Common defects of cold drawn steel pipe

During smelting or thermal processing, due to certain factors (such as non-metallic inclusions, gas, process selection or improper operation, etc.Internal or surface defects of steel are generated, which seriously affect the quality of materials or products, and sometimes lead to the scrapping of materials or products. Porosity, bubbles, shrinkage cavity residues, non-metallic inclusions, segregation, white spots, cracks and various abnormal fracture defects in cold drawn seamless tubing can be found through macroscopic 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, central segregation and point segregation.

General characteristics On the acid leaching sample, when the segregation is the accumulation 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-convex spot with a lighter color, an irregular shape, and a smoother appearance. During the solidification process of steel, the low-melting point substance finally solidifies, shrinks, and releases gas, resulting in pores, so it cannot be welded during thermal processing. According to its distribution, it can be divided into two categories: central type and general loose type.

On the transverse hot-dipped surface, the pores are irregular polygons with narrow pits at the bottom, usually at segregation. In severe cases, there is a tendency to become spongy. Foreign metal inclusions Metal bars, blocks and sheets falling into the ingot mold during pouring or unmelted ferroalloy added at the end of smelting. General features On etched flakes, there are many geometric shapes with sharp edges and distinct colors from their surroundings. Foreign non-metallic inclusions

During the casting process, the slag does not have time to float out, or the refractory material of the furnace lining and the inner wall of the gating system do not have time to peel off into the molten steel. Overall characteristics, larger non-metallic inclusions are easy to identify, while smaller inclusions peel off after corrosion, leaving small round holes. The semi-cured film on the surface of the steel ingot poured at the bottom of the overturned skin is rolled into molten steel. On the acid-leached sample, the color is different from the surrounding area, the shape is irregular, and there are oxide inclusions and pores around it.

The reason for the formation of shrinkage cavities or when the ingot is poured, due to the volume shrinkage during the final solidification, the liquid in the core is not replenished, and macroscopic cavities are formed in the ingot head or casting.

There are fine and short cracks on the transverse hot pickling specimen. On the longitudinal fracture, there are thick crystalline silver bright spots. Cracked cold-drawn seamless steel pipe causes cold-drawn seamless steel pipe on the transverse hot acid soaked sample, there is a crack oblique to the steel surface, the decarburization is serious nearby, and oxide skin is often sandwiched in the crack.

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