PEM Screw Manufacturers

How does the performance of screws change at different temperatures?

The performance changes of screws at different temperatures are mainly affected by their material properties and temperature range. Here are some key points about screw performance changes at different temperatures:

Low-temperature properties: Screws may become brittle in low-temperature environments, which results in reduced impact toughness and plasticity. For example, the performance of carbon and alloy steels is affected at temperatures below -50°C.
High Temperature Performance: As temperatures increase, the strength of screws typically decreases. This is because metal materials are prone to creep and stress relaxation at high temperatures, resulting in a decrease in preload force.
Effect of heat treatment: For high-strength screws, such as 42CrMo steel high-strength bolts, adjusting the heat treatment process parameters (such as carbon potential, water-soluble quenching agent concentration during quenching and cooling, and tempering temperature) can improve its overall performance in a specific use environment.
Plating effect: The coating of a screw can also affect its performance at different temperatures. For example, zinc-plated screws perform better below 230°C, while nickel-plated screws can be used at higher temperatures, with an upper limit of about 590°C.
Stainless steel screws: Due to their alloy composition, stainless steel screws generally have a wider operating temperature range, approximately between -120°C and 320°C.
Specialty Materials: For higher temperature applications, special alloy materials may be required, such as Inconel and nickel-based superalloys, which can maintain performance at higher temperatures.
Performance grade: The performance grade of the bolt, such as 8.8, 10.9, etc., also indicates its performance at different temperatures. A higher performance grade generally means the screw can withstand higher operating temperatures.
Coefficient of thermal expansion: Temperature changes can also cause differences in the coefficient of thermal expansion between screws and connections, which can affect clamping force and the stability of the connection.
Fire resistance: In building steel structures, the fire resistance of stainless steel bolts is also an important consideration, and its mechanical properties at high temperatures are crucial to the safety of the structure.

When designing and selecting screws, the temperature range of their working environment must be considered to ensure that the screws can work properly at that temperature and maintain the required mechanical properties. Exceeding the screw's operating temperature range may cause connection failure or other safety issues.

Why do screws undergo stress relaxation at high temperatures?

The reason why screws undergo stress relaxation at high temperatures is related to the physical properties and thermomechanical behavior of the material. The following are several key factors that cause stress relaxation in screws at high temperatures:

Creep effect: In a high temperature environment, even under the action of sustained stress below the yield strength, metal will undergo slow plastic deformation. This phenomenon is called creep. Creep causes the screw material to stretch less, thus reducing the preload force.
Time dependence: Stress relaxation is a time-dependent phenomenon in which the stress in the screw gradually decreases over time, even under constant stress.
Temperature effect: The stress relaxation rate of metal increases with temperature. At high temperatures, atomic mobility increases, making dislocation movement more likely to occur, resulting in accelerated stress relaxation.
Thermal activation process: High temperature provides the necessary heat energy, allowing dislocations in the metal to cross obstacles, promoting plastic deformation, and thus leading to stress relaxation.
Oxidation and corrosion: At high temperatures, oxidation and corrosion processes can also have a negative impact on the performance of screws, resulting in a reduction in material strength and stiffness.
Heat treatment process: The heat treatment process of screws has a significant impact on their high-temperature performance. Proper heat treatment can improve the stress relaxation resistance of screws, but this can also result in increased cost.
Material alloy composition: The material alloy composition of a screw is critical to its high temperature performance. Some alloying elements can improve stress relaxation resistance but may increase material cost.
ISO Standard Recommendations: The ISO 898-1 standard states that in applications between 150°C and 300°C, users are advised to consult an experienced fastener metallurgist to select appropriate bolt materials and designs.
High-temperature alloys and stainless steels: For high-temperature applications, specialized high-temperature alloys and stainless steel materials are often used, which are designed to maintain their mechanical properties at high temperatures.
Stress relaxation and operating temperature: The amount of stress relaxation depends not only on the operating temperature, but also on the operating time at the temperature. Over time, stress and preload will decrease, possibly causing connection failure.

Understanding these factors can help you consider the impact of high-temperature operating environments on their performance when designing and selecting screws to ensure the reliability and safety of your connections.