Key issues of austenitic stainless steel heat treatment

The core value of austenitic stainless steel lies in its excellent corrosion resistance, particularly in oxygen-rich oxidizing environments and acidic conditions. However, this corrosion resistance is not absolute; it heavily depends on whether the material surface can form and maintain a protective passive film. Heat treatment, especially the crucial solution treatment, plays a decisive role in ensuring the effectiveness of this passive film.

Since austenitic stainless steel undergoes no solid-state phase transformation from room temperature to high temperatures, the primary purpose of its heat treatment is not to alter hardness, but rather to address issues arising during processing and to optimize the microstructure. Solution treatment involves heating the steel to a sufficiently high temperature to fully dissolve carbides precipitated during processing back into the austenitic matrix, followed by rapid cooling to room temperature. This rapid cooling process is critical, it "freezes" the homogeneous solid solution state achieved at high temperatures, preventing the re-precipitation of carbides during cooling. This ensures the uniform distribution of alloying elements within the matrix, laying the foundation for the formation of a stable passive film.

Problems arise if the cooling rate during solution treatment is too slow. As the temperature decreases, the solubility of carbon in austenite decreases, causing carbon atoms to preferentially combine with chromium and precipitate chromium-rich carbides at grain boundaries. This directly leads to a significant reduction in chromium content in the regions near the grain boundaries, a phenomenon known as sensitization. Chromium-depleted grain boundaries become extremely vulnerable, triggering severe intergranular corrosion, which can occur even in ordinary corrosive environments. Sensitization also significantly weakens the material's resistance to pitting corrosion, as localized breakdown of the passive film allows corrosion to propagate inward. Furthermore, the non-uniform distribution of chromium reduces resistance to uniform corrosion.

Another common failure mode for austenitic stainless steel is stress corrosion cracking. It requires two necessary conditions: stress and a specific corrosive environment. For applications primarily utilizing its corrosion resistance rather than its strength, the hazards of residual stresses demand particular attention. Stress relief annealing can effectively eliminate residual stresses, thereby reducing the risk of stress corrosion cracking.

Once sensitization occurs, it can be remedied by reheating the steel above 850°C to redissolve the carbides, followed by rapid cooling again. Therefore, proper solution treatment (heating and rapid cooling) is the most critical heat treatment process for ensuring that austenitic stainless steel achieves its optimal corrosion resistance. Improper heat treatment, particularly that which leads to sensitization, significantly compromises the corrosion resistance upon which it relies.