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Analysis of failure of welding of internal plate and spare tightening bolt of smelting and casting equipment
Scanning electron microscopy (SEM) was used to examine the positions of the bolts on the copper plate. Based on the macroscopic appearance and color of the fracture surfaces, it was initially determined that bolt No. 3 was the first to fail. Bolts located on either side of No. 3 were labeled as No. 1 and No. 2. For comparative purposes, the corresponding bolts on another copper plate were also removed and designated as No. 4. The fracture surfaces of bolts No. 1, 2, 3, and 4 were analyzed using a QUANTA-400 scanning electron microscope. The SEM images revealed multiple welds in the fractures of samples No. 1, 2, and 3. Notably, the unwelded area in the fracture of sample No. 3 accounted for approximately half of the surface. Further magnification showed that the unwelded region was a free surface, covered with a significant amount of flux slag.
The fracture pattern at the weld joint closely resembled that of a manually knocked-out bolt. Additionally, a layer of high-temperature oxide scale was observed at the fracture joint of No. 3, confirming that this bolt was welded first. The open weld surface was likely exposed to high temperatures during the welding process, leading to oxidation.
Metallographic analysis was conducted on bolts No. 1 and No. 4. They were cut along their axis, polished, and electrolytically etched using a 10% oxalic acid solution. Observations under an optical microscope revealed a uniform copper-iron alloy transition layer at the weld. In contrast, no such layer was present in the non-welded regions (as seen in image 7). The matrix structure was identified as austenite.
Welding defects can significantly impact the integrity of the joint between the bolts and the copper plates. If a defect exists, it disrupts the continuity of the weld, reducing its strength and creating a potential fracture initiation point. During operation, vibrations from the continuous casting machine cause stress to concentrate at these weak points. When a bolt breaks, the bonding force between the copper plate and the support plate becomes unbalanced. Due to the differing thermal expansion coefficients of the two materials, the copper plate expands and deforms inwardly, applying tensile stress to the fastening bolts. As a result, bolts near defective welds are more prone to breakage, ultimately leading to loss of fastening function.
The location of the failed bolts—specifically in the second and third rows—corresponds to the position of the molten metal surface within the crystallizer, where temperatures are highest. This area is most affected by heat, causing the copper plate to expand and deform inwardly. The resulting tensile stress on the bolts increases the likelihood of failure, especially if there are existing welding defects.
In conclusion, the early failure of the bolts was primarily due to welding defects in the joints between the bolts and the copper plates. These defects compromised the weld continuity, reduced the structural integrity, and led to premature failure under operational stresses. It is recommended that suppliers investigate improvements in the welding process or consider alternative mechanical fastening methods to enhance reliability.