Seamless stainless steel pipes are produced through a series of steps involving heating, piercing, rolling, and finishing to create pipes with no welded seams. Here's a detailed explanation of the production process, followed by the testing methods and standards that ensure their quality.
Production Process of Seamless Stainless Steel Pipes
Raw Material Preparation
Billet Preparation: Stainless steel billets (cylindrical solid bars) are selected and cut to the required length. The billets are inspected for surface defects and prepared for heating.
Heating
Heating the Billet: The billet is heated in a rotary hearth furnace or an induction furnace to a temperature typically around 1200°C - 1300°C (2192°F - 2372°F) to soften the metal and make it more workable.
Piercing
Piercing Process: The heated billet is transferred to a piercing mill where it is pierced using a rotary piercer or a Mannesmann piercing mill. The billet is rotated and pushed over a piercing mandrel or plug to create a hollow shell. This forms the rough tube with an initial hollow center.
Rolling
Elongation or Pilger Mill Rolling: The hollow shell is rolled in an elongator or pilger mill to further reduce the diameter and wall thickness. The pipe passes through a series of rolling stands with tapered rollers that gradually reduce its thickness and elongate its length.
Mandrel Mill Process: A mandrel (a long cylindrical rod) is inserted into the hollow pipe while it passes through the rolling stands. This helps in achieving the desired internal and external dimensions while maintaining a smooth internal surface.
Sizing and Stretch Reducing
Sizing Mill: The pipe goes through a sizing mill to achieve precise outer diameter and wall thickness. The sizing operation ensures dimensional accuracy.
Stretch Reducing Mill: In some cases, the pipe undergoes stretch reducing, where it is further reduced in diameter and fine-tuned for thickness.
Heat Treatment
Annealing: The pipe is subjected to heat treatment or annealing to relieve internal stresses, enhance ductility, and improve the mechanical properties. The annealing process involves heating the pipe to a specific temperature and then cooling it rapidly (quenched) or slowly, depending on the desired properties.
Straightening and Cutting
Straightening: The pipe is passed through a straightening machine to correct any bends or deviations from the desired shape.
Cutting: The pipe is cut to the required lengths using cutting machines, such as saws or flame cutters.
Surface Finishing
Descaling and Pickling: The pipe surface is cleaned using acid solutions (pickling) or abrasive blasting to remove scale, oxides, and other impurities.
Polishing: The pipe is polished to achieve the required surface finish, particularly for applications where a smooth finish is critical.
Inspection and Testing
Visual and Dimensional Inspection: Pipes are visually inspected for surface defects, and their dimensions are checked to ensure they meet the required specifications.
Testing Methods and Standards for Seamless Stainless Steel Pipes
Seamless stainless steel pipes are subjected to a range of tests to verify their quality and compliance with standards.
Common Testing Methods:
Non-Destructive Testing (NDT):
Ultrasonic Testing (UT): Uses high-frequency sound waves to detect internal flaws or discontinuities.
Eddy Current Testing (ECT): Detects surface and near-surface defects using electromagnetic induction.
Magnetic Particle Inspection (MPI): Detects surface and slightly subsurface discontinuities using magnetic fields and iron particles (applicable only to ferromagnetic materials).
Dye Penetrant Inspection (DPI): Uses a liquid penetrant to reveal surface defects that may not be visible to the naked eye.
Destructive Testing:
Tensile Testing: Measures the pipe's strength by pulling it until it breaks.
Hardness Testing: Determines the material's resistance to indentation.
Flattening Test: Assesses the pipe's ability to withstand flattening without cracking.
Bend Test: Evaluates the ductility and ability of the pipe to withstand bending without breaking.
Impact Testing: Assesses the pipe's toughness or ability to absorb energy during fracture.
Hydrostatic Testing:
The pipe is filled with water or another liquid and pressurized to check for leaks, defects, or structural weaknesses. It verifies the pipe's ability to withstand its maximum rated pressure.
Standards for Seamless Stainless Steel Pipes:
Several international standards specify the manufacturing, testing, and quality requirements for seamless stainless steel pipes:
ASTM Standards:
ASTM A312: Standard Specification for Seamless, Welded, and Heavily Cold Worked Austenitic Stainless Steel Pipes.
ASTM A213: Standard Specification for Seamless Ferritic and Austenitic Alloy Steel Boiler, Superheater, and Heat-Exchanger Tubes.
ASTM A269: Standard Specification for Seamless and Welded Austenitic Stainless Steel Tubing for General Service.
ASTM A790: Standard Specification for Seamless and Welded Ferritic/Austenitic Stainless Steel Pipe.
ASME Standards:
ASME SA312, SA213, SA269, SA790: These are specifications equivalent to the ASTM standards but published by the American Society of Mechanical Engineers for use in pressure piping applications.
EN (European Norm) Standards:
EN 10216-5: Seamless steel tubes for pressure purposes – Technical delivery conditions – Part 5: Stainless steel tubes.
EN 10297-2: Seamless circular steel tubes for mechanical and general engineering purposes – Technical delivery conditions – Part 2: Stainless steel.
ISO Standards:
ISO 1127: Stainless steel tubes – Dimensions, tolerances, and conventional masses per unit length.
ISO 2037: Stainless steel tubes for the food industry.
These standards provide guidelines for material properties, dimensions, tolerances, surface finish, mechanical properties, and testing methods to ensure that the seamless stainless steel pipes meet the required performance and safety criteria for their intended applications.