Why do 304 stainless steel pipes crack

(1) Non-metallic inclusions in the microstructure of 304 stainless steel pipes can damage the integrity of the passivation film on the metal surface, reducing the corrosion resistance of the 304 stainless steel pipe. Therefore, the grade of non-metallic inclusions in 304 stainless steel pipes should be strictly controlled to below level 1.5.

(2) The Cr content in the base metal and weld metal of 304 stainless steel pipes is too low. Cr depletion reduces the density of the chromium passivation film on the stainless steel surface. Therefore, the quality requirements for 304 stainless steel pipes and welding materials should be improved. Strict testing of the raw material composition upon arrival at the factory should be conducted to ensure that the weld metal composition of the 304 stainless steel pipe is not weaker than that of the base metal. During the welding process, the welding parameters of the 304 stainless steel pipe should be strictly controlled, and the welding heat input should be minimized as much as possible to prevent sensitization of the 304 stainless steel pipe, which would lead to Cr precipitation along the grain boundaries to form Cr23C6, thus avoiding intergranular corrosion.

(3) In a marine atmospheric environment, the chloride ion content in the air is high, and there are suitable temperatures and humidity that accelerate corrosion. This makes the oxide film on the surface of 304 stainless steel pipe easily damaged, resulting in electrochemical corrosion on the surface of 304 stainless steel pipe. The corrosion microcracks expand rapidly under the action of residual stress, causing stress corrosion cracking. Therefore, the on-site air environment should be strictly controlled, and the salt spray environment should be isolated (e.g., by coating or adding a protective layer) to prevent the destructive effect of chloride ions. The damage to the 304 stainless steel pipe is stress corrosion cracking. Stainless steel has poor thermal conductivity. After welding, the 304 stainless steel pipe generates residual stress due to high temperature. The corrosion microcracks generated after corrosion of the 304 stainless steel pipe expand rapidly under the action of residual stress, forming stress corrosion cracking.