Hydrogen Embrittlement Prevention Technology and Connection Reliability Assurance for High-Strength Bolts Used in Rail Fasteners
1. What are the core causes of hydrogen embrittlement and main failure characteristics of high-strength bolts for rail fasteners?
The core cause of hydrogen embrittlement of high-strength bolts for rail fasteners is that hydrogen atoms invade and remain inside the steel during processing links such as cold working, electroplating and pickling, forming hydrogen-induced cracks under tensile stress and eventually leading to brittle fracture. During cold working, plastic deformation of steel produces lattice defects, where hydrogen atoms tend to accumulate; during electroplating (such as galvanizing), hydrogen atoms in the electrolyte are reduced at the cathode and penetrate into the bolt surface; during pickling derusting, hydrogen atoms generated by the reaction between acid solution and steel also invade the substrate. When the bolt is in service, under the combined action of fastening preload and train vibration tensile stress, the residual hydrogen atoms inside will diffuse to the stress concentration area, making the plasticity and toughness of the steel drop sharply. When the hydrogen content exceeds the critical value (≥2ppm), hydrogen embrittlement fracture will form at the microcracks. The main characteristic of hydrogen embrittlement failure is sudden fracture without plasticity. The fracture surface is flat and bright, showing cleavage or quasi-cleavage morphology, without obvious necking and plastic deformation traces. Fracture mostly occurs at stress concentration parts such as the root of the bolt thread and the transition fillet between the head and the shank, and the fracture often occurs in a short time after bolt fastening without obvious pre-damage signs. It is a sudden failure and poses a great threat to the safety of rail connection.
2. What are the core process links of the whole-process hydrogen embrittlement prevention and control for high-strength rail bolts?
The core process links of the whole-process hydrogen embrittlement prevention and control for high-strength rail bolts are divided into three stages: processing hydrogen control, aging dehydrogenation and finished product hydrogen prevention, covering the whole process of hydrogen invasion, hydrogen residue and hydrogen re-invasion, realizing precise control of hydrogen content. The processing hydrogen control stage focuses on reducing the initial invasion of hydrogen atoms: warm heading is adopted to replace traditional cold heading in cold working, the processing temperature is controlled at 150-200℃ to reduce the generation of lattice defects and the probability of hydrogen atom accumulation; low-hydrogen pickling process with hydrogen inhibitor is adopted before electroplating, the acid solution temperature is controlled at 40-50℃, and the pickling time is shortened to 5-8 minutes to reduce hydrogen atom invasion; cyanide-free alkaline galvanizing is adopted to replace traditional acid galvanizing to reduce the amount of hydrogen precipitation in the electrolyte, and hydrogen remover is added to the electroplating solution to reduce the reduction of hydrogen atoms at the cathode. The aging dehydrogenation stage is the key of prevention and control: immediately after bolt processing, low-temperature aging dehydrogenation treatment is carried out, the bolt is placed in an oven at 200-220℃ for 24-30 hours, and the diffusion and escape of hydrogen atoms are accelerated by temperature rise, so that the hydrogen content inside the bolt is reduced to below 0.5ppm, far lower than the critical value. The finished product hydrogen prevention stage focuses on preventing the re-invasion of hydrogen atoms: after dehydrogenation treatment, the bolt is sealed and coated with a special hydrogen-proof sealant coating with a thickness of 10-15μm to form a dense protective film, which prevents hydrogen atoms in the external environment (such as hydrogen in humid environment) from penetrating into the bolt again and improves the anti-corrosion performance of the bolt at the same time.
3. What are the adaptation requirements of hydrogen embrittlement prevention and control parameters for high-strength rail bolts of different strength grades?
The core of the adaptation requirements of hydrogen embrittlement prevention and control parameters for high-strength rail bolts of different strength grades is to adjust the temperature and time of aging dehydrogenation and the hydrogen control strength of processing technology to match the hydrogen embrittlement sensitivity of steel of different strength grades. Grade 8.8 bolts are medium strength grade with low hydrogen embrittlement sensitivity, and the adapted prevention and control parameters are: warm heading temperature 150℃, low-hydrogen pickling time 8 minutes, aging dehydrogenation at 200℃ for 24 hours after alkaline galvanizing, hydrogen content after dehydrogenation controlled at ≤0.8ppm, and surface hydrogen-proof coating thickness 10μm. Grade 10.9 bolts are high strength grade with moderate hydrogen embrittlement sensitivity, and the adapted prevention and control parameters are: warm heading temperature 170℃, low-hydrogen pickling time 6 minutes, increased amount of hydrogen remover during galvanizing, aging dehydrogenation at 210℃ for 26 hours, hydrogen content after dehydrogenation ≤0.6ppm, and surface hydrogen-proof coating thickness 12μm. Grade 12.9 bolts are ultra-high strength grade with high carbon content, many lattice defects and extremely high hydrogen embrittlement sensitivity, which are the key of prevention and control. The adapted prevention and control parameters are: warm heading temperature 200℃, low-hydrogen pickling time 5 minutes, hydrogen-free galvanizing process replacing traditional alkaline galvanizing, aging dehydrogenation at 220℃ for 30 hours, and secondary hydrogen content detection after dehydrogenation to ensure hydrogen content ≤0.5ppm, surface hydrogen-proof coating thickness 15μm. At the same time, fillet strengthening treatment is carried out on the root of the bolt thread to reduce stress concentration and the probability of hydrogen-induced crack initiation. Bolts of all strength grades require aging dehydrogenation to be completed within 4 hours after electroplating to avoid the diffusion of hydrogen atoms into the steel.
4. What are the core detection methods and judgment standards for hydrogen embrittlement of high-strength rail bolts?
The core detection methods for hydrogen embrittlement of high-strength rail bolts are divided into two categories: hydrogen content detection and hydrogen embrittlement sensitivity detection, which cooperate with each other to realize a comprehensive judgment of hydrogen embrittlement risk, covering the whole links of raw materials, processing and finished products. Hydrogen content detection adopts the thermal extraction method: the bolt sample is placed in a vacuum heating furnace and heated to 600-800℃ to completely escape the internal hydrogen atoms, and the content of escaped hydrogen is detected by a hydrogen analyzer. This method has a detection accuracy of 0.1ppm, which is the core means for quantitative detection of hydrogen content. The hydrogen content of raw materials shall be ≤1.0ppm, processing semi-finished products ≤0.8ppm, and finished products ≤0.5ppm (Grade 12.9)/≤0.8ppm (Grade 8.8). Hydrogen embrittlement sensitivity detection adopts slow strain rate tensile test (SSRT) and constant load tensile test. In the SSRT, the bolt sample is stretched to fracture at a low speed of 1×10⁻⁶s⁻¹. If the elongation after fracture and reduction of area of the sample decrease by ≤10% compared with the hydrogen-free sample, the hydrogen embrittlement sensitivity is judged to be low. In the constant load tensile test, a tensile stress of 75% of the nominal tensile strength is applied to the sample and kept for 200 hours at room temperature. If the sample has no fracture, it is judged to be qualified. In addition, the finished bolts also need to undergo appearance and metallographic detection: no cracks and pitting on the appearance, no hydrogen-induced microcracks in metallographic detection, and no abnormal lattice distortion in stress concentration areas such as the root of the thread. The sampling inspection ratio of hydrogen content detection for each batch of bolts is ≥5%, and hydrogen embrittlement sensitivity detection is batch-by-batch detection. All unqualified batches are reworked and dehydrogenated again.
5. What are the key points of hydrogen embrittlement prevention and control in on-site installation and use of high-strength rail bolts?
The key points of hydrogen embrittlement prevention and control in on-site installation and use of high-strength rail bolts mainly focus on avoiding secondary hydrogen absorption, controlling fastening stress and preventing stress concentration to ensure that hydrogen embrittlement failure is not caused during the service of the bolts and the connection reliability is guaranteed. Before installation, check the state of the surface protective layer of the bolt, strictly prohibit the use of bolts with damaged and peeling coating to prevent hydrogen atoms in media such as humid air and rainwater from invading the bolt through the damaged part. If the coating has slight damage, repair it with a special hydrogen-proof repair agent before use. During bolt fastening, the preload must be strictly controlled, and a torque wrench is used to tighten according to the design torque: the tightening torque of Grade 8.8 bolts is ≤300N·m, Grade 10.9 ≤450N·m, Grade 12.9 ≤600N·m. Avoid excessive tensile stress on the bolt caused by over-torque fastening, which induces hydrogen embrittlement fracture. At the same time, the "step-by-step tightening method" is adopted for tightening, which is tightened to the design torque in 2-3 times to avoid stress mutation. During installation, ensure the matching accuracy of the bolt with the nut and pressure plate, the thread engagement gap is ≤0.1mm without jamming and eccentric load, to prevent additional stress concentration at the root of the thread and aggravate the accumulation of hydrogen atoms. During service, the inspection cycle is 3 months, focusing on checking whether the bolt has cracks and fracture signs, especially at the thread root and head transition. If microcracks appear on the bolt surface, replace it immediately and it is strictly forbidden to continue using it. In addition, in hydrogen-absorbing environments such as coastal and high-humidity areas, regularly apply hydrogen-proof and anti-corrosion grease on the exposed parts of the bolts to strengthen secondary protection, prevent hydrogen atom invasion, and clean the accumulated water and corrosive media around the bolts in time to reduce the inducing factors of hydrogen embrittlement.