Why does 304 stainless steel pipe change color when exposed to high temperatures?

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304 stainless steel pipe is widely used in various fields due to its good corrosion resistance and oxidation resistance. However, 304 stainless steel pipe manufacturers have found that in fact, when 304 stainless steel pipe is used above 450 ° C, the surface of the steel pipe will appear yellowish.

So, what are the principles and influencing factors of high-temperature discoloration on the surface of 304 stainless steel pipes?

The oxide film on the surface of the stainless steel welded pipe is actually a five-color transparent film, and its color change is affected by the principle of light interference. When light interferes, which color appears is mainly caused by the change in the thickness of the oxide film on the surface. Therefore, the oxidation resistance of 304 stainless steel pipe is an effective way to solve its high-temperature discoloration.

The factors that affect oxidation resistance mainly include the influence of chemical elements, the influence of oxide film, and the influence of surface roughness.

1. Chemical elements

The carbon content in the 304 tubes is low, and carbon atoms are easy to diffuse into the stainless steel matrix structure during the oxidation process in a high-temperature environment, so the oxidation process of the tube is also accompanied by the carburizing process.

Therefore, the test can choose steel with different contents such as chromium, silicon, nickel, etc. to do a high-temperature cycle test. After the test is over, measure the increased ratio of carbon content in various types of steel, and use the amount of carburization to judge the degree of oxidation of the material.

In order to further verify the influence of silicon on the carburizing resistance of stainless steel pipes, only the proportion of silicon is adjusted in 310S, and then high-temperature cycle tests are performed. The final test results show that when the mass fraction of chromium element is increased to more than 22% or the mass fraction of silicon element is increased to more than 2%, the high-temperature oxidation resistance of the steel pipe has been significantly improved.

2. Effect of an oxide film

The tough oxide film on the surface can prevent the oxidant from invading the stainless steel substrate, that is, prevent the oxidation from continuing to occur, while the loose oxide film will make the oxide film continue to thicken. In the continuous production line, the continuous pickling method is used to perform forced oxidation on stainless steel pipes in order to enhance the compactness of the oxide film.

3. Effect of surface roughness

Oxidation of 304 stainless steel pipes in high-temperature environments will thicken the surface oxide film. This situation is directly related to the oxidation resistance of the material itself and the heat absorption capacity of the material. Surface roughness, surface anti-color, etc. will affect the heat absorption per unit area of the pipe and affect the degree of discoloration.

The practice has shown that when the surface roughness of the thin-walled stainless steel pipe is less than 0.022 μm, its heat absorption capacity will be significantly reduced, the steel pipe will not be further oxidized, the thickness of the oxide film will basically remain unchanged in the high-temperature environment, and the degree of discoloration will be very slight.