Why Stainless Steel Pipe Fittings Need Bright Annealing Heat Treatment?

  • By:The thin-walled stainless steel pipe fittings are made of 304 (06Cr19Ni10) austenitic stainless steel. After forming, they are welded into a blank pipe, and then reduced and stretched for multiple passes. After the pipes are washed and dried ...
  • Date:2021/09/23

The thin-walled stainless steel pipe fittings are made of 304 (06Cr19Ni10) austenitic stainless steel. After forming, they are welded into a blank pipe, and then reduced and stretched for multiple passes. After the pipes are washed and dried, they are annealed ( Solution treatment). Annealing is divided into two methods: general annealing and bright annealing.

It refers to the annealing performed without the protective gas, such as a continuous annealing furnace heated by an open flame. Such annealed stainless steel pipe fittings also need to be pickled to remove the oxide scale formed on the surface during the annealing process. Corrosion, so the gloss is poor, the surface looks white, not bright, and the toughness is poor.

Bright annealing

There are two types. One is annealing under the protection of full hydrogen. This hydrogen comes from electrolysis or third-party supply, high purity and low dew point. The other is decomposition with ammonia. The decomposed gas is dried and enters the furnace. As a protective gas, the purity and dew point are relatively inferior. However, the two methods are similar in that they both use hydrogen as a protective gas. The furnace structure is special. There is a thing called "muffle" inside. The flame first heats the "muffle", and then the heat is transferred by heat. Conducted to stainless steel pipe fittings, so as to avoid oxidation of stainless steel pipe fittings, so the brightly annealed stainless steel pipe fittings do not need to be pickled, so brightly annealed stainless steel pipe fittings are brighter than ordinary annealed stainless steel pipe fittings and have better corrosion resistance.

After bright annealing, our products can increase the toughness of the pipe and prevent the pipe from rebounding during the compression process.

In the production of thin-walled stainless steel pipe fittings, due to various factors such as production management, operation, process technology, raw and auxiliary materials, some quality problems are likely to occur during bright annealing of pipes, which affects the brightness of pipe fittings.

So, what are the factors that affect the brightness of thin-walled stainless steel pipe annealing?

1. Impact of protective atmosphere
Liquid ammonia decomposition gas is used as a protective atmosphere, and the decomposition reaction of liquid ammonia will occur above 800 ° C. 2NH3 → 3H2 + N2 hydrogen is used as reducing gas, and liquid ammonia decomposition atmosphere is used as protective gas, which can make the surface of the welded pipe white and smooth.

2. Sealability of the furnace body
In order to ensure that the thin-walled stainless steel pipe is annealed, the bright annealing furnace should be closed and isolated from the outside air.

3. Solution temperature
Solution temperature is one of the most important parameters of the solid-liquid process. Too high or too low will directly affect the quality of the pipeline. If the temperature is too high, the structure of the welded pipe will be coarsened, the performance will decrease, and the temperature will be too low. To eliminate incompleteness, choose 1080 ℃ ± 10 ℃, and carry out proper heat preservation, so that the carbide can be fully dissolved.

4. Protective gas pressure
In order to prevent slight leakage, the protective gas in the furnace should maintain a certain positive pressure. If it is a hydrogen protective gas, it is generally required to be above 20kBar.

5. Water vapor in the furnace
Excessive water vapor content in the furnace will cause atmospheric damage and affect the annealing brightness of thin-walled stainless steel pipe fittings. Water vapor comes from the furnace material on the one hand, and water stains from the stainless steel pipe fittings entering the furnace on the other.

Basically these are the factors that affect whether the thin-walled stainless steel pipe is bright. Under normal circumstances, the thin-walled stainless steel pipe that has been brightly annealed will be very bright. Therefore, we must focus on these points in the product design and production process. Based on the in-depth understanding of production processes and processes, Seco's products are designed with the above points in mind. Therefore, Hangao Tech (SEKO Machinery) intelligent advance online bright annealing furnace for stainless steel pipe machine machine has been affirmed and supported by customers for many years. Its quality is reliable and trustworthy.    

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