The toughness of carbon steel hexagonal bolts at low temperatures directly affects the reliability and safety of their connection structure, especially in extremely cold regions or low-temperature industrial settings. Insufficient toughness can easily lead to brittle fracture, causing equipment failure or even safety accidents. Ensuring low-temperature toughness hinges on material selection, optimized heat treatment processes, microstructure control, and environmentally adaptable design, requiring a multi-dimensional toughness assurance system.
Material composition is fundamental to low-temperature toughness. Ordinary carbon steel is prone to the ductile-brittle transition at low temperatures, increasing the risk of fracture. Adding alloying elements such as nickel and manganese can significantly lower the ductile-brittle transition temperature. For example, the ductile-brittle transition temperature of nickel-containing steel can be lowered to below -60℃, while ordinary carbon steel is close to the brittle fracture critical point at -20℃. Furthermore, controlling the content of impurities such as sulfur and phosphorus, and reducing grain boundary segregation, can prevent intergranular fracture at low temperatures. Precise material composition is the primary prerequisite for improving the low-temperature toughness of carbon steel hexagonal bolts.
Heat treatment processes are crucial for controlling toughness. Quenching and tempering are crucial steps. Excessive quenching temperature leads to grain coarsening and reduced toughness; insufficient temperature prevents the formation of a uniform martensitic structure. Tempering temperature must be strictly controlled within the range of 250-400℃ to avoid temper brittleness. If tempering at 450-600℃ is used, rapid cooling is necessary to prevent the precipitation of brittle phases at grain boundaries. By optimizing heat treatment parameters, carbon steel hexagonal bolts can maintain high impact absorption energy at low temperatures, reducing the risk of brittle fracture.
Microstructure refinement is an effective means of improving toughness. The finer the grains, the more complex the crack propagation path, and the higher the toughness. Through thermomechanical deformation processes, such as medium-temperature die rolling, a fibrous structure can be formed within carbon steel, inhibiting the propagation of microcracks at crack tips. Simultaneously, grain refinement reduces stress concentration and decreases the fracture driving force. Experiments show that combining heat treatment with mechanical deformation significantly improves the low-temperature toughness of 45 steel, making it suitable for bolt manufacturing in extremely cold environments.
Environmental adaptability design must be tailored to the application scenario. In low-temperature, high-humidity environments, ice layers easily form on bolt surfaces, increasing the risk of stress concentration. Surface treatments, such as galvanizing or applying anti-rust oil, can isolate moisture and prevent ice from directly contacting the substrate. Furthermore, designing anti-loosening structures, such as double nuts or spring washers, can reduce loosening caused by vibration and prevent brittle fracture due to stress concentration at low temperatures. For carbon steel hexagonal bolts used outdoors, snow removal must be considered to prevent snowmelt from seeping into the thread gaps and causing freezing damage.
Low-temperature impact testing is a crucial step in verifying toughness. According to relevant standards, bolts must be subjected to impact tests under preset low-temperature conditions, using V-notch or U-notch specimens. The material toughness is determined by the impact energy. If the test results do not meet the standards, the material composition or heat treatment process needs to be adjusted. For example, a petrochemical project required bolts to have an impact energy of no less than 27J at -45℃. By optimizing the tempering temperature, the product ultimately met the requirements, ensuring safe use in low-temperature environments.
In practical applications, ensuring the low-temperature toughness of carbon steel hexagonal bolts needs to be integrated throughout the entire design, manufacturing, and installation process. From material selection and heat treatment parameter setting to microstructure control and environmental adaptability improvement, every step requires strict control. For example, in the construction of Arctic oil and gas platforms, by selecting low-alloy steel, optimizing heat treatment processes, and adding surface rust prevention treatments, carbon steel hexagonal bolts maintained good toughness at -50℃, ensuring the long-term stable operation of the platform structure.
Ensuring the toughness of carbon steel hexagonal bolts in low-temperature environments requires a multi-layered toughness assurance system based on materials science, utilizing process optimization, and supplemented by environmental adaptability design. By precisely controlling material composition, heat treatment parameters, microstructure, and environmental factors, the low-temperature toughness of bolts can be significantly improved, providing reliable support for engineering safety in extreme environments.
