In what state is the material of square tube called alloy square tube:

Square alloy tube with ferrite structure in service. Chromium content is 11%~30%, with body centered cubic crystal structure. This kind of steel generally does not contain nickel, and sometimes contains a small amount of elements such as Mo, Ti, Nb, etc. This kind of steel has the characteristics of high thermal conductivity, small shrinkage coefficient, good oxidation resistance, stress corrosion resistance, etc. It is mostly used to manufacture parts and components resistant to atmospheric, steam, water and oxidizing acid corrosion. The application of this kind of alloy square tube is limited because of its poor plasticity, significantly reduced plasticity and corrosion resistance after welding. The application of secondary refining technology (AOD or VOD) can greatly reduce the interstitial elements such as carbon and nitrogen, thus making this kind of steel widely used.

It is an alloy square tube with about half of austenite and half of ferrite. Under the condition of low C content, Cr content is 18%~28% and Ni content is 3%~10%. Some steels also contain alloying elements such as Mo, Cu, Si, Nb, Ti and N. This kind of steel has the characteristics of both austenite and ferrite alloy square tubes. Compared with ferrite, it has higher plasticity and toughness, no room temperature brittleness, significantly improved intergranular corrosion resistance and welding performance, and also adheres to the 475 ℃ brittleness of ferrite alloy square tubes, high thermal conductivity, superplasticity and other characteristics. Compared with the austenitic alloy square tube, it has higher strength and significantly improved resistance to intergranular corrosion and chloride stress corrosion. Dual phase alloy square tube has good pitting corrosion resistance and is also a nickel saving alloy square tube.    

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