Seamless steel pipes can be divided into hot-rolled pipes, cold-rolled pipes, cold-drawn pipes, extruded pipes, etc. according to different production methods. Seamless steel pipes are an important product formed by continuous calcination and smelting under very high temperature conditions. They also maintain relatively good performance in related industries, have relatively important applications, and play a relatively large role in all walks of life. In fact, seamless steel pipes have both advantages and disadvantages, so we should look at this issue correctly.
Causes and detection methods of common failures of seamless steel pipes
1. The inner surface of the steel pipe has straight or intermittent nail-shaped raised skin. It often appears at the head of the hair tube and is easy to peel off.
Causes: Improper adjustment of perforator parameters. Steel sticking to the top. Iron oxide scale accumulation in the rough pipe, etc.
Inspection: The inner surface of the steel pipe is allowed to have peeling skin without roots that is easy to peel off (or can be burned during heat treatment). The peeling skin with roots should be ground or removed.
2. There are linear protrusions with corresponding width and depth on the inner surface of the steel pipe.
Cause: The core rod is severely worn, the grinding is not smooth or too deep, etc.
Inspection: The casing and pipeline are allowed to have an inner edge with a height not exceeding 8% of the wall thickness and a maximum height not exceeding 0.8mm that does not affect the diameter. Out-of-tolerance pipes should be repaired, ground and re-cut.
General pipes and line pipes are allowed to have inner edges with a height not exceeding 8% of the wall thickness (maximum height is 0.8mm)
For L2 grade (i.e. N5) flaw detection requirements for steel pipes, the inner edge height shall not exceed 5% (the maximum height is 0.5mm)
The sharp inner edges should be smoothed.
3. The inner surface of the steel pipe shows spots and scars, which generally do not take root and are easy to peel off.
Causes: Impurities in graphite lubricant. Iron ear at the rear end of the rough pipe is pressed into the inner wall of the steel pipe, etc.
Inspection: The inner surface of the steel pipe is not allowed to exist. The pipe ends should be ground and re-cut. The grinding should not exceed the negative deviation required by the standard. The actual wall thickness shall not be less than the smaller value required by the standard. Scars inside the entire length shall be deemed scrapped.
4. There are linear scratches of corresponding width and depth on the inner surface of the steel pipe.
Causes: low rolling temperature, hard metal objects sticking to the mandrel, impurities in the graphite, etc.
Inspection: The inner straightness of casing and general pipe is allowed to be no more than 5% (maximum depth of 0.4mm for pressure vessels). The inner straightness should be ground or removed. The inner straightness with sharp edges should be ground smooth.
5. There are linear, spiral or semi-spiral serrated defects on the inner surface of the steel pipe.
Causes: tube billet: loose center, segregation; serious shrinkage hole residue; non-metallic inclusions exceed the standard. Uneven heating of tube billet, too high or too low temperature, too long heating time. Perforation area: serious wear of the plug; improper adjustment of perforator parameters; aging of perforating roller, etc.
Inspection: Inner folds are not allowed on the inner surface of the steel pipe. The inner folds at the pipe ends should be ground or re-cut. The actual wall thickness at the ground part shall not be less than the minimum value required by the standard.
Conclusion
Seamless steel pipes can destroy the casting structure of steel ingots, refine the grains of steel, and remove microstructural defects, thereby making the steel structure dense and improving mechanical properties. This improvement is mainly reflected in the rolling direction, so that the steel is no longer isotropic to a certain extent. Bubbles, cracks and looseness formed during pouring can also be welded under high temperature and pressure. Seamless steel pipes are processed through various heat treatments and calcinations. Among various seamless steel pipes, since their use temperature and calcination temperature are relatively high, the requirements for seamless steel pipes are relatively high, especially in temperature control. The higher the requirements, the common basic temperatures and technologies must be controlled in various steel pipes.