What are the differences in performance between the bright annealed stainless steel seamless tubes

The seamless stainless steel tubes subjected to bright annealing and those that were not annealed exhibit significant differences in performance. These differences are mainly attributed to the influence of the annealing process on the material's microstructure. Here is a detailed comparison: 

II. Corrosion Resistance

1. Tempered Annealed Tube

Surface Oxide Film: During the annealing process, oxidation is inhibited by protective gases (such as H₂, N₂), resulting in a uniform and dense passivation film (Cr₂O₃) with high surface finish (Ra ≤ 0.8 μm), reducing the adhesion of corrosive media.

Intergranular Corrosion Risk: For austenitic stainless steels (such as 304, 316L), if solution annealing (rapid cooling at high temperature) is adopted, carbides (Cr₂₃C₆) can be prevented from precipitating at the grain boundaries, reducing the tendency of intergranular corrosion.

Stress Corrosion: Elimination of cold working stress reduces the risk of stress corrosion cracking (SCC), especially suitable for corrosive environments containing chloride ions, etc.

2. Unannealed Tube

Surface Defects: After cold processing, the surface may still contain lubricants, oxides or microcracks, which are prone to become the starting point of corrosion.

Stress Concentration: The remaining processing stress may accelerate the penetration of corrosive media, and corrosion is more likely to occur in acidic or salt spray environments. 

III. Organization and Structure

1. Bright annealing tube

Crystal grain state: Annealing re-crystallizes the cold-worked grains, forming uniform equiaxed crystals and eliminating texture and lattice distortion (for example, martensitic stainless steel can partially transform into ferrite + pearlite after annealing).

Residual stress: Basic elimination of residual stress generated during cold processing (residual stress ≤ 50 MPa), better dimensional stability, and less deformation over a long period of use.

2. Unannealed tube

Cold processing structure: Retains the processed fibrous structure, with grains elongated or distorted (such as austenitic stainless steel after cold drawing may produce deformed martensite), and high-density dislocations exist.

Stress state: High residual stress (up to 200-300 MPa), which may cause cracking due to aging during storage or use of the tube. 

IV. Surface Quality and Functional Characteristics

1. Bright Annealed Tube

Surface finish: The surface is as smooth as a mirror (roughness Ra ≤ 0.4 μm), without oxide scale or scratches, suitable for clean fields such as food and medicine.

Electrical conductivity and thermal conductivity: After uniformization of the structure, the electrical / thermal conductivity performance is more stable, suitable for heat exchangers and electronic equipment tubes.

2. Unannealed Tube

Surface defects: May have oxide color, scratches, pits, etc., requires additional acid washing or polishing treatment.

Functional limitations: The surface is rough and prone to accumulation of dirt, not suitable for high cleanliness or fluid resistance sensitive scenarios (such as pharmaceuticals, high-purity gas transportation). 

V. Comparison of Typical Application Scenarios

Summary

Bright annealing significantly enhances the plasticity, toughness, corrosion resistance and processing adaptability of stainless steel seamless tubes, making them more suitable for high-end manufacturing and harsh environments. Unannealed tubes, on the other hand, retain the characteristics of cold processing and are only suitable for scenarios with lower performance requirements (such as ordinary mechanical structures and temporary uses). When choosing, it is necessary to make a comprehensive judgment based on specific working conditions (such as the corrosiveness of the medium, pressure, and subsequent processing requirements). 

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Tags: bright annealed, stainless steel seamless tubes, unannealed