1.What are the effects of welding processes?
Flattening seam welding (for thin plates): For deep-drawing cold-rolled thin plates with a thickness of 0.5-1.5mm, studies show that when high-speed flattening seam welding is used, and the welding process parameters are appropriate and there are no welding defects (such as spatter or insufficient fusion), the joint strength can be higher than that of the base material after recrystallization annealing. The tensile fracture occurs in the base material region, exhibiting ductile fracture characteristics.
Resistance spot welding (for high-strength steel): For high-strength cold-rolled plates such as DH590, resistance spot welding performed within a suitable process window shows that the main failure mode of the weld is pull-out fracture. Numerous shear dimples are observed on the fracture surface, indicating ductile fracture and reliable weld strength.
2.How do micro-organisms evolve?
Weld Zone Strengthening: After welding cold-rolled strip steel, the microhardness of the weld zone is usually the highest, determined by the granular bainite, lath bainite, and small amount of martensite formed in the weld zone.
Heat-Affected Zone (HAZ) Weakening: The real weak point often lies in the critical coarse-grained zone of the HAZ. This region exhibits a large amount of polygonal ferrite, leading to a significant decrease in microhardness, gradually approaching the hardness of the base metal. Cracks often originate in this region during cupping tests.
Special Characteristics of Stainless Steel: For cold-rolled and annealed 316L stainless steel, although the welded joint maintains a certain strength (tensile strength of approximately 608 MPa for a single weld), it is significantly lower than that of the cold-rolled base metal (approximately 1059 MPa). The weld microstructure is typical equiaxed dendritic austenite.
3.What are the key influencing factors?
Heat treatment condition: Whether or not post-weld annealing is performed significantly affects strength. Recrystallized joints can have strength exceeding that of the base metal; while non-recrystallized joints have lower strength than the base metal, and fracture occurs at the weld toe, exhibiting brittle fracture.
Welding defects: The strength of welded joints is mainly affected by welding defects such as internal spatter and insufficient fusion. Only defect-free joints can achieve their intended mechanical properties.
4.Under what circumstances is the weld strength of cold-rolled coil sufficient?
The weld strength of cold-rolled coils is sufficient under optimized process and defect-free conditions, and can even exceed that of the base material through heat treatment. However, in practical applications, special attention needs to be paid to the performance degradation of the heat-affected zone and the precise control of welding process parameters to ensure that the weld does not become the starting point of fracture during subsequent rolling or stamping processes.
5.What are the strength design targets for rolled coils during rolling or deep processing?
For welds in cold-rolled coils during rolling or deep processing, the strength design target is to match the base material, but key points need to be considered in practical applications.