For high-temperature resistant bolts and nuts operating in high-temperature environments, creep is a key factor affecting connection reliability. Creep is a slow plastic deformation of materials under high temperature and sustained stress, leading to bolt elongation, loss of preload, and even connection failure. To reduce the impact of creep on connections, a comprehensive approach is needed, encompassing material selection, structural design, manufacturing processes, installation control, and maintenance management.
Material selection is fundamental to mitigating creep effects. High-temperature resistant bolts and nuts should prioritize materials with high creep strength and good thermal stability, such as nickel-based superalloys, iron-based superalloys, or heat-resistant steels that have undergone special heat treatment. These materials maintain high tensile and yield strength at high temperatures, delaying creep. For example, GH2132 alloy, through aging precipitation of strengthening phases, exhibits excellent endurance and creep strength below 650℃, making it suitable for high-temperature applications such as aero-engines and gas turbines. Furthermore, the material's coefficient of thermal expansion should match that of the connected components to reduce additional stress caused by differences in thermal expansion.
Structural design is crucial for reducing the impact of creep. The geometric parameters of bolts and nuts need to be optimized for high-temperature operating conditions. For example, increasing the bolt diameter can reduce working stress, thereby slowing down the creep rate; using a rounded thread root design can reduce stress concentration and lower the risk of crack initiation; appropriately increasing the bolt length or adding a sleeve can provide elastic deformation space to compensate for elongation caused by creep. For high-temperature flange connections, a heat-insulating ring can be installed on the gasket or inside the flange to reduce heat transfer to the bolt, lower its operating temperature, and thus delay creep.
Manufacturing process control directly affects the creep resistance of bolts. Heat treatment is a key step; solution treatment and aging treatment can optimize the grain structure and strengthening phase distribution of the material, improving its high-temperature strength and creep resistance. For example, solution treatment followed by aging treatment of nickel-based alloys can form stable carbides at grain boundaries, preventing grain boundary slippage and thus enhancing creep resistance. Furthermore, strict control of surface quality is necessary during manufacturing to avoid defects such as scratches and cracks, preventing creep cracks from initiating from surface defects.
Installation control is an important aspect of reducing the impact of creep. The preload of high-temperature bolts needs to be precisely calculated based on the material's creep characteristics. This ensures a tight connection while preventing premature creep fracture due to excessive preload. Specialized tools should be used during installation to ensure even distribution of preload and prevent localized stress concentration. For critical connections, hydraulic tensioners or a torque + angle method can be used for preload to improve installation accuracy. Furthermore, the installation sequence must be appropriate; for example, flange bolts should be tightened symmetrically to avoid loosening due to uneven force.
Environmental factors have a significant impact on creep. High temperatures, oxidation, and corrosion accelerate creep damage to bolt materials. Therefore, protective treatments such as copper plating, ceramic coatings, or anti-oxidation coatings are necessary to reduce high-temperature oxidation and corrosion. In high-temperature steam environments, chrome plating on the steam side or stainless steel overlay welding can improve the bolt's corrosion resistance. Optimizing equipment heat dissipation design to reduce bolt operating temperature is also an effective way to delay creep.
Regular maintenance and monitoring are crucial for ensuring the long-term reliability of high-temperature bolts. A regular inspection system should be established to promptly detect cracks or defects on the bolt surface using methods such as ultrasonic testing, magnetic particle testing, or penetrant testing. For critical connections, strain gauges or fiber optic sensors can be installed to monitor bolt preload changes and creep elongation in real time. If creep damage exceeds acceptable limits, the bolt must be replaced immediately to prevent connection failure and accidents.
Mitigating the creep effects of high-temperature resistant bolts and nuts requires a comprehensive approach throughout their entire lifecycle, including design, manufacturing, installation, operation, and maintenance. Through appropriate material selection, structural optimization, process control, precise installation, and regular maintenance, the creep resistance of bolts at high temperatures can be significantly improved, ensuring the safe and stable operation of equipment.
