What are the main process flows for manufacturing seamless ferritic stainless steel pipes and welded ferritic stainless steel pipes

Ferritic stainless steel pipes are widely used in automotive exhaust systems, heat exchangers, and household appliances due to their excellent resistance to stress corrosion cracking (SCC), low thermal expansion coefficient, and cost advantages. Their manufacturing processes are primarily categorized into seamless and welded pipes. These processes differ significantly, yet both are crucial to the mechanical properties and corrosion resistance of the final product.

Seamless Ferritic Stainless Steel Pipe Manufacturing Process

The key to seamless pipe manufacturing lies in achieving high uniformity in material structure and properties across the entire pipe wall through hot piercing and precision cold working, thereby avoiding the introduction of any welding defects.

1. Preparation and Piercing

Raw Material Selection: High-purity ferritic stainless steel round billets are used. Because ferritic grades (such as 430, 439, and 444) generally have lower ductility than austenitic steels, the metallurgical quality of the billets is extremely high, with strict control of inclusions and segregation.

Heating: The billet is heated to the piercing temperature. Precise temperature control ensures ductility while avoiding coarsening of grains or surface oxidation.

Piercing: A rotary piercer is used to punch a solid steel billet into a hollow shell. This is the most critical step in seamless pipe manufacturing, as the quality of the piercing directly determines the difficulty of subsequent processes and the quality of the internal and external surfaces of the pipe.

2. Rolling and Drawing

Hot Rolling/Extrusion: The shell then enters a pipe mill (such as a Pilger mill) for further hot rolling to reduce the outer diameter and wall thickness while improving the internal and external surface quality and dimensional accuracy, resulting in a rough tube. For certain high-alloy grades, extrusion may be used.

Cold Working Preparation: The rough tube is pickled to remove oxide scale and prepare for cold working.

Cold Working: This is a key step in achieving high-precision seamless pipe. It primarily includes cold rolling and cold drawing. Cold drawing pulls the tube through a die, reducing its dimensions and improving its surface finish. Cold working significantly increases the tube's strength, but it also causes work hardening and reduces elongation.

3. Heat Treatment and Finishing

Annealing: After cold working, the tube must undergo solution annealing (or intermediate annealing) to eliminate work hardening and residual stresses, restore the ductility of the ferritic stainless steel, and optimize its corrosion resistance. The annealing temperature and holding time significantly affect the grain size of ferritic tubes.

Straightening: This eliminates the bends introduced during heat treatment.

Finishing and Inspection: This includes cutting, chamfering, pickling, polishing, and crucially, non-destructive testing (NDT), such as eddy current testing and ultrasonic testing, to ensure the tube is free of internal defects such as cracks or interlayers.

Welded Ferritic Stainless Steel Pipe Manufacturing Process

Welded pipe manufacturing is based on strip (coil), offering the advantages of high production efficiency and dimensional accuracy. However, the metallographic structure in the weld area must be consistent with that of the base material and intergranular corrosion must be avoided.

1. Preparation and Forming

Raw Material Preparation: Finished ferritic stainless steel cold-rolled coil (cold rolled coil) or hot-rolled coil is used. The edge quality and thickness tolerance of the strip are critical, directly affecting the stability of subsequent welding.

Slitting: The coil is cut longitudinally into strips of a specific width, precisely corresponding to the circumference of the desired pipe.

Continuous Forming: The strip passes through a series of rollers, gradually bending it into an open, round tube shape, known as a tube blank. This process must be uniform and continuous to prevent stress concentrations.

2. Welding

High-Frequency Induction Welding (HFIW) or Plasma Arc Welding (PAW): This is the most commonly used welding method for ferritic stainless steel pipe.

HFIW uses a high-frequency electric current to generate heat, fusing the edges of the pipe blank. Because ferritic stainless steel grades (particularly stabilized grades) generally have excellent weldability, HFIW can achieve fast, high-quality autogenous welding (without the addition of filler metal).

The key to the welding process is precise control of heat input and extrusion to ensure grain refinement in the weld area, prevent the formation of brittle phases such as martensite, and minimize weld oxidation.

Bead Trimming: After welding, protruding weld beads inside and outside the weld must be removed immediately to meet dimensional and fluid resistance requirements.

3. Sizing and Finishing

In-Line Bright Annealing: For ferritic stainless steel, in-line continuous bright annealing is typically performed immediately after welding. Heat treatment in a protective atmosphere (such as hydrogen or a nitrogen-hydrogen mixture) aims to restore the microstructure of the weld and its heat-affected zone (HAZ), eliminate residual stresses, and maintain the surface finish of the pipe, eliminating the need for further pickling.

Sizing and Straightening: After annealing, the pipe passes through a sizing mill for final sizing and roundness correction, followed by straightening.

Eddy Current Testing: Eddy current testing of the weld area is a key quality control step, ensuring the weld is free of defects such as incomplete penetration, porosity, and cracks.