The manufacturing process of bright annealed stainless steel seamless tubes requires integrating the characteristics of stainless steel with the requirements of the annealing process, achieving dual improvements in tube performance and surface quality through multiple process steps. The following are the core process flow and key control points:
I. Raw Material Preparation
1. Raw Material Selection
Steel Grades: Commonly used grades include austenitic (304/304L, 316/316L), ferritic (430), and duplex stainless steel (2205), etc., which should be selected based on corrosion resistance, strength, and other requirements.
Raw Material Forms:
Round Bar (Hot Rolled / Forged) : Used for the piercing process, with diameters typically ranging from φ50 to φ300mm. Sulfur (S) and phosphorus (P) impurities must be controlled (S ≤ 0.03%, P ≤ 0.035%).
Continuous Cast Billet: High-precision billets can reduce subsequent machining, but attention should be paid to defects such as center porosity.
2. Pre-Treatment of Raw Materials
Surface Cleaning: Remove the oxide scale (through sandblasting and acid washing) to prevent impurities from embedding into the tube material during piercing.
Heating: Before piercing, the billet must be heated to 1100–1250°C (for austenitic stainless steel), ensuring that the billet has good ductility.
II. Piercing and Forming (Basic Process for Hot-Rolled Seamless Tubes)
1. Types of Piercing Machines
Two-Roller Skew Rolling Piercing Machine: The most commonly used type, which uses the cooperation of rollers and a piercing point to transform round bars into hollow tubes. The extension ratio can reach 1.8–2.5.
Three-Roller Piercing Machine: Suitable for high-alloy steels, reducing defects on the inner surface of the raw tube.
2. Key Parameters
Piercing Point Material: Heat-resistant alloy (such as Cr24Ni7Si2), which needs to be replaced regularly to control the quality of the inner surface.
Raw Tube Dimensions: The wall thickness is typically 2–3 times that of the finished tube, with an outer diameter tolerance of ±1.5%.
III. Rolling and Cold Working (Dimensional Precision and Performance Optimization)
1. Hot Rolling Extension (Optional)
Automatic Tube Mill: Reduces the thickness of the raw tube, improving wall thickness uniformity (tolerance from ±10% to ±8%).
Continuous Tube Mill (MPM): Continuous rolling improves efficiency, suitable for mass production, with wall thickness tolerance reaching ±5%.
2. Cold Working (Determines Final Precision)
Cold Drawing / Cold Rolling:
Cold Drawing: Reduces diameter and wall thickness through die drawing, with deformation per pass ranging from 15% to 30%, suitable for small-diameter tubes (φ6–φ150mm).
Cold Rolling: Multi-roll rolling (e.g., LG mill) achieves higher precision (wall thickness tolerance ±0.1mm), with surface roughness Ra ≤ 1.6 μm.
Intermediate Annealing : If deformation exceeds 20%–30% during cold working, stress-relief annealing (at 300–450°C, air-cooled) is required to prevent cracking caused by work hardening.
IV. Bright Annealing (Core Process)
1. Purpose of Annealing
Microstructure Optimization: Eliminate deformation-induced martensite and dislocations caused by cold working, achieving recrystallization of the grains (grain size for austenitic steel: 5–8 levels).
Surface Brightness: By using a protective atmosphere to inhibit oxidation, a uniform passivation film is formed, avoiding the residual scale left by traditional annealing.
2. Annealing Equipment and Process Parameters
Furnace Type:
Continuous Bright Annealing Furnace: Suitable for large-scale production, using belt or roller conveyance, with furnace lengths ranging from 20–50 meters.
Box-Type Bright Annealing Furnace: Suitable for small-batch, multi-specification production, requiring high sealing performance.
Protective Atmosphere:
Hydrogen (H₂): Purity ≥ 99.99%, strong reducing properties, can reduce oxide films, commonly used for austenitic stainless steel (e.g., 304), but explosion prevention must be noted (hydrogen concentration > 4% is explosive).
Nitrogen-Hydrogen Mixture (N₂ + H₂): Hydrogen content 5%–10%, reduces costs while maintaining reducing properties, suitable for ferritic stainless steel.
Pure Nitrogen (N₂): Used only for short-duration annealing or in scenarios where surface quality requirements are not high (e.g., some ferritic steels).
Temperature and Time:
Solution Annealing (Austenitic Steel): 1050–1100°C, with holding time adjusted based on wall thickness (e.g., φ50 × 3mm tube held for 15–20 minutes), followed by rapid cooling (such as water quenching or air cooling) to inhibit carbide precipitation.
Recrystallization Annealing (Ferritic / Martensitic Steel): 700–900°C, held for 30–60 minutes, then cooled in air to promote grain uniformity.
3. Key Control Points
Furnace Atmosphere Dew Point: ≤ -40°C (low dew point to avoid oxidation caused by moisture).
Heating Rate: Cold-worked tube materials require slow heating (50–100°C/h) to prevent stress cracking.
Cooling Rate: Austenitic steel requires rapid cooling (cooling rate > 50°C/s), while ferritic steel can be cooled more slowly.
V. Finishing and Inspection (Ensuring Finished Product Quality)
1. Dimensional Finishing
Straightening: Roller straighteners eliminate bending (straightness ≤ 1.5 mm/m), or tension straightening improves overall performance.
Cutting: Saw cutting or flying saw length adjustment (length tolerance ±5 mm), with end burrs removed through grinding.
2. Surface Treatment
Pickling and Passivation (Optional): If there is slight oxidation after annealing, clean using a nitric acid + hydrofluoric acid solution to form a denser passivation film (increased Cr³⁺ content).
Polishing: Mechanical polishing (using sand belts or centerless grinding) or electrolytic polishing can reduce surface roughness to Ra ≤ 0.2 μm, meeting ultra-high cleanliness requirements (e.g., for the semiconductor industry).
3. Performance Testing
Mechanical Properties: Tensile testing (yield strength, tensile strength, elongation), hardness testing (HV/HB).
Corrosion Resistance: Intergranular corrosion test (e.g., ASTM A262 E method), salt spray test (5% NaCl solution, no corrosion after ≥ 48 hours).
Surface Quality: Visual inspection (no cracks, folds, oxidation discoloration), endoscope inspection for internal defects.
Dimensional Accuracy: Outer diameter (±0.5%), wall thickness (±5%), straightness (≤1 mm/m).
VI. Comparison of Typical Process Routes
|
Tube Type |
Typical Process Flow |
Characteristics |
|
Small-Diameter Precision Tubes |
Piercing → Cold Drawing (Multiple Passes) → Intermediate Annealing → Bright Annealing → Straightening → Polishing → Inspection |
High precision (tolerance ±0.05 mm), bright surface. |
|
Large-Diameter Thick-Wall Tubes |
Piercing → Hot Rolling (MPM) → Cold Expansion → Solution Annealing → Pickling → Inspection |
High efficiency, suitable for industrial large-scale production needs. |
|
Ultra-High-Purity Tubes (Semiconductor) |
Piercing → Cold Rolling (Multi-Roll Rolling) → Vacuum Bright Annealing (H₂ Atmosphere) → Electrolytic Polishing → Helium Leak Testing → Clean Packaging |
Impurity content ≤ 10 ppm, internal surface roughness Ra ≤ 0.2 μm. |
Summary
The manufacturing of bright-annealed stainless steel seamless tubes requires balancing forming precision with surface and performance control. The core lies in eliminating cold working stress and refining grain size through protective atmosphere annealing, while ensuring final quality through precise cold working and inspection. This process is widely used in high-end manufacturing fields, with technical challenges primarily involving atmosphere control, temperature uniformity, and the adaptability of different steel grades to the process.
