Factors affecting bright annealing of seamless stainless steel pipes

There are many factors that affect whether seamless stainless steel pipes are bright after annealing, mainly including the temperature, atmosphere, sealing, air pressure and water vapor content in the furnace. These factors are introduced in detail below.

1. Annealing temperature
First, look at the annealing temperature to see if it has reached the corresponding specified temperature. Because the solution heat treatment usually adopted by seamless stainless steel pipes during heat treatment, which is usually called annealing treatment, the optimal temperature range during treatment is 1040-1120℃, or when observing through the observation hole of the annealing furnace, it is found that the stainless steel pipe in the annealing zone appears in an incandescent state but does not soften and sag.

2. Annealing atmosphere

Pure hydrogen is usually used as the annealing atmosphere during annealing treatment. The purity of the atmosphere is preferably greater than 99.99%. If part of the atmosphere is inert gas, the purity of the overall atmosphere can be appropriately lower, but there must be no excess oxygen and water vapor.

3. Furnace sealing
Because the bright annealing furnace should be closed and isolated from the outside air, it uses hydrogen as a protective gas. Generally, there is only one exhaust port that is open to the outside, which is used to ignite the exhausted hydrogen. When checking the airtightness of the furnace, you can use soapy water to apply to the joints and gaps of the annealing furnace to observe whether there is any gas leakage; the most prone area to gas leakage is the inlet and outlet of the annealing furnace. The sealing rings in this area are very easy to wear after long-term use and need to be checked frequently. If there is a problem, they should be replaced immediately.

4. Working pressure of protective gas
To avoid micro-leakage, the furnace should ensure that the gas should always maintain a certain positive pressure. If it is hydrogen protective gas, it is usually required to be above 20kBar.

5. Water vapor content in the furnace
On the one hand, check whether the furnace body material is dry. After the first installation of the furnace, the furnace body material must be dried quickly; secondly, check whether there is too much water stains left on the stainless steel seamless pipe entering the furnace, especially if there are holes in the pipe, water cannot leak in, otherwise it will completely destroy the atmosphere in the furnace.

If all of the above aspects are done well, under normal annealing treatment, the stainless steel seamless pipe about 20 meters after the furnace is opened will begin to shine and reflect light.

Read more: Stainless Steel ERW vs Seamless Pipe or Carbon steel tube vs Stainless steel tube