When chromium-nickel austenitic stainless steel is exposed to temperatures between 450 and 800°C, a phenomenon known as intergranular corrosion often arises. This occurs because carbon atoms tend to precipitate from the austenite matrix in the form of Cr23C6, depleting the grain boundaries of chromium. Consequently, preventing chromium depletion at these boundaries is key to avoiding intergranular corrosion.
In stainless steel alloys, elements are typically ranked by their affinity for carbon, starting with titanium, niobium, molybdenum, chromium, and manganese. Given titanium's stronger affinity for carbon compared to chromium, adding titanium to the alloy ensures that carbon binds preferentially with titanium to form titanium carbide. This effectively inhibits the formation of chromium carbides and subsequent grain boundary chromium depletion, thus safeguarding against intergranular corrosion.
However, titanium's interactions with other elements like nitrogen and oxygen impose limitations on its use. Titanium reacts with nitrogen to form titanium nitride and with oxygen to produce titanium dioxide. These reactions necessitate a controlled addition of titanium, usually kept around 0.8% in stainless steel to prevent intergranular corrosion.
To ensure resistance to intergranular corrosion, titanium-containing stainless steel must undergo stabilization post-solid solution treatment. During solid solution treatment, the steel achieves a single-phase austenite structure, but this state becomes unstable upon heating beyond 450°C. At elevated temperatures, carbon in the steel tends to precipitate as carbides. While Cr23C6 forms at 650°C, TiC forms at 900°C. To minimize intergranular corrosion, carbides should ideally exist solely in the form of TiC.
Titanium exhibits superior carbide stability compared to chromium carbides. When stainless steel is heated above 700°C, chromium carbides start converting into titanium carbides. Stabilization involves heating the steel to 850–930°C for one hour. This process ensures complete decomposition of chromium carbides into stable titanium carbides, improving the material's resistance to intergranular corrosion. Additionally, titanium promotes the precipitation of Fe2Ti intermetallic compounds under specific conditions, thereby enhancing the stainless steel's high-temperature strength.
Despite its benefits, titanium isn't entirely benign in stainless steel. Excessive amounts of titanium can lead to the formation of undesirable inclusions like TiO2 and TiN, which negatively affect purity. These inclusions can also degrade the surface quality of steel ingots, increasing grinding requirements and potentially causing wastage. Furthermore, titanium's presence often leads to poor polishing characteristics and challenges in achieving high-precision finishes.
In conclusion, while titanium offers significant advantages in mitigating intergranular corrosion in stainless steel, careful consideration must be given to its quantity and potential drawbacks during alloy design and processing.
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