Factors affecting bright annealing of seamless stainless steel pipes

The brightness of seamless stainless steel pipes after annealing is determined by several key factors, including annealing temperature, furnace atmosphere, sealing conditions, protective gas pressure, and moisture content inside the furnace. Below is a detailed breakdown of these critical elements:

1. Annealing Temperature

The first consideration is whether the annealing temperature has reached the specified range. For seamless stainless steel pipes, solution heat treatment—commonly referred to as annealing—requires an optimal temperature range of 1040–1120°C. When viewed through the furnace observation window, pipes in the annealing zone should appear incandescent without signs of softening or sagging.

2. Furnace Atmosphere

A controlled atmosphere is essential for achieving a bright finish. Pure hydrogen is typically used as the protective gas, with a preferred purity of 99.99% or higher. If an inert gas is partially substituted, the overall atmosphere purity can be slightly lower, but there must be no residual oxygen or water vapor, as these can lead to oxidation and discoloration.

3. Furnace Sealing

The annealing furnace must be tightly sealed to prevent external air contamination. Since hydrogen is used as a protective gas, the furnace should remain closed except for a single exhaust port designed to safely release and ignite excess hydrogen.

To check for potential leaks:

Apply soapy water to the furnace’s joints and gaps and observe for bubbling, which indicates gas leakage.

Pay special attention to the inlet and outlet sealing rings, as these are prone to wear over time. Regular inspections are necessary, and damaged seals should be replaced immediately to maintain airtight conditions.

4. Protective Gas Pressure

To prevent micro-leakage, the protective gas inside the furnace must always be maintained at a positive pressure. For hydrogen gas environments, the recommended pressure is typically above 20 kBar, ensuring a consistent protective atmosphere and preventing unwanted oxidation.

5. Moisture Content Inside the Furnace

Excess moisture inside the furnace can significantly impact the annealing results. To minimize water vapor contamination:

Ensure the furnace materials are fully dried before initial use, especially after installation or maintenance.

Check for residual water on stainless steel pipes before they enter the furnace. Pipes with internal cavities must be completely dry, as trapped moisture can disrupt the furnace atmosphere and lead to oxidation.

Achieving a Bright Finish

When all the above conditions are properly controlled, seamless stainless steel pipes exiting the furnace—typically after about 20 meters of processing—will begin to exhibit a bright, reflective surface, meeting the desired annealing quality.