Stainless steel seamless pipe is a long steel material with a hollow cross-section and no seams around the perimeter. The thicker the wall thickness of the stainless steel seamless pipe, the more economical and practical it is, while thinner walls lead to a significant increase in processing costs.
The process of this product determines its limited performance. Generally, seamless steel pipes have low precision: uneven wall thickness, low brightness of inner and outer surfaces, high fixed-length cost, and difficult removal of spots and black spots on both inner and outer surfaces. Its testing and shaping must be done offline. Therefore, it demonstrates its superiority in high pressure, high strength, and mechanical structure materials. Stainless steel pipes are divided into hot-rolled, hot-extruded, and cold-drawn stainless steel pipes according to the rolling method.
According to the different stainless steel metallographic structures, they are divided into semi-ferritic and semi-martensitic stainless steel seamless pipes, martensitic stainless steel seamless pipes, austenitic stainless steel seamless pipes, and austenitic-ferritic duplex stainless steel seamless pipes, etc.
Specifications and appearance quality
- A. According to GB14975-2002 “Stainless Steel Seamless Steel Pipe” regulations, the usual length of steel pipes (non-fixed length) for hot-rolled steel pipes is 1.510m, and for hot-extruded steel pipes, it is equal to or greater than 1m. For cold-drawn (rolled) steel pipes with a wall thickness of 0.51.0mm, the lengths are 1.07m; for those with a wall thickness greater than 1.0mm, the lengths are 1.58m.
- B. There are a total of 45 diameters ranging from 54mm to 480mm for hot-rolled (hot-extruded) steel pipes; there are 36 wall thicknesses ranging from 4.5mm to 45mm. There are a total of 65 diameters ranging from 6mm to 200mm for cold-drawn (rolled) steel pipes; there are 39 wall thicknesses ranging from 0.5mm to 21mm.
- C. The inner and outer surfaces of the steel pipe must be free of cracks, folds, tortuosity, cracks, wrinkles, rolling defects, delamination, and scabs. These defects should be completely removed (except for pipes for mechanical processing), and after removal, they should not cause the wall thickness and outer diameter to exceed the negative deviation. Other minor surface defects that do not exceed the allowable negative deviation may not be removed.
- D. Allowable straightness depth: For hot-rolled, hot-extruded steel pipes with a diameter less than or equal to 140mm, the straightness depth shall not exceed 5% of the nominal wall thickness, and the maximum depth shall not exceed 0.5mm; for cold-drawn (rolled) steel pipes, the straightness depth shall not exceed 4% of the nominal wall thickness, and the maximum depth shall not exceed 0.3mm.
- E. Both ends of the steel pipe should be cut at right angles and burrs removed.
Manufacturing process of stainless steel seamless pipe
Hot rolling
Hot rolling (extruding seamless steel pipe): Round billet → heating → piercing → three-roll skew rolling, continuous rolling or extrusion → descaling → sizing (or reducing) → cooling → straightening → hydrostatic test (or flaw detection) → marking → warehousing.
Raw Material Preparation
The raw material for rolling seamless pipes is a round billet. The round billet needs to be cut into billets about 1 meter long by a cutting machine and then conveyed to the furnace for heating.
Heating and Piercing Process
The billet is heated to approximately 1200 degrees Celsius inside the furnace, with hydrogen or acetylene used as fuel. Temperature control inside the furnace is critical. After heating, the round billet undergoes piercing by a pressure piercing machine.
Piercing and Rolling/Extrusion
The commonly used piercing machine is a conical roller piercing machine, known for its high production efficiency and ability to pierce various steel grades. Following piercing, the round billet is successively rolled or extruded by three-roll skew rolling, continuous rolling, or extrusion.
Descaling and Sizing
After extrusion, descaling and sizing are required. The sizing machine uses a high-speed rotating conical drill head to punch holes in the billet to form a steel pipe. The inner diameter of the steel pipe is determined by the outer diameter length of the sizing machine drill head.
Cooling and Straightening
After sizing, the steel pipe enters the cooling tower and is cooled by water spraying. Following cooling, the steel pipe needs to be straightened for further processing.
Quality Inspection and Finishing
After straightening, the steel pipe is sent to a metal flaw detection machine (or hydrostatic test) for internal flaw detection by a conveyor belt. If there are cracks, bubbles, and other problems inside the steel pipe, they will be detected. After quality inspection, the steel pipe undergoes strict manual selection. Following inspection, the steel pipe is painted with numbers, specifications, production batch numbers, etc., before being lifted into the warehouse by a crane.
Cold drawing
Cold drawing (rolling) seamless steel pipe: Round billet → heating → piercing → heading → annealing → acid pickling → oiling (copper plating) → multiple cold drawing (cold rolling) → billet pipe → heat treatment → straightening → hydrostatic test (flaw detection) → marking → warehousing.
The rolling method of cold-drawn (rolled) seamless steel pipes is more complicated than that of hot-rolled (extruded) seamless steel pipes. The production process of the first three steps is basically the same. The difference starts from the fourth step. After the round billet is pierced and headed, it needs to undergo annealing. After annealing, it needs to be pickled with special acidic liquid. After pickling, oil is applied. Then, it is followed by multiple cold drawing (cold rolling), billet pipe production, and special heat treatment. After heat treatment, it needs to be straightened.
Conclusion
In conclusion, the production process of stainless steel seamless pipes involves several intricate steps, from raw material preparation to quality inspection and finishing. Each stage requires precision and adherence to strict standards to ensure the production of high-quality pipes suitable for various applications. As technology advances and industry standards evolve, manufacturers continue to refine their processes to meet the growing demands for stainless steel seamless pipes in various industries. With meticulous attention to detail and continuous improvement, the stainless steel seamless pipe industry remains vital in supporting infrastructure development and industrial applications worldwide.
