Unexpected failures of screws can lead to significant damage and high costs for the final product. One potential cause of these failures is hydrogen embrittlement.
Hydrogen embrittlement (HE) refers to the irreversible loss of ductility in a metal or alloy due to the infiltration of hydrogen atoms into its structure.
This condition, when combined with applied loads or tensile stress, can result in brittle fractures that may occur after a delay—sometimes hours or days after installation—often without visible signs. These fractures do not occur immediately after installation or several months later.
Hydrogen embrittlement is categorized into two main types based on the source of hydrogen: internal hydrogen embrittlement (IHE) and environmental hydrogen embrittlement (EHE). Internal hydrogen embrittlement arises from residual hydrogen absorbed during manufacturing processes, such as steelmaking or electroplating. Environmental hydrogen embrittlement occurs when hydrogen is introduced from external sources while the metal is under stress, such as in-service fasteners.
1. Why do hydrogen embrittlement failures occur in fasteners?
Hydrogen embrittlement failures occur when three key conditions are present simultaneously. These conditions are appropriately illustrated in the Venn diagram.
When a crack begins to form under tensile stress, there is a significant concentration of stress at the crack tip. If enough hydrogen is present, it interacts with the crack tip and triggers the growth of the crack. (Fig 1.)
2. How does the process of hydrogen embrittlement occur in fasteners?
Hydrogen Absorption: At room temperature, hydrogen atoms can be absorbed by carbon steel alloys during processes such as acid cleaning, electroplating, or exposure to hydrogen-containing environments.
Diffusion: Once absorbed, hydrogen atoms are highly mobile and can diffuse to areas of highest stress within the material, such as grain boundaries, dislocations, or inclusions. Hydrogen forms voids at these metal grain boundaries, resulting in a loss of ductility and increased brittleness, making the material more prone to cracking.
Embrittlement: Given enough time, when the fastener is subjected to high tensile stress, these voids apply additional pressure on the metal grains, leading to the formation of initial cracks. These cracks can grow along weakened grain boundaries, ultimately causing the fastener to break. This process is known as intergranular cracking.
- Hydrogen absorption
2. Hydrogen in preferential sties like grain boundaries, dislocations or impurities
3. Attacks growth and leads to cracking under stress
When a crack begins to form under tensile stress, there is a significant concentration of stress at the crack tip. If enough hydrogen is present, it interacts with the crack tip and triggers the growth of the crack. (Fig 1.)
3. How can the risk of internal hydrogen embrittlement in screws be minimized?
Hydrogen embrittlement is both invisible and unpredictable. While manufacturing processes are designed to reduce the risk, there is no method that can entirely prevent it.
Standards for electroplated screws, such as ISO 4042 and ASTM F1941/F1941M, require a mandatory baking process for fasteners with a hardness above 390 HV. This baking is conducted at temperatures between 190°C and 230°C for up to 24 hours, depending on the screw’s size, strength, and hardness. Although this process does not completely eliminate internal hydrogen embrittlement, it significantly lowers the risk. (Fig 2.)
During electroplating, hydrogen is generated and absorbed by the screw. The baking process aims to remove as much hydrogen as possible through effusion and trap the remaining hydrogen within the steel structure to prevent it from becoming mobile. This helps reduce the amount of hydrogen that can lead to embrittlement.
For screws used in high-mechanical-demand applications where hydrogen embrittlement is a concern, it is advisable to use organic coatings that do not involve electrolysis during the coating process.
4. How can KEBA Fastenings solutions help prevent hydrogen embrittlement in your assemblies?
Hydrogen embrittlement can cause unexpected failures, making it crucial to address this issue during the design and manufacturing stages.
KEBA Fastenings can assist in preventing hydrogen embrittlement failures in your assemblies by:
- Advising on the use of materials that are less vulnerable to hydrogen embrittlement.
- Improving joint designs to minimize stresses such as tensile, shear, and bending forces.
- Selecting the optimal coating for screws that meets your corrosion protection needs while lowering the risk of hydrogen embrittlement.
For example, coatings like zinc plating (ASTM B633) provide a protective layer that limits hydrogen diffusion. More porous coatings facilitate greater hydrogen movement, which can help in reducing the likelihood of embrittlement.
