What are the common defects of cold drawn seamless carbon steel pipes?

What are the common defects of cold drawn seamless carbon steel pipes?

During the smelting or thermal processing process, materials or products are sometimes scrapped due to the influence of certain factors (such as non-metallic inclusions, gases, and improper process selection or operation, etc.).
Through macroscopic inspection, looseness, bubbles, shrinkage cavity residues, non-metallic inclusions, segregation, white spots, cracks and various abnormal fracture defects of cold-drawn seamless steel pipes can be found. Macroscopic inspection methods are divided into two methods: acid leaching inspection and fracture inspection. A brief summary of common macroscopic defects revealed by acid leaching is as follows:

1. Isolation
Causes:
During the casting and solidification processes, some elements aggregate due to selective crystallization and diffusion, resulting in uneven chemical composition. According to the different distribution positions, it can be divided into ingot segregation, central segregation and point segregation.
Macroscopic features:
On acid-leached samples, when they segregate into erodibles or gas inclusions, they are dark in color, irregular in shape, slightly concave, flat at the bottom, and have many dense micropore spots. If resist elements accumulate, light-colored, irregularly shaped, relatively smooth, slightly raised spots will form.

2. loose
Cause: During the solidification process of steel, due to the final solidification shrinkage of low-melting point materials, gas is released and cavities are generated, resulting in the steel being unable to be welded during thermal processing. According to its distribution, it can be divided into two categories: central loose type and general loose type.
Macroscopic features:
The side pores of hot acid leaching are irregular polygons and narrow pits at the bottom, which mostly appear in segregation spots. In severe cases, there is a tendency to connect into a spongy shape.

3. Reasons for the formation of inclusions
① Metal inclusions
Causes:
During the casting process, metal strips, blocks, and plates fall into the ingot mold or the iron alloy added at the end of smelting is not melted.
Macroscopic features:
On the etched sheet, there are many geometric shapes with sharp edges, and the colors are obviously different from the surroundings.
 
② Foreign non-metallic inclusions
Causes:
During the pouring process, no slag should float out, and no refractory material on the furnace lining and inner wall of the pouring system should peel off into the molten steel.
Macroscopic features:
Larger non-metallic inclusions can be easily identified, while smaller inclusions will corrode and flake away, leaving small, round holes.

4. Lift skin
Causes:
The semi-solidified film on the surface of the bottom ingot is rolled into the molten steel.
Macroscopic features:
On acid-leached samples, the color is different from that of the surroundings, and the shape is an irregular curved narrow strip, often surrounded by oxide inclusions and pores.

5. Shrinkage
Causes:
When the ingot or casting is poured, due to the volume shrinkage during final condensation, the liquid in the core cannot be replenished, forming a macroscopic cavity at the head of the ingot or casting.
Macroscopic features:
On the side acid-leached samples, the shrinkage cavity is located in the center, and there are often segregation, inclusions or loose spots around it. Sometimes caves or gaps can be seen before etching, and after etching, parts of the holes become darker and appear as irregular wrinkled holes.

6. Bubbles
Causes:
Defects caused by gases produced and escaped during ingot casting.
Macroscopic features: There are cracks roughly perpendicular to the surface on the transverse specimen, with slight oxidation and decarburization nearby. Subcutaneous bubbles: Subcutaneous bubbles that exist below the surface are called subcutaneous bubbles, and deeper subcutaneous bubbles are called pinholes. During the forging process, these unoxidized and unwelded pores extend into thin tubes with isolated small pinholes in their cross-sections. The cross-section resembles regularly arranged point-like segregations, but the darker ones are internal honeycomb bubbles.

7.White spots
Causes:
It is generally believed to be the effect of hydrogen and structural stress. Segregation and inclusions in steel also have a certain impact and are a type of crack.
Macroscopic features:
Short, thin cracks were present on the transverse hot acid-leached samples. There are large bright white spots of crystallized silver on the longitudinal fracture.

8. Reasons for the formation of cracks in cold drawn seamless steel pipes:
Axial intergranular cracks: When the dendritic structure is serious, cracks will 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.

9.Fold
Formation reason:
The surface scars of cold-drawn seamless steel pipes or steel ingots are uneven and the edges and corners of the tip are superimposed on the cold-drawn seamless steel pipe during forging and rolling, or ears are formed due to improper pass design or operation, and are stacked when continuing to roll. combined.
Macro Features:

Cold-drawn seamless steel pipe has an oblique crack with the surface of the steel on the transverse hot acid dipping sample, and there is serious decarburization nearby, and oxide scales are often contained in the crack.