The valve stem is an important part of the valve, used for transmission, connecting the actuator or handle, moving or rotating directly under the valve core to realize the switching or adjustment of the valve. During the opening and closing of the valve, the valve stem is not only a moving part, a stressed part, but also a sealing part. At the same time, it will be impacted and corroded by the medium, and it will also generate friction. Therefore, when selecting the valve stem material, it must be ensured that it has sufficient strength at a specific temperature, good impact toughness, wear resistance and corrosion resistance. The valve stem is a wearing part, and attention should also be paid to the machinability and heat treatment properties of the material when selecting it. Therefore, many valve stems are usually made of Inconel 718 material.
Inconel 718 stem is resistant to acid corrosion in harsh service environments and high temperature conditions, so it can be used in chemical desulfurization and deacidification of coal in corrosive or acid environments, especially in corrosive environments containing carbon dioxide and hydrogen sulfide. Inconel 718 has very good mechanical properties after machining, so this alloy is widely used in petroleum production, coal mining, aerospace equipment and machinery manufacturing industries. The alloy contains over 50% nickel, about 20% chromium and 3% molybdenum. It plays a vital role in the production of coal chemical industry and the use of equipment. Nickel element makes Inconel 718 more stable, and can prevent chloride ion stress corrosion cracking in strong oxidizing environment. The presence of chromium element improves the oxidation resistance of the alloy, and also improves the corrosion resistance of the material. The presence of molybdenum element reduces the corrosion phenomenon of the alloy and enhances the pitting corrosion resistance in harsh environments.
Stress corrosion cracking is a form of failure caused by the interaction of stress and a corrosive environment. Once stress corrosion cracking is formed, it will propagate at a very high rate, causing structural members to rupture within a short period of time, which is a very dangerous phenomenon. External environmental factors have a very large impact on the stress corrosion performance of Inconel 718 alloy. Inconel 718 alloy has strong stress corrosion sensitivity in boiling magnesium chloride solution at 154℃, so it limits the maximum temperature of Inconel 718 alloy valve stem used in coal chemical industry. In addition, the presence of hydrogen sulfide in the environment also promotes the stress corrosion cracking susceptibility of Inconel 718 alloy in high temperature chlorine-containing environments. At high strength levels, stress corrosion cracking susceptibility increases with increasing hydrogen sulfide partial pressure. Stress corrosion susceptibility is very high at high partial pressures of hydrogen sulfide, especially when the solution contains sulfur.
Inconel 718 alloy is used in the production of coal chemical valve stem material due to its excellent corrosion resistance. Inconel 718 alloy valve stem material generally does not have uniform corrosion and pitting corrosion in the coal chemical production environment. Inconel 718 alloy stems have excellent uniform corrosion resistance in environments containing carbon dioxide and hydrogen sulfide, with corrosion rates below 0.025 mm per year, even at high temperatures. Inconel 718 alloy exhibits excellent corrosion resistance in sour environments. The corrosion rate of Inconel 718 alloy valve stem material in hydrochloric acid solution is 0.1mm/h, and the corrosion resistance of this solution is comparable to that of alloys such as 825 SM2550 and G3. Objectively speaking, the corrosion resistance of Inconel 718 alloy in acid chemical processing environment is sufficient, especially in this working environment, corrosion inhibitors are added to reduce the corrosion rate of the alloy. When the aging temperature increased, the corrosion rate of Inconel 718 alloy in 5% boiling hydrogen sulfide solution decreased slightly, indicating a slight improvement in its resistance to uniform corrosion. However, after the aging temperature reaches 788 °C, the corrosion rate increases and the resistance to uniform corrosion decreases with the increase of the aging time.
Inconel 718 alloy has excellent corrosion resistance, but sometimes it is deeply corroded in harsh acidic environments. Usually, the deeply corroded environment often contains sulfur and oxygen elements. Therefore, inconel corrosion in Inconel 718 alloy stem material often occurs in extremely harsh acid chemical environments. The pitting corrosion resistance of Inconel 718 alloy stem material is comparable to that of duplex stainless steel. Due to insufficient corrosion resistance, stainless steels such as 410 stainless steel, 9Cr1Mo 304 stainless steel, CA6NM and 17-4PH cannot be used. If pitting corrosion occurs on the Inconel 718 alloy valve stem, first review the heat treatment process. Considering that the heat treatment process affects its structure and reduces the pitting corrosion resistance of the alloy valve stem, the heat treatment that has a greater impact on it The process is an aging process, and the aging process involves aging temperature and aging time. Reasonable changes in aging temperature and aging time help to improve the pitting corrosion resistance of alloy valve stems. It is worth noting that the temperature of pitting corrosion does not increase with the prolongation of heat treatment time, but the temperature of pitting corrosion decreases with the prolongation of heat treatment time. The ability to resist pitting corrosion decreases, and the appearance of this phenomenon is related to carbide precipitation and element segregation caused by aging. Aging time and temperature are two important factors in effective aging treatment. Increasing the aging temperature and prolonging the aging time will cause the pitting corrosion resistance of the valve stem alloy material to decrease significantly. On the other hand, the effect of pitting corrosion resistance on the performance of Inconel 718 valve stem in an acidic service environment is not very important, and it is not a key factor limiting the completion of the valve stem function unless the valve stem works in an extremely special environment.
To sum up, the corrosion and cracking behavior of Inconel 718 material valve stem is ubiquitous in the service environment of high acid coal chemical industry, and the corrosion and cracking mechanism is more complicated. It is necessary to carry out innovative research on valve stem material and optimize the production process of coal chemical industry. , timely remove the hidden trouble of valve stem material corrosion and cracking failure in the process of coal chemical production.
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