BA-Grade Stainless Steel Seamless Pipe Production Process

BA-Grade stainless steel seamless pipe (bright annealed stainless steel seamless pipe) is a high-precision stainless steel pipe. Its core feature is the bright annealing process, which achieves an oxidation-free surface and high gloss, while maintaining excellent mechanical properties and corrosion resistance. It is widely used in industries such as semiconductors, food and medicine, and precision instruments, where cleanliness and surface quality are extremely demanding. The production process requires strict control of precision and cleanliness at every step. It is divided into six core stages: raw material preparation, hot rolling, cold rolling and finishing, bright annealing, finishing inspection, and finished product packaging. Details of each stage are as follows:

First, Raw Material Preparation for BA-Grade Stainless Steel Seamless Pipe

BA-grade stainless steel pipe requires extremely high raw material purity (for example, 304/316L stainless steel must have a C ≤ 0.03% and an S ≤ 0.015% to prevent impurities from affecting corrosion resistance and surface quality). This stage focuses on screening qualified billets and pre-processing them:

1. BA-Grade Stainless Steel Seamless Pipe Billet Selection: Round stainless steel billets that meet ASTM or GB standards (such as austenitic stainless steels like 304, 316L, and 317L) are used. The billet diameter is determined by the specifications of the finished pipe (usually 80-200mm). Spectroscopic analysis is performed to check the chemical composition of the billets to ensure that the content of alloying elements such as Cr, Ni, and Mo meets the required standards, and to avoid internal defects such as inclusions and shrinkage cavities. 2. BA Grade Stainless Steel Seamless Pipe Billet Pretreatment

Surface Cleaning: Shot blasting or pickling (nitric acid + hydrofluoric acid mixture) removes scale, oil, and impurities from the billet surface to prevent defects from transferring to the finished product during rolling.

3. Heating of BA Grade Stainless Steel Seamless Pipe: The cleaned billet is fed into a continuous heating furnace and uniformly heated at 850-1200°C (depending on the steel grade) to soften the billet to a plastic state in preparation for subsequent piercing. (The heating atmosphere must be controlled to avoid oxidation.)

Second, the hot rolling stage of BA-grade seamless stainless steel pipe.

This stage involves hot working the round billet into a "rough tube" (preliminary seamless pipe). The key is to ensure uniform wall thickness and crack-free steel pipe. The specific steps are:

1. Piercing: The heated round billet is fed into a two-roll cross-roll piercing mill. Two rotating rolls and a centering mandrel work together to roll the solid round billet into a hollow "rough tube" (the inner diameter is determined by the mandrel diameter, and the outer diameter is controlled by the roll spacing). This step requires strict control of roll speed and mandrel position to avoid defects such as uneven wall thickness and internal folding in the rough tube.

2. Hot Rolling & Elongation: The rough tube is fed into a continuous pipe mill (MPM) or automatic rolling mill (AZM). Through continuous rolling using multiple sets of rolls, the outer diameter and wall thickness of the rough tube are reduced to near-finished specifications (the "rough tube"). Rolling oil is used during the rolling process to reduce roll wear and surface scratches. 3. Sizing & Reducing: The rough pipe is fed into a sizing mill, where precision rollers calibrate the outer diameter to the standard size (tolerance ≤ ±0.1mm). If a smaller outer diameter is required, it is fed into a reducing mill, where multiple rolling passes reduce the outer diameter while ensuring uniform wall thickness.

4. Cooling & Straightening: After hot rolling, the rough pipe is first cooled to room temperature by water or air to prevent high-temperature oxidation. It is then fed into a multi-roll straightening mill to correct the curvature of the pipe (BA-grade seamless stainless steel pipe requires a straightness of ≤1mm/m), ensuring accuracy in subsequent processing.

Third, the cold-rolling finishing stage for BA-grade seamless stainless steel pipes.

The surface roughness of the hot-rolled rough pipe (Ra ≈ 5-10μm) does not meet BA-grade requirements, requiring further surface refinement and dimensional tolerance control through cold rolling:

1. Pickling and Degreasing: Before cold rolling, remove the surface oxide scale and rolling oil from the rough pipe.

- Pickling: Soak in a mixture of nitric acid and hydrofluoric acid to dissolve the surface oxide scale (to prevent residual oxide layers that affect the cold-rolled surface quality).

- Degreasing: Use an alkaline detergent or organic solvent to remove rolling oil to prevent oil from being embedded in the surface during cold rolling.

2. Cold Rolling: The pickled raw tube is fed into a cold rolling mill (such as an LD or LG mill). Using a combination of rollers and mandrels, the steel tube undergoes multiple cold rolling passes (with diameter and wall reduction controlled at 10%-20% per pass). This ultimately achieves:

- Dimensional accuracy: OD tolerance ≤ ±0.05mm, wall thickness tolerance ≤ ±5%;

- Surface roughness: Ra reduced to 0.8-1.6μm (preparing for subsequent bright annealing).

The cold rolling process requires the use of cold rolling lubricant (typically extreme pressure lubricant) and a controlled rolling speed (10-30m/min) to avoid surface scratches.

3. Degreasing and Cleaning: After cold rolling, any residual cold rolling oil is immediately removed through spray cleaning and ultrasonic cleaning to ensure that residual oil content is ≤5mg/m². (Oil contamination can cause dark spots on the surface during bright annealing, affecting gloss.)

Fourth, bright annealing of BA-grade seamless stainless steel pipes.

Bright annealing is the core difference between BA-grade seamless stainless steel pipes and ordinary stainless steel pipes. Annealing in a protective atmosphere eliminates cold-rolling internal stresses, improves mechanical properties, and prevents surface oxidation. The specific process is as follows:

1. Atmosphere Preparation: Use a high-purity inert gas (such as 99.999% hydrogen, nitrogen, or a hydrogen-nitrogen mixture) as the protective atmosphere. Ensure that the oxygen content in the annealing furnace is ≤10 ppm (excessive oxygen content can cause surface oxidation and discoloration).

2. Annealing Process Control: Degreased cold-rolled tubes are fed into a continuous bright annealing furnace, with the "heating-holding-cooling" curve strictly controlled:

- Heating Phase: Heating at a rate of 5-10°C/min to 900-1100°C (typically 1050°C for 304 steel and 1080°C for 316L steel);

- Holding Phase: Holding at the target temperature for 30-60 minutes to eliminate the work hardening caused by cold rolling and restore the toughness of the stainless steel (hardness ≤ HV200 after annealing);

- Cooling Phase: Rapid cooling (such as water cooling or forced air cooling) is used to cool the tube from the annealing temperature to below 200°C to prevent grain growth that affects mechanical properties while maintaining surface gloss.

3. Surface Inspection: After annealing, the surface of the tube should exhibit a uniform silvery-white luster, free of oxidation discoloration (such as blue, yellow, or black spots), and a surface roughness of Ra ≤ 0.4μm (some high-end BA tubes require Ra ≤ 0.2μm).

Fifth, BA-Grade Stainless Steel Seamless Pipe Finishing and Inspection Stage

As a high-precision product, BA-grade stainless steel seamless pipes undergo multi-dimensional inspection to eliminate substandard products and undergo meticulous finishing:

1. Pipe Cutting and Chamfering: Based on the order length (e.g., 6m, 3m), precision cutting is performed using a CNC pipe cutting machine (such as laser cutting or plasma cutting), with a cut perpendicularity error of ≤0.5°. The pipe ends are then chamfered (both internal and external) to prevent scratches and burrs from sharp edges.

2. Nondestructive Testing (NDT)

- Ultrasonic Testing (UT): Detects internal defects such as cracks and inclusions (detection sensitivity ≥ Φ2.0mm flat-bottom hole);

- Eddy Current Testing (ET): Detects surface and near-surface defects (such as scratches and pinholes, with a depth of ≤0.1mm).

- Hydrostatic Testing: Hydrostatic testing (HP) is performed on the pipe (test pressure ≥ 1.5 times the design pressure), maintaining pressure for 30 seconds to ensure leak-proof sealing.

3. Dimensional and Surface Inspection

- Dimensional Measurement: Use a laser caliper and wall thickness micrometer to measure the outside diameter and wall thickness to ensure tolerances meet standards (such as ASTM A269 and GB/T 14976).

- Surface Inspection: Visual inspection (under strong light) or a surface roughness tester to verify surface quality. Defects such as oxidation discoloration, scratches, and pits are not permitted.

4. Chemical Composition and Mechanical Property Re-inspection: Samples undergo spectral analysis (to confirm chemical composition) and mechanical property testing (tensile testing and hardness testing) to ensure tensile strength ≥ 515 MPa, yield strength ≥ 205 MPa, and hardness ≤ HV200 (304 steel standard).

Sixth, Finished Product Packaging of BA-Grade Stainless Steel Seamless Pipes

BA-grade stainless steel seamless pipes require extremely high cleanliness standards, and packaging must avoid surface contamination and scratches:

- Cleaning: After passing inspection, the interior of the pipe is cleaned with pure water to remove residual impurities. Some high-end applications (such as semiconductors) require ultrasonic cleaning and vacuum drying to ensure internal cleanliness of the pipe is ≤ Class 100 (ISO 14644).

- Packaging:

Inner Packaging: Individual pipes are wrapped in food-grade polyethylene film and sealed at both ends with plastic plugs to prevent dust and moisture from entering the pipe.

- Outer Packaging: Multiple pipes are packed into wooden boxes or stainless steel pallets. The box is lined with moisture-proof paper, and the pallet is wrapped with stretch film. Product information, including specifications, batch number, and material, is labeled.