Rectangular nuts, as high-strength fasteners, are susceptible to hydrogen embrittlement during the manufacturing process due to hydrogen atom accumulation, resulting in decreased toughness and even fracture.
Hydrogen embrittlement occurs when hydrogen atoms diffuse into grain boundaries or defects, forming high-pressure micro-zones that weaken metal bonds and ultimately trigger intergranular fracture. During processes such as cold heading, machining, and electroplating, tensile stresses easily build up on the surface of rectangular nuts, creating traps for hydrogen atom accumulation. When the hydrogen concentration exceeds a critical value, the material is susceptible to delayed fracture under static or alternating stresses. The core goal of stress relief annealing is to eliminate residual stresses through heat treatment, reduce the driving force for hydrogen atom accumulation, and promote hydrogen escape.
Annealing temperature is a key parameter influencing the effectiveness of hydrogen embrittlement mitigation. Rectangular nuts are typically made of low-carbon alloy steel or medium-carbon steel. Annealing temperature must be controlled below the recrystallization temperature to avoid microstructure coarsening and resulting strength loss. Generally, the annealing temperature is set between 500°C and 650°C, which stimulates hydrogen diffusion without inducing phase transformation. If the temperature is too low, the hydrogen atoms will diffuse insufficiently, limiting the dehydrogenation effect. Excessively high temperatures may damage the surface coating or induce oxidation. Therefore, precise control is required based on the material composition and process requirements.
The holding time directly affects the efficiency of hydrogen escape and the degree of residual stress elimination. Rectangular nuts must be kept in the annealing furnace for a sufficient period of time to allow hydrogen atoms to diffuse from grain boundaries or defects to the surface and volatilize. The holding time is typically set at 2 to 4 hours, depending on the thickness and hydrogen content of the nut. If the holding time is insufficient, hydrogen atoms cannot fully escape, and residual stress may still cause hydrogen embrittlement. If the holding time is too long, grain growth may occur, reducing mechanical properties. Optimal holding parameters must be determined through experiments in actual production.
The cooling method has a significant impact on the annealing effect. Rectangular nuts must be cooled slowly after annealing to prevent the generation of new residual stresses due to thermal stress. Furnace cooling or air cooling is generally used, with a cooling rate of 5°C to 10°C per minute. Rapid cooling can lead to a large temperature difference between the surface and the interior, causing thermal cracking; excessively slow cooling can extend production cycles and increase costs. Some processes employ staged cooling, initially maintaining a slower cooling rate at high temperatures and then accelerating cooling after the temperature drops below 300°C, to balance efficiency and quality.
The synergistic effect of stress relief annealing with other processes can further enhance hydrogen embrittlement prevention. For example, annealing before electroplating can reduce residual stress in the substrate and minimize hydrogen absorption during the electroplating process. Low-temperature annealing immediately after electroplating can promote hydrogen release from the coating, preventing delayed fracture. Furthermore, combining annealing with surface treatments (such as Dacromet coating) can create a physical barrier to prevent hydrogen permeation from the environment, mitigating the risk of hydrogen embrittlement at the source.
In actual production, stress relief annealing for rectangular nuts requires process optimization based on material properties and application scenarios. For example, the aerospace industry places extremely high demands on the reliability of nuts, necessitating stricter annealing parameters (such as higher temperatures and longer hold times). Meanwhile, the automotive industry must balance cost and performance, selecting cost-effective process solutions. Through long-term data accumulation and failure analysis, a library of annealing processes tailored to different materials can be established, providing standardized guidance for hydrogen embrittlement prevention and control in rectangular nuts.
Stress relief annealing effectively reduces the susceptibility of rectangular nuts to hydrogen embrittlement by precisely controlling temperature, time, and cooling methods. This process not only eliminates residual stresses generated during manufacturing, reducing the driving force for hydrogen atom aggregation, but also physically blocks the path to hydrogen embrittlement by promoting hydrogen escape. Combined with other preventative measures, stress relief annealing provides a reliable guarantee for the safe use of high-strength fasteners.
