1.What are the testing methods and testing points for cold bending tests?
Objective: To simulate actual processing conditions and evaluate the bending performance of materials under controlled conditions using a standard test.
Method:
Using a bending testing machine, according to national standards (e.g., GB/T 232), industry standards, or mutual agreements, the specimen is bent around a mandrel (indenter) of a certain diameter to a specified angle (e.g., 90°, 180°).
Common types include three-point bending and spiral bending.
Test Points:
After bending, the outer side of the specimen (the surface under tensile stress) is directly observed with the naked eye or using a low-magnification magnifying glass (5-10x).
Check for cracks, fissures, or complete fracture. Cracks may appear transversely (perpendicular to the bending direction) or longitudinally.
Result Judgment: Record the mandrel diameter, bending angle, and whether cracks have occurred. This is the most basic acceptance criterion.
2.How about macroscopic observation and dimensional measurement?
Visual inspection: Conduct an overall observation of the cracked part, recording the location of the crack (distance from the edge, whether it appears regularly along its length), its direction, and its macroscopic morphology.
Dimensional measurement: Use calipers and micrometers to measure the actual thickness of the cracked area and compare it with the nominal thickness to check for serious negative thickness deviations or localized thinning.
3.What are the objectives and methods of penetrant testing?
Objective: To detect and make more visible microcracks opening on the sample surface.
Method: Apply red colorant to the curved area, clean it, and then apply a developer. The microcracks will appear as clear red lines. Suitable for precise laboratory analysis of crack direction and origin.
4.What are the methods for testing hardness?
Objective: To check whether the material hardness is uniform or whether there are localized hard spots.
Method: Perform Vickers or Rockwell hardness tests from the edge of the rolled sheet to the core, or along the vicinity of the crack path.
Analysis: Excessive hardness or uneven hardness distribution (usually caused by uneven microstructure or localized work hardening) significantly reduces plasticity, leading to bending cracks.
5.How are chemical components analyzed?
Although the chemical composition of cold-rolled coils generally meets the standards, the content and ratio of trace elements (such as N, Al, Ca, S, and P) have a significant impact on bending performance. For example, when the nitrogen content is high and the aluminum content is insufficient, brittle AlN is easily formed and precipitated at the grain boundaries, leading to "aging brittleness" and intergranular cracking.