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Tundish Trunnion Defects and Nondestructive Testing
Release Date:
2022/04/08
The ladle is a critical vessel in the metallurgical industry, primarily used in steelmaking and foundry operations to receive molten steel at the furnace front and to carry out refining and treatment of molten steel and molten iron. Among its components, the ladle trunnion plays an indispensable role: it must support the entire weight of the ladle and, in certain applications, also withstand the forces generated by gentle tilting of the ladle. Moreover, the trunnion is in constant friction with the hook lining, making it prone to scratching and the development of crack defects.
As a critical load-bearing component of the ladle, the trunnion plays a pivotal role in the safe operation of the steel ladle and directly affects the overall safety of the equipment. If defects in the trunnion are not detected in a timely manner, they can lead to serious safety accidents.
1. Major defects of the ladle trunnion
The primary defects of the trunnion stem arise from shrinkage cavities, cracks, and slag inclusions formed during casting, as well as fatigue cracks that develop during service due to relative motion between the trunnion stem’s friction pairs.
The ladle is subjected to frequent intermittent operation, resulting in alternating loads on the trunnion bearings. During lifting, starting, and stopping, the ladle’s own weight generates substantial acceleration forces, which in turn cause elastic deformation of the wire ropes, leading to repeated vibrations that increase the number of stress cycles. Consequently, transverse fatigue cracks predominantly develop in the ladle trunnions. Moreover, the connection between the trunnions and the ladle is made by direct welding; if cracks are not detected promptly, they can easily propagate under high loading conditions, posing a serious risk of slippage and detachment.
2. Hazards of Fatigue Cracks in Ladle Trunnions
Fatigue cracks pose a significant hazard. If ignored, they can at best reduce the load-carrying capacity of the ear shaft, and at worst lead to its outright fracture.
Internal defects in the ear shaft can also develop fatigue cracks under continuous tensile stress; however, in most cases, fatigue cracks are more likely to initiate at locations of surface stress concentration—such as the root of the ear shaft and grooves formed by friction with the bushing—under cyclic loading. If these small defects are not inspected during service or are overlooked during inspection, the ear shaft is prone to catastrophic fracture at the crack location under gravitational loads, potentially leading to serious safety accidents.
3. Flange Shaft Nondestructive Testing Method for Ladle
The main nondestructive testing methods for the ladle ear shaft include penetrant testing, ultrasonic testing, and magnetic particle testing.
During inspection, the inspection method for the trunnion shall be determined based on its external dimensions, shape, surface condition, and the characteristics of any defects (including type, size, location, and orientation).
Since the load on the trunnion is applied perpendicular to its longitudinal axis, and most fatigue cracks are stress-related, fatigue cracks on the trunnion typically initiate at the root of the trunnion. Magnetic particle indications generally exhibit a broad central region that tapers symmetrically toward both ends, with the cracks predominantly oriented perpendicular to the trunnion’s longitudinal axis. Moreover, the trunnion surface often bears numerous scratches caused by abrasive wear; these scratches are typically wide and shallow. Therefore, magnetic particle inspection is an appropriate method for examining the trunnion.
For newly formed fatigue cracks, since they have not yet propagated into the ear shaft and remain confined to the surface, the yoke method is employed for surface inspection. When the magnetic indications are well-defined and sharply outlined, with fine, delicate lines, this confirms the presence of a crack; conversely, if the magnetic indications are blurred and the lines are broad, this indicates a surface mechanical scratch.
Internal cracks and other defects in the trunnion can be detected by ultrasonic testing. If the reflected wave is the bottom echo, the trunnion is free of internal defects; if defects are present, the reflected wave will appear at the defect location. During inspection, scanning should be performed in two mutually perpendicular directions—along the trunnion axis and radially—to cover the entire volume of the trunnion as thoroughly as possible. This is because such defects can readily reduce the trunnion’s strength, thereby compromising the safe operation of the ladle.
Conclusion
The use of ladles is of paramount importance in the production process. To prevent major safety incidents, thorough and meticulous inspections of ladles are essential to ensure their safe operation. The trunnion is a critical component of the entire ladle structure, serving as the primary lifting element; its structural integrity directly affects the safe operation of the entire equipment system. Therefore, regular safety inspections of ladle trunnions are even more crucial.
The “Safety Regulations for Ironmaking” stipulate that the trunnions shall undergo nondestructive testing once a year. Trunnions are the most frequently used components and are also highly susceptible to wear. Of course, the entire vessel is not scrapped simply because the trunnions are damaged or have reached the end-of-life threshold; from an economic standpoint, various repair measures are typically undertaken.
The repair plan must be carefully evaluated, with technical methods such as strength verification being employed; once repaired, the component must meet all safety requirements. For this very reason, inspection of the trunnion is highly meaningful.
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