Study On The Mechanism Of Stress Corrosion Cracking In Shell Cover Of 316L Austenitic Stainless Steel Van Tanks

Cr-Ni austenitic stainless steel, such as 18-8 and 12-18-Mo which was widely used, has become an important research topic in the field of stainless steel in both China and abroad, and the important engineering problems that urgently to be solved in various industry departments which stainless steel was applied for a long time as well. So in this paper mainly with the method of metallographic analysis, and the aid of optical microscope(OM), scanning electron microscope(SEM), X-ray diffraction(XRD), vickers hardness tester and other equipments at the same time, the mechanism of stress corrosion cracking in shell cover of 316 L austenitic stainless steel van tanks hasbeenstudied. The results showed that: Cracks of stainless steel tank near the weld has obvious characteristics of stress corrosion crack, which propagated in intergranular and transgranular mixed model. Cracks in thickness direction are deep. Besides, there are a large number of corrosion products within cracks and the δ-ferrite strips in austenitic stainless steel have delayed action on the propagation of the crack. Crack fracture was of brittle fracture characteristics, and the crack sections are covered with a layer of corrosion products. On the surface of the stainless steel tank sample, other defects such as corrosion pit, intergranular corrosion cracks and fusion zone cracks are also found which play a promote role in the occurrence of stress corrosion cracking. Results of metallographic analysis showed that the microstructure of austenitic stainless steel storage tanks is mainly composed of austenitic and δ-ferrite. And in the process of long-term service, M23C6 carbides precipitate out along the deformation zone and the ferritic/austenitic interface. There are also σ-phase precipitate out inside the ferrite and around the M23C6 carbides, which result in the deterioration of mechanical properties and corrosion resistance of stainless steel storage tanks. Stress corrosion test and oxalic acid electrolytic etching test show that the corrosion resistance of heat affected zone deteriorates in the most.