In the process of smelting or hot working, due to some factors (such as non-metallic inclusions, gases, process selection or improper operation, etc.), internal or surface defects of
seamless tubes are generated, which seriously affect the quality of materials or products, and sometimes lead to material defects. or product obsolescence. Looseness, air bubbles, shrinkage cavity residues, non-metallic inclusions, segregation, white spots, cracks and various abnormal fracture defects in cold drawn seamless tubes can be found through macro inspection.
Macro inspection methods are divided into acid leaching inspection and fracture inspection.
During solidification, some elements aggregate due to selective crystallization and diffusion, resulting in inhomogeneous chemical composition. According to the different distribution positions, it can be divided into ingot type, center segregation and point segregation.
General characteristics: On the acid leaching sample, when the segregation is the aggregation of corrosive substances or gas inclusions, the color is darker, the shape is irregular, slightly concave, the bottom is flat, and there are many dense microporous spots. If it is an accumulation of corrosion-resistant elements, it is a micro-protrusion with a lighter color, an irregular shape and a smoother appearance. During the solidification of the steel, the low melting point material eventually solidifies, shrinks and outgassing, creating pores and thus cannot be welded during hot working. According to its distribution, it can be divided into two categories: central type and general loose type.
General characteristics: On the lateral hot-dip plated surface, the pores are irregular polygons with narrow pits at the bottom, which usually appear at segregation. Seriously, there is a tendency to become spongy. Foreign Metal Inclusions During the casting process, the metal strips, blocks and sheets fall into the ingot mold or the ferroalloy added at the end of the smelting is not melted. The general feature is that on the etched sheet, there are many geometric shapes with sharp edges and a distinct color difference from the surrounding.
Foreign non-metallic inclusions:
During the pouring process, the slag does not have time to surface, or the refractory material of the furnace lining and the inner wall of the pouring system do not have time to peel off into the molten steel.
General characteristics: Larger non-metallic inclusions are easily identified, while smaller inclusions flake off after corrosion, leaving small round holes. The semi-cured film on the surface of the steel ingot poured at the bottom of the inverting skin is rolled into the molten steel. On the acid leached sample, the color is different from the surrounding, the shape is irregular, and there are oxide inclusions and pores around.
The reason for the formation of shrinkage cavities: When the ingot is poured, due to the volume shrinkage during final solidification, the liquid in the core cannot be replenished, and macroscopic voids are formed in the ingot head or casting.
There are small and short cracks on the transverse hot acid leaching specimen. On the longitudinal fracture, there are coarse crystalline silver bright white spots. The reason for the formation of cracked cold-drawn seamless steel pipe is that the cold-drawn seamless steel pipe has a diagonal crack on the steel surface on the transverse hot acid soaked sample, and the nearby decarburization is serious, and the crack is often sandwiched with oxide scale.
