In harsh industrial environments such as petrochemical plants, fastener failure frequently leads to severe production safety incidents. At a certain oil refinery, a double-ended stud fracture occurred after 16 years of service in the floating-head cooler situated at the top of the regeneration tower within a gas purification unit. In this shell-and-tube condenser, the shell-side medium consisted of circulating water at a temperature of 90–100°C, while the tube-side medium was acidic gas under a pressure of 0.4 MPa. As a critical fastener connecting core components, the double-ended stud in question was fabricated from 35CrMoA alloy steel. However, over its prolonged service life, these fasteners ultimately failed to withstand the rigors of the complex operating conditions, thereby necessitating the present failure analysis.
Macroscopic examination of the fractured studs revealed that the fracture sites were predominantly concentrated at the roots of the threads. The fracture surfaces were coated with a substantial amount of black corrosion products; furthermore, the cross-sections appeared relatively flat and exhibited no discernible plastic deformation-characteristics typical of brittle fracture. Upon cleaning, the fracture surfaces clearly displayed features indicative of fatigue cracking: the fatigue origin was located at the root of the thread, propagating inward at a specific angle before entering a zone of rapid propagation characterized by a radial pattern. This mode of failure is not uncommon in double-ended studs subjected to prolonged alternating loads and is frequently the result of the synergistic interaction of multiple contributing factors.
To further investigate the root cause of the fracture, technicians conducted a spectral chemical analysis and hardness testing on the bolt. The results indicated that the bolt's chemical composition complied with the national standard requirements for 35CrMoA material. However, the hardness test results revealed a potential latent defect: the base metal hardness of the double-ended stud reached a high level of 350–360 HV1. According to relevant standards established by NACE (National Association of Corrosion Engineers) and API (American Petroleum Institute), steel components subjected to load-bearing conditions in hydrogen sulfide-containing environments must have their hardness controlled below 22 HRC to effectively resist hydrogen sulfide stress corrosion cracking. Evidently, the hardness of this specific batch of double-ended threaded studs was excessively high, placing them within a sensitive range where they were highly susceptible to hydrogen embrittlement and stress corrosion fracture.
Metallographic examination further substantiated the material-level defects. Axial cross-sections of the samples revealed that, due to improper control of the heat treatment process, the thread surface had developed a fully decarburized layer approximately 31 μm thick, characterized by a ferrite microstructure; conversely, the core microstructure consisted of tempered sorbite. This surface decarburization not only diminished the strength of the surface layer but also rendered it highly prone to the initiation of microcracks during the tightening process, thereby serving as the starting point for fatigue fracture. For precision fasteners such as these double-ended machine screws, the rigor and precision of the heat treatment process directly determine their ultimate microstructure and mechanical properties.
Scanning Electron Microscopy (SEM) observations of the fracture surface morphology provided even more microscopic evidence. The entire original fracture surface was blanketed in a substantial layer of corrosion products, and distinct fatigue striations were clearly visible within the crack initiation zone. As the crack propagated deeper into the material-with its propagation rate accelerating-the fracture morphology gradually transitioned from characteristics typical of corrosion fatigue to those indicative of stress corrosion cracking. This suggests that during its service life, the double-ended hex bolt was subjected to the dual assault of cyclic stress fluctuations and a corrosive medium, thereby accelerating both the initiation and propagation of the crack.
Energy Dispersive Spectroscopy (EDS) analysis of the corrosion products successfully identified the culprit among the environmental factors. The resulting spectral data revealed that the corrosion products contained a significant concentration of sulfur (S) elements, alongside a minor presence of chlorine (Cl) elements. This directly demonstrates that the H2S, moisture, and other corrosive saline media present in the heat exchanger's shell-side feedstock have permeated the threaded mating surfaces. These deleterious elements constitute critical preconditions for stress corrosion in low-alloy steel components, causing ordinary screw bolts to undergo insidious intergranular corrosion and embrittlement-even during seemingly stable operational periods.
Taken together, this failure incident was the unfortunate result of the combined interplay of three major factors: stress, material properties, and environmental conditions. Regarding stress, in addition to the axial preload applied during assembly, the mass of the floating head itself generated radial bending stresses; furthermore, thermal stresses induced by equipment start-up and shut-down cycles-coupled with vibrations caused by pulsating process media-subjected the bolts to a complex state of alternating stress over an extended period. For this specific type of Double-Ended Socket Screw, the combination of an excessively high yield-to-tensile strength ratio and superimposed residual stresses significantly reduced its fatigue life.
To prevent a recurrence of such unfortunate incidents, enterprises must enhance their inspection protocols for critical fasteners during equipment maintenance. In particular, for components such as Double-Ended Wing Screws-which perform vital connection functions-periodic hardness re-verification, magnetic particle testing, and torque calibration are indispensable preventive maintenance measures, serving to nip potential safety hazards in the bud.
In summary, whether dealing with general-purpose Dual-Head Stud Bolts or specialized corrosion-resistant fasteners, safe operation is inextricably linked to scientific material selection and meticulous management. If you have any further questions regarding the selection of industrial fasteners, failure analysis, or corrosion-prevention solutions, please feel free to contact us at any time for professional technical support!
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