Purpose and Requirements of Stainless Steel Bright Annealing

Objectives

1. Eliminate Work Hardening-Purpose and Requirements of Stainless Steel Bright Annealing
   • Obtain a satisfactory metallographic structure.

Process Overview

Bright Annealing Furnace

• Used for the heat treatment of finished stainless steel under a protective atmosphere.
• Metallographic structure and heat treatment processes vary based on performance requirements.

300 Series Austenitic Stainless Steel

• Heat Treatment Process: Solution treatment.
• Heating: Carbides dissolve into austenite at 1050-1150°C.
• Cooling: Rapid cooling below 350°C to form a supersaturated solid solution with uniform austenite structure.
• Cooling Rate: 55°C/s to pass through re-precipitation temperature zone (550-850°C) quickly.
• Holding Time: Minimal to prevent grain coarsening and maintain surface finish.

400 Series Ferritic Stainless Steel

• Heating Temperature: Around 900°C.
• Cooling: Slow cooling to obtain an annealed softened structure.

Martensitic Stainless Steel

• Heat Treatment: Annealing or segmented quenching and tempering.

Cooling Equipment Requirements

• Large adjustment range for cooling section equipment.
• Strong convection cooling with three adjustable cooling sections.
• Air volume is adjustable to control the plate shape along the width of the strip.

Uniform Structure Requirement

• Muffle Bright Annealing Furnace: Uses a large-size muffle tube for even heating.
• Strip Speed: Constant linear speed in the heating furnace.
• Tension Adjustment: Precisely adjustable roller tension device before and after the furnace to ensure strip speed and shape requirements.

Non-Oxidized Bright Surface

Protective Atmosphere

• Heat Treatment: Conducted under H2 protective atmosphere to avoid oxidation.
• Alloy Components: Fe, Cr, Ni, Mn, Ti, Si.
  • Main Concerns: Oxidation of Cr, Mn, Si, and Ti within the heating temperature range.
  • Chromium Oxidation: Dechromizes the strip surface, reducing corrosion resistance.
  • Protective Measures: Maintain H2 dew point below -60°C for Cr (17-18%) and Ti (0.5%) to avoid oxidation at 800-1150°C.

Maintaining Purity

• Muffle Cover: Ensures good sealing and maintains a pure protective atmosphere.
• Furnace Design: Vertical furnace with inlet and outlet at the bottom to minimize air entry risk compared to horizontal furnaces.
• Sealing Requirements: Inlet and outlet sealing boxes, strip conveying sections, tension adjustment rollers, and furnace top guide roller boxes must meet parts per million (ppm) level sealing requirements to prevent oxygen and water vapor entry and gas leakage.