Due to the large size of the head and the limitations of the existing steel plate width specification of 2.2 meters, the manufacturing process prioritized maximizing the use of stainless steel scraps. Initially, stainless steel plates were manually welded using arc welding. Subsequently, three stainless steel plates and three carbon steel plates were joined using explosive welding to create a composite board. The composite boards were then manually arc-welded to form a single composite plate blank.
The composite plate elliptical head was shaped using a cold spinning process. To investigate the cause of cracking in the composite plate head and develop effective countermeasures to prevent similar issues in the future, engineers conducted an in-depth analysis.
1. Analysis of the Inner Wall of the Composite Plate Head
The damage to the inner wall of the head was confined to the splicing welds between stainless steel plates 1 and 2. No issues were observed at the splicing welds of the composite plate itself. The cracks on the stainless steel splicing welds were located in the center depressions of the welds and were primarily caused by overload tearing due to excessive plastic deformation.
Analysis of the Outer Wall of the Composite Plate Head
On the carbon steel side of the outer wall, cracking was accompanied by visible signs of excessive deformation, necking, and splitting. These deformations corresponded precisely to the splicing welds of stainless steel plates 1 and 2 on the inner wall. Observation revealed that the main crack surface aligned with the splicing welds of the inner stainless steel plates. Additionally, the crack propagation exhibited steps where cracks from different layers intersected.
This indicates that the cracking during the cold spinning process resulted from multi-crack source overload tearing. As the cracks propagated, they intersected with cracks at different levels, altering their direction momentarily before returning to the primary crack’s path. This primary path corresponded to the inner wall’s stainless steel splicing welds, highlighting that this weld area was the weakest point in the composite plate.
The main fracture displayed a distinct stress rest line, indicative of a fibrous plastic fracture caused by repeated overloading and deformation. The fracture originated from the stainless steel plate’s inner wall and extended into the carbon steel substrate. Upon close inspection, multiple crack sources were identified on the stainless steel plate, confirming that the main fracture was a multi-crack source failure.
Microanalysis
Further microanalysis was conducted to delve deeper into the material and structural factors contributing to the cracking.
2. Metallographic Analysis of the Carbon Steel Substrate
The void morphology in the carbon steel substrate of the composite plate before spinning resembles that of the material before processing. However, after cold spinning, the number and diameter of voids increase significantly. This occurs because, during cold spinning, the carbon steel base undergoes cold work hardening due to repeated force applications. This leads to dislocation accumulation and changes in the metallographic structure. These changes in void size and distribution are the primary cause of material tearing and failure.
2.2 Metallographic Analysis of Stainless Steel Splicing Welds
The splicing welds on the stainless steel plate were created using double-sided manual arc welding, resulting in double-sided formations without any repair welding. The weld on the side bonded to the carbon steel base is the secondary weld, while the weld on the inner wall surface of the head is the primary weld. A geometric defect, less than 2 mm in size, exists between the two welds due to insufficient penetration during the second weld. This defect forms a continuous linear welding imperfection along the weld center.
These welding defects became crack initiation points during the cold spinning process. Further magnification of these defects reveals preferential crack initiation at their edges. Thus, the linear welding defect in the stainless steel plate is identified as the primary source of internal tearing in the composite plate head.
2.3 Scanning Electron Microscopy (SEM) Analysis
SEM analysis of the fracture samples revealed that the stainless steel composite plate fractures exhibit a characteristic plastic equiaxed dimple structure, indicative of an overload plastic fracture. Similarly, the carbon steel substrate fractures also show a plastic dimple structure, but with significant size variations. Many abnormally large dimples were observed, which were linked to material inconsistencies and voids in the metallographic structure. These voids are a major source of the large dimples in the fractures.
Summary
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Welding Defects as a Source of Cracking
The linear defects in the double-sided manual arc welds of the stainless steel plate, distributed along the weld center, were a critical source of cracking during the manufacturing of the DN6200 composite plate head. Repeated forces during the spinning process caused these cracks to propagate, ultimately leading to head failure due to overload.
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Carbon Steel Substrate Cavities
The numerous voids in the carbon steel substrate were responsible for the formation of abnormally large dimples during overload fractures. These voids contribute to the uneven properties of the material and are areas where cracking is most likely to occur. The increase in void number and size is primarily attributed to the repeated external forces during the cold spinning process, which caused cold work hardening, dislocation accumulation, and changes in the metallographic structure.
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Recommendations
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Control of Voids: Efforts should be made to minimize the number and size of voids in the carbon steel substrate to reduce potential defects exacerbated by cold working.
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Welding Quality: The quality of splicing welds must be strictly controlled. In particular, during manual arc welding of stainless steel plates, thorough inspection of splicing welds is essential to eliminate hidden internal defects and ensure reliable weld integrity.
