Common defects and causes of cold drawn seamless tubes

In the process of smelting or hot working steel, due to certain factors (such as non-metallic inclusions, gases, and improper process selection or operation, etc.). Resulting in defects in the interior or surface of the seamless tube, which will seriously affect the quality of materials or products, and sometimes lead to scrapping of materials or products.

Looseness, 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. The methods of macroscopic inspection are divided into two methods: acid leaching inspection and fracture inspection. A brief introduction to common macroscopic defects revealed by acid leaching is as follows:

1. Segregation

Causes of formation: During the casting and solidification process, the aggregation of certain elements caused by selective crystallization and diffusion, resulting in uneven chemical composition. According to the different positions of the distribution, it can be divided into ingot type, center and point segregation.

Macroscopic characteristics: On the acid leaching sample, when the segregation is erodible material or gas inclusions, it is dark in color, irregular in shape, slightly concave, flat at the bottom and has many dense microporous spots. If the resist elements are aggregated, it will be light-colored, irregularly shaped, and relatively smooth micro-convex spots.

2. Looseness

Formation reason: During the solidification process, the steel fails to be welded during the hot working process due to the final solidification shrinkage of the low melting point material and the release of gas to generate voids. According to their distribution, they can be divided into two categories: central and general loose.

Macroscopic features: On the lateral hot acid leaching surface, the pores are irregular polygons and pits with narrow bottoms, which usually appear in segregation spots. In severe cases, there is a tendency to connect into a spongy shape.

3. Inclusions

Formation reason:

① Foreign metal inclusions

Cause: During the pouring process, the metal strips, blocks and sheets fall into the ingot mold or the ferroalloy added at the end of the smelting stage is not melted.

Macroscopic features: On the etched sheet, there are mostly geometric shapes with clear edges and significantly different colors from the surrounding.

② Foreign non-metallic inclusions

Cause: During the pouring process, there is no slag floating out or the refractory material of the furnace lining and the inner wall of the pouring system that has been peeled off into the molten steel.

Macroscopic features: Larger non-metallic inclusions are well identifiable, while smaller inclusions corrode and flake off, leaving small circular holes.

③Flip the skin

Cause of formation: The semi-solidified film on the surface of the bottom-casting ingot is involved in the molten steel.

Macroscopic features: On the acid leaching sample, the color is different from the surrounding, and the shape is irregularly curved and narrow strips, and there are often oxide inclusions and pores around the periphery.

4. Shrinkage

Causes of formation: When the ingot or casting is poured, the liquid in the core cannot be replenished due to the volume shrinkage during final condensation, and macroscopic cavities are formed in the head of the ingot or casting.

Macroscopic features: The shrinkage cavity is located in the center of the transverse acid leaching sample, and the surrounding area is often segregated, mixed or loosely dense. Sometimes caves or crevices can be seen before etching, and after etching, the parts of the holes are darkened and appear as irregularly wrinkled holes.

5. Bubbles

Cause of formation: Defects caused by gas generated and released during the casting of the ingot.

Macroscopic features: On the transverse specimen, there are cracks roughly perpendicular to the surface, with slight oxidation and decarburization nearby. The presence of subcutaneous bubbles below the surface is called subcutaneous bubbles, and the deeper subcutaneous bubbles are called pinholes. During the forging process, these unoxidized and unwelded pores are extended into thin tubes with isolated small pinholes on the cross section. The cross section is similar to the regular point segregation, but the darker color is the inner honeycomb bubble.

6. White Spot

Cause of formation: It is generally considered to be the effect of hydrogen and structural stress, and the segregation and inclusions in the steel also have a certain influence, which is a kind of crack.

Macroscopic features: On the transverse hot acid leaching sample, there are short and thin cracks. On the longitudinal fracture, there are coarse crystalline silver bright white spots.

7. Cracks

Causes of formation: Axial intergranular cracks. When the dendritic structure is serious, cracks occur in the large-sized billet along the main branch of the dendritic structure and between the branches.

Internal cracking: cracking due to improper forging and rolling process.

Macroscopic features: On the cross section, the axial position cracks along the intergranular, showing a spider web shape, and radial cracking in severe cases.

8. Fold

Causes of formation: The surface scars of the cold-drawn carbon steel tube or ingot are uneven and the edges and corners of the tip are superimposed on the cold-drawn seamless tube during forging and rolling, or ears are formed due to improper pass design or operation, and continue rolling. Superimposed during production.

Macroscopic characteristics: On the transverse hot acid dipping sample of cold-drawn seamless steel tube, there is an oblique crack with the surface of the steel, there is serious decarburization in the vicinity, and oxide scales are often contained in the crack.

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