the tube be lower than that of the tube sheet, so that the tube undergoes plastic deformation while the tube sheet undergoes elastic deformation, thereby achieving a strong connection. In contrast, this is exactly the case after aluminizing the tubes; therefore, strong expansion jointing cannot be performed between the aluminized steel heat exchange tube bundles and the tube sheet.

Hi Haidou on the third floor, what is the approximate temperature for the medium-temperature range you are referring to? Is the corrosion resistance after aluminizing mainly related to wear resistance?

The purpose of aluminizing heat exchange tubes is to improve their resistance to high-temperature oxidation and corrosion.

I’ve learned it, thanks for sharing above