What are the specific parameter requirements for the annealing process of stainless steel BA pipes and sanitary pipes?

The core difference in the annealing process between stainless steel BA tubes (bright annealed tubes) and sanitary grade tubes lies in surface quality control and atmosphere requirements. Parameters must be adjusted in conjunction with material (e.g., 304/316L), tube diameter and wall thickness, and annealing equipment (continuous / batch). The following are specific parameter requirements:

I. Core Premise (Commonality)

The purpose of annealing is to eliminate work hardening, restore plasticity, and prevent intergranular corrosion, while ensuring the surface is free of oxidation/decarburization; the mainstream materials are 304/304L, 316/316L austenitic stainless steel, with a preference for continuous bright annealing furnaces and vacuum annealing furnaces.

Secondly, stainless steel BA tube (Bright Annealed) annealing process parameters

BA Tube core requirements are a glossy, non-oxidized inner and outer surface, with parameters focusing on "non-oxidizing atmosphere + precise temperature control/cooling."

Annealing Temperature

304/304L: Continuous Furnace 1050~1100°C; Batch Furnace 1080~1120°C

316/316L: Continuous furnace 1080~1150°C, batch furnace 1100~1150°C (higher temperature required for molybdenum content to achieve austenitization).

Additional Note: Thin-walled tubes (≤1mm) should take the lower temperature limit, thick-walled tubes (>2mm) should take the upper limit, and it is strictly prohibited to exceed 1200℃ (as it may lead to coarse grain formation and overburning).

Insulation Duration

Continuous Furnace: Insulation time is 1-2 minutes per 1mm wall thickness (e.g., for a 0.8mm wall thickness, 0.8-1.6 minutes); for pipe diameters ≤ 25mm, 1 minute is taken, and for pipe diameters > 25mm, 1.5-2 minutes are taken.

Batch Furnace: Overall insulation for 20~60 minutes, with short thin-walled tubes and long thick-walled tubes, ensuring uniform heating of the tubes.

Preserve the atmosphere

Purity Requirements: High purity (H₂ ≥ 99.999%) or hydrogen-nitrogen mixture (H₂ 5~20%, N₂ balanced), dew point ≤ -60℃ (moisture < 1ppm).

Pressure Control: Intra-furnace slight positive pressure (20~50 mbar) to prevent air intrusion.

Additional: A small amount of protective gas should be introduced through the tube interior to ensure the inner wall remains bright.

Heating / Cooling Rate

Heating: Continuous furnace: 50~100℃/min (avoid rapid heating to prevent deformation); Batch furnace: ≤50℃/min (after 800℃, slow down to 20~30℃/min).

Cooling: Rapid cooling in the high-temperature section (annealing temperature → 800℃) at a rate of ≥50℃/min for continuous furnace gas quenching or ≥30℃/min for batch furnace to prevent the precipitation of chromium carbides; natural cooling in the low-temperature section (800℃ → room temperature) or at a rate of ≤20℃/min to reduce thermal stress.

Key Limit Values

Oxygen content inside the furnace ≤ 5ppm

Temperature sensor error ≤ ±5°C.

Section 3: Sanitary Tube Annealing Process Parameters

Core requirements for hygiene-grade piping are "clean internal surface + low roughness (Ra≤0.8μm)," which balance luster and compatibility with subsequent polishing.

Annealing Temperature

304/304L: 1040~1090℃; 316/316L: 1070~1140℃ (5~10℃ lower than BA pipe to prevent surface roughness increase).

Insulation Duration

Continuous Furnace: Insulation time of 0.8~1.5 minutes per 1mm wall thickness (shorter than BA pipe, reduces high-temperature exposure)

Batch Furnace: An additional 10~20% insulation time is required for the welding seam area of sanitary grade pipes, which takes 15~45 minutes.

Preserve the ambiance

A Broader Range of Options: High purity N₂ (≥99.99%, dew point ≤ -50℃), hydrogen-nitrogen mixture (H₂ content 3~10%), or vacuum annealing (vacuum ≤ 1×10⁻³Pa, no atmosphere contamination, higher cleanliness).

Heating / Cooling Rate

Heating: Continuous furnace 40~80℃/min, batch furnace ≤40℃/min (priority given to ensuring even heating on the inner surface).

Cooling: High-temperature section (annealing temperature → 800℃): ≥40℃/min (continuous furnace), ≥25℃/min (batch furnace); low-temperature section: 15~25℃/min gradual cooling (ensuring roundness, reducing deformation).

Additional Requirements

The furnace is free of oil污 and dust, preventing the accumulation of impurities.

Post-annealing, it is necessary to adapt polishing processes to avoid excessively thick oxide layers, which can increase polishing difficulty.

Additional Points to Consider

Low-Carbon Stainless Steel (304L/316L): Annealing temperature can be slightly lower (5~10℃), and holding time can be shortened (carbon content ≤0.03%, no need to worry about carbide precipitation).

Small diameter (≤10mm): require more precise temperature control to avoid localized overheating; Large diameter (>50mm): extend insulation time to ensure Austenitization of the core.

Finished Product Requirements: BA tubes should achieve a bright surface directly after annealing without polishing; hygienic-grade tubes must be polished to the corresponding roughness after annealing.