34 stainless steel pipe, expressed as 1Cr25ni20Si2 by Chinese GB steel number, belongs to austenitic stainless steel pipe. Generally speaking, the corrosion resistance of stainless steel mainly comes from chromium. Experiments show that the corrosion resistance of steel will be greatly improved only when the chromium content exceeds 12%. Therefore, the chromium content in stainless steel is generally not less than 12%. Due to the increase of chromium content, it also has a great influence on the structure of steel. When the chromium content is high but the carbon content is low, chromium will make the iron and carbon balance, and the υ phase area on the figure will shrink or even disappear. This stainless steel is ferrite structure, and it does not undergo phase transformation when heated, so it is called ferrite stainless steel. When the chromium content is low (but higher than 12%) and the carbon content is high, the alloy is easy to form martensite when cooled from high temperature, so this kind of steel is called martensitic stainless steel. Nickel can expand the υ phase region and make steel have austenite structure. If the nickel content is enough to make the steel have austenitic structure at room temperature, it is called austenitic stainless steel. 34 stainless steel pipe, expressed as 1Cr25ni20Si2 by Chinese GB steel number, belongs to austenitic stainless steel pipe. Generally speaking, the corrosion resistance of stainless steel mainly comes from chromium. Experiments show that the corrosion resistance of steel will be greatly improved only when the chromium content exceeds 12%. Therefore, the chromium content in stainless steel is generally not less than 12%. Due to the increase of chromium content, it also has a great influence on the structure of steel. When the chromium content is high but the carbon content is low, chromium will make the iron and carbon balance, and the υ phase area on the figure will shrink or even disappear. This stainless steel is ferrite structure, and it does not undergo phase transformation when heated, so it is called ferrite stainless steel.

When the chromium content is low (but higher than 12%) and the carbon content is high, the alloy is easy to form martensite when cooled from high temperature, so this kind of steel is called martensitic stainless steel.

Nickel can expand the υ phase region and make steel have austenite structure. If the nickel content is enough to make the steel have austenitic structure at room temperature, it is called austenitic stainless steel.