Bolt failure is a typical reliability issue in petrochemical and high-temperature, high-pressure heat exchange equipment during long-term operation. Double-ended bolts, as key fastening components in the sealing structures of floating head heat exchangers, condensers, and pressure vessels, directly impact the overall operational stability of the equipment. A floating head cooler in the regeneration system of an oil refinery experienced bolt fracture failure after 16 years of operation. This case exhibits typical characteristics of corrosion fatigue and stress corrosion coupling, providing important reference value for understanding the failure mechanism of fasteners in wet H2S environments.
The equipment operated under the following conditions: shell-side circulating water temperature of 90–100℃, tube-side acidic gas temperature of 30–35℃, and operating pressures of 0.2 MPa and 0.4 MPa, respectively. The medium contained a certain proportion of H2S and salt corrosive media. The bolt specification was M20×2.5mm, and the material was 35CrMoA quenched and tempered steel. Under the combined effects of long-term alternating thermal stress, assembly preload, and medium corrosion, the double-ended bolt eventually fractured at the root of the thread.
Macroscopic observation revealed that the fracture surface was located in the thread root region, with a relatively smooth morphology and no obvious macroscopic plastic deformation. Black corrosion products adhered to the surface. After cleaning, the fracture surface exhibited typical fatigue propagation characteristics, with the crack originating at the thread root and propagating inward at an angle of approximately 20°. It then entered a rapid propagation zone exhibiting a radial pattern, ultimately resulting in instantaneous fracture. This morphological characteristic indicates that the failure process of the Double Ended Machine Screw involved multiple stages: corrosion fatigue, stress corrosion cracking, and instantaneous fracture.
Chemical composition analysis showed that the material as a whole met the requirements of the 35CrMoA standard, indicating that the failure was not caused by abnormal composition. Hardness testing showed that the Double Ended Lag Bolt had a relatively high overall hardness, with the matrix structure mainly composed of tempered sorbite, reaching a local level of 350–360 HV, which is higher than the recommended range for conventional fasteners. Metallographic analysis further revealed a certain degree of decarburization on the thread surface, with a decarburized layer of approximately 31 μm. The surface microstructure was predominantly ferrite. This type of heterogeneous structure easily leads to localized stress concentration under stress, thus inducing crack initiation.
Scanning electron microscopy (SEM) analysis further revealed the fracture mechanism of the Double Ended Hex Bolt. Obvious fatigue striations were observed in the crack initiation region, and corrosion products filled the crack during propagation. As the crack propagation rate increased, the fracture surface gradually transitioned from fatigue propagation to stress corrosion cracking characteristics, exhibiting typical quasi-cleavage and radial propagation morphology, indicating that the corrosive medium participated in the crack propagation process. Energy dispersive spectroscopy (EDS) analysis showed that the corrosion products contained high levels of sulfur and small amounts of chloride, indicating significant sulfide and chloride corrosion in the service environment
Comprehensive analysis indicates that this failure is a typical corrosion fatigue and sulfide stress corrosion cracking problem caused by the coupling of multiple factors. Its formation mechanism mainly manifests in three aspects.
First, stress factors. The double-end threaded stud experiences high preload during assembly, resulting in significant axial tensile stress at the thread root. Simultaneously, the floating head structure introduces additional bending stress. During operation, thermal stress from expansion and contraction, vibration loads from medium flow, and cyclic stress during start-up and shutdown keep the screw bolt in a state of high stress fluctuation for extended periods, accelerating crack initiation and propagation.
Secondly, material factors play a role. While 35CrMoA steel possesses high strength, it is highly susceptible to hydrogen-induced cracking and sulfide stress corrosion at high hardness. Relevant standards indicate that in H2S-containing environments, the stress corrosion resistance of high-strength steel significantly decreases if the hardness exceeds a certain range. In this case, the double-ended socket screw has excessive hardness and exhibits localized decarburization defects, further reducing surface crack resistance and forming a typical crack initiation area.
Thirdly, environmental factors are significant. The medium contains H2S, moisture, and salts, leading to complex corrosion reactions under electrochemical conditions. The anodic process generates Fe2+, which combines with sulfur ions to form FeS corrosion products. Simultaneously, hydrogen atoms generated by the cathodic reaction can diffuse into the steel, inducing hydrogen embrittlement and blistering effects. Under the combined action of stress and corrosion, cracks gradually propagate at the root of the dual-end screw thread, accelerating failure.
Furthermore, the volume expansion effect of corrosion products further exacerbates the crack propagation of dual-threaded screws. The volume of corrosion products is typically 2 to 4 times the original volume, creating additional expansion stress within the closed crack. This causes the stress at the crack tip to continuously increase, thus accelerating the fracture process. This "corrosion-stress-propagation" positive feedback mechanism is a key characteristic of this type of failure.
In summary, this type of double-ended screw failure is essentially the result of the combined effects of structural stress concentration, material sensitivity, and the corrosive environment. For similar conditions, comprehensive improvements should be made, including reducing the hardness sensitivity of M10 double-ended screws, optimizing heat treatment processes, controlling preload levels, and improving sealing and corrosion protection design, to enhance the long-term service reliability of the structure.
For further information on the failure mechanism analysis of dual-head stud bolts or customized fastener selection recommendations, please contact our technical team for engineering support and solutions tailored to your specific needs.
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