Analysis of the hydrogen embrittlement fracture mechanism of bolts
Bolts, as core components of mechanical connections, directly affect equipment safety and operational efficiency through their reliability. However, hydrogen embrittlement fracture, a typical mode of bolt failure, has long been a technical challenge in engineering due to its insidious nature, suddenness, and catastrophic consequences. This paper systematically analyzes hydrogen embrittlement fracture from four dimensions: its mechanism, typical characteristics, influencing factors, and prevention strategies. Combined with practical cases from wind power, automotive, and aerospace fields, it reveals the inherent laws governing hydrogen embrittlement fracture.
I. Physicochemical Mechanism of Hydrogen Embrittlement Fracture
The essence of hydrogen embrittlement fracture is the degradation of material properties caused by the diffusion, aggregation, and interaction of hydrogen atoms in the metal lattice. Its core mechanisms can be divided into two categories:
1. Hydrogen Atom Migration and Crack Initiation
After hydrogen atoms penetrate the metal matrix through processes such as electroplating, pickling, and heat treatment, they migrate to high-stress areas (such as the thread root and the headstock transition zone) driven by stress and concentration gradients. When hydrogen atoms aggregate at defects (such as pores, inclusions, and dislocations) to form hydrogen molecules, the local hydrogen pressure can reach hundreds of megapascals, leading to microcrack nucleation. For example, a 12.9-grade wind turbine bolt was not dehydrogenated after electroplating. Hydrogen atoms accumulated at the root of the thread to form hydrogen molecules. The hydrogen pressure caused the microcracks to expand, eventually leading to fracture within 48 hours after installation.
2. Synergistic Effect of Hydrogen-Enhanced Local Plasticity (HELP) and Hydrogen-Enhanced Exfoliation (HEDE)
Hydrogen atoms promote dislocation movement and crack propagation by reducing interatomic bonding forces (weak-bond theory) and surface energy (surface-energy-reduction theory). At the crack tip, hydrogen atoms aggregate to form Cottrell gas clusters, reducing the resistance to dislocation movement and inducing local plastic deformation (HELP effect); simultaneously, hydrogen atoms weaken grain boundary bonding forces, leading to intergranular fracture (HEDE effect). In a fatigue test of a high-strength automotive bolt, hydrogen atoms accelerated crack initiation through the HELP effect, and then caused the crack to propagate rapidly along the grain boundaries through the HEDE effect, ultimately leading to brittle fracture.
II. Typical Characteristics of Hydrogen Embrittlement Fracture
Hydrogen embrittlement fracture has unique macroscopic and microscopic characteristics, which can be comprehensively determined through fracture morphology, hydrogen content detection, and process traceability:
1. Macroscopic Fracture Characteristics
• Delayed Fracture: Hydrogen embrittlement fracture usually occurs within hours to months after bolt installation, without obvious signs of plastic deformation. For example, a 10.9 grade bolt in a wind power project suddenly broke three months after installation, with the fracture surface flush and perpendicular to the direction of the maximum normal stress.
Fault No. 1
Fault No. 2
• Multi-regional stratification: The fracture surface can be divided into a crack initiation zone, a crack propagation zone, and a fracture zone. The crack initiation zone is located at the root of the thread or at stress concentration points, appearing as dark gray crystalline structures; the propagation zone is silvery-gray with visible radial striations; the fracture zone exhibits ductile fracture due to stress concentration, showing dimples.
2. Microscopic fracture characteristics
• Intergranular fracture: Microcracks exist at the grain boundaries in the crack initiation zone, exhibiting a candy-like morphology, with “chicken claw marks” visible on the grain surface. For example, in the fracture surface of a 12.9 grade bolt, the grain boundary spacing in the crack initiation zone is widened, and the accumulation of hydrogen atoms leads to grain boundary weakening.
• Quasi-cleavage fracture: The propagation zone exhibits cleavage steps and river patterns, with dimples in some areas, indicating localized plastic deformation during crack propagation.
• Hydrogen bubble traces: In cases of improper electroplating or pickling processes, residual hydrogen bubbles are visible on the fracture surface. Hydrogen bubbles were observed on the fracture surface of an oil dipstick, indicating incomplete dehydrogenation and hydrogen embrittlement.
3. Hydrogen Content Detection
The hydrogen content of the bolt matrix is measured using an oxygen, nitrogen, and hydrogen analyzer, combined with process tracing to identify the hydrogen source. For example, a self-tapping screw with a surface hardness of 690 HV, far exceeding the standard requirement, was not dehydrogenated after electroplating, resulting in a significantly increased hydrogen content and leading to hydrogen embrittlement fracture.
III. Factors Affecting Hydrogen Embrittlement Fracture
Hydrogen embrittlement fracture results from the combined effects of material, process, and environmental factors. Its sensitivity can be assessed through the following dimensions:
1. Material Factors
• Strength Grade: High-strength steels with a tensile strength ≥1050 MPa are sensitive to hydrogen embrittlement. For example, the risk of hydrogen embrittlement in 12.9 grade bolts (tensile strength ≥ 1080 MPa) is significantly higher than that in 8.8 grade bolts (tensile strength ≥ 830 MPa).
• Metallographic structure: Tempered martensite is the most sensitive to hydrogen embrittlement, followed by upper bainite and lower bainite, while austenite exhibits the best resistance. A certain wind turbine bolt uses tempered martensite, which is highly susceptible to hydrogen embrittlement, requiring strict control of the heat-treatment process.
• Chemical composition: Elements such as carbon, sulfur, and phosphorus increase the sensitivity to hydrogen embrittlement. For every 0.1% increase in carbon content, the hydrogen embrittlement threshold decreases by approximately 20%.
2. Process factors
• Electroplating process: Electroplating zinc, cadmium, and other cathode plating layers easily induces hydrogen embrittlement, requiring dehydrogenation treatment at 200-300℃ within 24 hours after electroplating. For example, an 8.8-grade bolt that was not dehydrogenated after electroplating fractured due to hydrogen embrittlement, resulting in an excessive actual strength.
• Pickling process: Excessive pickling time or high acid concentration can exacerbate hydrogen permeation. A gasket manufacturer used pickling rather than shot peening, leading to hydrogen-induced fractures in high-strength gaskets.
• Heat treatment process: Excessively high quenching temperatures or excessively rapid cooling rates can increase the hydrogen diffusion rate. A 12.9-grade bolt was heated to 950℃ during quenching, resulting in increased hydrogen permeation and fracture.
3. Environmental Factors
• Stress State: Tensile stress is a necessary condition for hydrogen embrittlement, while compressive stress can suppress it. Insufficient preload during the installation of a bolt led to a hydrogen embrittlement fracture due to alternating tensile stress during service.
• Temperature: The hydrogen atom diffusion rate decreases at low temperatures (<100℃), increasing susceptibility to hydrogen embrittlement fracture. A wind power project experienced a significant increase in bolt hydrogen embrittlement fracture rate during winter construction.
IV. Prevention Strategies for Hydrogen Embrittlement Fracture
To address the mechanisms and influencing factors of hydrogen embrittlement fracture, a systematic solution must be developed across four areas: material design, process optimization, environmental control, and monitoring and maintenance.
1. Material Selection and Modification
• Prioritize the use of hydrogen embrittlement-resistant materials, such as austenitic stainless steel (304L, 316L), low-strength steel (≤8.8 grade), or surface-modified alloys. For example, a car engine uses 304L stainless steel bolts; a high-cooling-rate process refines the grain size, thereby reducing hydrogen-trap density.
• Additive Manufacturing Technology: 316L stainless steel bolts are manufactured using Directed Energy Deposition (DED), followed by annealing to eliminate residual stress and reduce hydrogen embrittlement fracture sensitivity.
2. Process Optimization and Control
• Dehydrogenation Annealing: Immediately after electroplating, a dehydrogenation treatment at 200-300℃ for 4-24 hours is performed to reduce the hydrogen content to a safe threshold (<2ppm). For example, a wind turbine bolt, after electroplating, underwent a dehydrogenation process at 220℃ for 8 hours, resulting in a 90% reduction in hydrogen embrittlement fracture rate.
• Surface Coating Technology: Nickel plating or ceramic coatings are used to block hydrogen permeation. Nickel plating can reduce hydrogen permeability to less than 1/10 that of the raw material, thereby lowering the risk of hydrogen embrittlement.
• Hydrogen-Free Surface Treatment: Mechanical methods such as shot peening and polishing are used instead of pickling to reduce hydrogen penetration. A gasket manufacturer completely solved the problem of hydrogen embrittlement fractures after switching to shot peening.
3. Environmental and Stress Synergistic Control
• Hydrogen-Containing Environment Isolation: Bolts are prevented from contacting high-temperature, high-pressure hydrogen gas, acidic solutions, or humid environments. The inner wall of the oil pipeline is coated with a polymer to reduce direct contact between hydrogen and metal.
• Stress Concentration Optimization: Localized stress is reduced through structural design. For example, increasing the fillet radius at bolt connections homogenizes stress distribution and reduces the risk of hydrogen-induced cracking.
4. Monitoring and Maintenance
• Real-time Hydrogen Content Monitoring: The amount of hydrogen absorbed by materials in high-pressure environments is monitored using gas chromatography or hydrogen probe technology. When the hydrogen concentration approaches the critical threshold, an emergency dehydrogenation procedure is initiated.
• Microstructure Characterization: Hydrogen distribution and dislocation structure are observed using transmission electron microscopy (TEM) and electron backscatter diffraction (EBSD) techniques to guide process parameter optimization.
V. Conclusion
Hydrogen embrittlement fracture of bolts is a complex phenomenon resulting from the combined effects of materials, processes, and environmental factors. Its prevention requires a comprehensive approach throughout the entire lifecycle, from design and manufacturing to service. By selecting anti-hydrogen embrittlement materials, optimizing process parameters, controlling environmental stress, and implementing dynamic monitoring and maintenance, the risk of hydrogen embrittlement fracture can be significantly reduced. In the future, as additive manufacturing, intelligent sensing, and digital twin technologies develop, the prediction and prevention of bolt hydrogen embrittlement fracture will become more precise and reliable, providing a solid guarantee for the safe operation of high-end equipment.