Hydrogen embrittlement testing of rail bolts and service safety of high-strength bolts
Why are high-strength track bolts more prone to hydrogen embrittlement fracture than ordinary bolts?
The higher the strength grade of high-strength bolts, the higher their matrix hardness and tensile strength, the lower the material's toughness, and the stronger the sensitivity to hydrogen atoms. High-strength bolts of grade 8.8 and above undergo quenching and tempering treatment, forming a dense martensitic structure inside, where hydrogen atoms are highly likely to accumulate at grain boundaries. When the hydrogen atom concentration reaches the critical value, "hydrogen-induced cracking" will occur, resulting in sudden brittle fracture without obvious plastic deformation. Ordinary bolts (such as grade 4.8) have a ferrite-pearlite matrix structure with good toughness, where hydrogen atoms are not easy to accumulate, and the risk of hydrogen embrittlement is extremely low.
Mainstream detection methods include constant load tensile test, delayed fracture test and ultrasonic testing, each with different application scenarios. The constant load tensile test is the arbitration method specified by the Chinese standard, suitable for factory batch testing. A load of 75% of the yield strength is applied, and no fracture within 200 hours is qualified. The delayed fracture test is suitable for the research and development stage, testing the hydrogen embrittlement sensitivity of bolts by simulating extreme load environments. Ultrasonic testing is a rapid on-site screening method, suitable for the detection of installed bolts. It quickly finds hidden danger bolts by identifying the reflected waves of hydrogen-induced cracks, avoiding disassembly losses.
Different surface treatment processes have significant differences in hydrogen embrittlement risk, and the core lies in whether hydrogen atoms are introduced. Electroplating (such as electro-galvanizing) produces a large number of hydrogen atoms, with extremely high hydrogen embrittlement risk; ordinary electroplating processes are strictly prohibited for high-strength bolts. The amount of hydrogen atom infiltration in hot-dip galvanizing is small, but high temperatures will cause hydrogen atom diffusion, which needs to be combined with dehydrogenation treatment. Zinc flake (Dacromet) and zinc diffusion coating processes are hydrogen-free processes, almost not introducing hydrogen atoms, and are the preferred surface treatment methods for high-strength track bolts. In addition, improper control of the pickling process can also be a major source of hydrogen atoms, so it is necessary to optimize the pickling formula and add a dehydrogenation process.
The typical characteristic of hydrogen embrittlement fracture is "brittle fracture without plastic deformation", with a flat, bright gray crystalline fracture surface and no obvious fatigue striations. The fracture location mostly occurs at the thread root or shank stress concentration area of the bolt, and there are no precursors such as loosening or deformation before fracture, which is a sudden fracture. The fatigue fracture surface is divided into fatigue source, propagation zone and instantaneous fracture zone. The propagation zone has clear fatigue striations, and slight plastic deformation or loosening signs will appear before the bolt fractures. Through fracture morphology analysis, the two fracture forms can be quickly distinguished, providing a basis for accident analysis.
In terms of emergency measures, it is necessary to immediately block the line section, conduct a comprehensive investigation on the fracture of bolts of the same batch, replace all unbroken bolts of the same batch, and strictly prohibit continued use. In terms of prevention and control measures, it is necessary to trace the surface treatment process and hydrogen embrittlement test report of the batch of bolts; if there are process violations or missing tests, the supplier should be included in the blacklist. In subsequent procurement, high-strength bolts treated by hydrogen-free processes such as Dacromet and zinc diffusion coating should be preferred, and suppliers should be required to provide third-party hydrogen embrittlement test reports. At the same time, a bolt warehousing inspection system should be established on site to eliminate hydrogen embrittlement hidden dangers from the source.