Intergranular corrosion of austenitic stainless steels

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Seen from other sources: 1. Intergranular corrosion of austenitic stainless steel. When austenitic stainless steel is held at temperatures between 450 and 850°C or cooled slowly, intergranular corrosion occurs. The higher the carbon content, the greater the tendency to intergranular corrosion. Furthermore, intergranular corrosion can also occur in the heat-affected zone of welded parts. This is due to the precipitation of Cr-rich Cr23C6 at the grain boundaries. This results in a chromium-depleted zone forming in the surrounding matrix, thereby creating a corrosion galvanic cell. This intergranular corrosion phenomenon also exists in the ferritic stainless steels mentioned earlier. In engineering, the following methods are commonly used to prevent intergranular corrosion: (1) Reducing the carbon content in steel so that it is below the saturated solubility of carbon in austenite at equilibrium, thereby fundamentally solving the problem of chromium carbides (Cr23C6) precipitating at the grain boundaries. Generally, reducing the carbon content in steel to below 0.03% is sufficient to meet the requirements for intergranular corrosion resistance. (2) By adding elements such as Ti and Nb that can form stable carbides (TiC or NbC), Cr23C6 precipitation at the grain boundaries can be prevented, thereby avoiding intergranular corrosion in austenitic stainless steels. (3) By adjusting the ratio of austenite-forming elements to ferrite-forming elements in the steel, a dual-phase structure of austenite + ferrite is obtained, with ferrite accounting for 5% to 12%. This dual-phase structure is less prone to intergranular corrosion. (4) By adopting appropriate heat treatment processes, intergranular corrosion can be prevented, thereby achieving optimal corrosion resistance. 2. Stress corrosion of austenitic stainless steels: Cracking that results from the combined effect of stress (mainly tensile stress) and corrosion is known as stress corrosion cracking, abbreviated as SCC (Stress Crack Corrosion). Austenitic stainless steels are prone to stress corrosion in corrosive media containing chloride ions. When the nickel content reaches 8%–10%, austenitic stainless steels exhibit the highest tendency to stress corrosion. As the nickel content is increased further to 45%–50%, this tendency to stress corrosion gradually decreases until it disappears. The most effective way to prevent stress corrosion in austenitic stainless steels is to add 2%–4% Si and to keep the nitrogen content below 0.04% during manufacturing. In addition, the content of impurities such as P, Sb, Bi, and As should also be reduced as much as possible. Alternatively, A-F dual-purpose steel can be used, as it is insensitive to stress corrosion in Cl- and OH- media. When the initial microcracks encounter the ferrite phase, they stop expanding further, and the ferrite content should be around 6%. 3. Deformation strengthening of austenitic stainless steels: Single-phase austenitic stainless steels possess good cold deformation properties, allowing them to be drawn into very fine wires or rolled into extremely thin steel strips or pipes. After significant deformation, the strength of steel increases greatly, especially when rolling at low temperatures. The tensile strength can exceed 2,000 MPa. This is because, in addition to the cold work hardening effect, deformation-induced M transformation is also present. Austenitic stainless steel can be used to manufacture stainless springs, watch springs, and wire ropes in aviation structures after being strengthened through deformation. If welding is required after deformation, only spot welding can be used, as the deformation increases the tendency to stress corrosion. Ferromagnetism is also generated due to some γ->M transitions, which should be taken into account when it is used (such as in instrument components). The recrystallization temperature changes with the degree of deformation; when the degree of deformation is 60%, the recrystallization temperature drops to 650°C. The recrystallization annealing temperature for cold-deformed austenitic stainless steel ranges from 850°C to 1050°C. At 850°C, an holding time of 3 hours is required, while at 1050°C, sufficient heating time is sufficient, followed by water cooling. 4. Heat treatment of austenitic stainless steels. The common heat treatment processes for austenitic stainless steels include solution treatment, stabilization treatment, and stress relief treatment. (1) Solution treatment. Heating the steel to 1050–1150°C and then quenching it in water serves primarily to dissolve the carbides in austenite, maintaining this state at room temperature; this significantly improves the steel’s corrosion resistance. As mentioned above, to prevent intergranular corrosion, solution treatment is typically used to dissolve Cr23C6 in austenite, followed by rapid cooling. Air cooling can be used for thin-walled components, while water cooling is generally employed. (2) Stabilization treatment. It is generally carried out after solution treatment, and is commonly used in 18-8 steel containing Ti and Nb. After solution treatment, the steel is heated to 850–880°C and then cooled in air; at this temperature, the chromium carbides dissolve completely, whereas the titanium carbides do not dissolve completely and precipitate during cooling, preventing the formation of new carbides. This approach effectively eliminates intergranular corrosion. (3) Stress relief treatment. Stress relief is a heat treatment process used to eliminate residual stresses in steel after cold working or welding; it generally involves tempering at 300–350°C. For steels that do not contain the stabilizing elements Ti and Nb, the heating temperature should not exceed 450°C to prevent the precipitation of chromium carbides, which could lead to intergranular corrosion. For cold-worked and welded parts of ultra-low carbon and Ti-, Nb-composite stainless steels, heating at 500–950°C followed by slow cooling is required to relieve stress (the upper temperature limit is used for removing welding stress), which can reduce the tendency to intergranular corrosion and enhance the steel’s resistance to stress corrosion.