Which stainless steels require solution treatment and stabilization?
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As the title suggests, I wonder whether stainless steel doesn’t need to undergo stabilization treatment after solution treatment. However, I’ve seen that some types of stainless steel require both solution treatment and stabilization treatment; I don’t understand this. I’m waiting for experts to explain it
The solubility of carbon in austenite grains in austenitic stainless steel is highly dependent on temperature. When austenitic stainless steels are used in the temperature range of 400°C to 850°C (the sensitization temperature range), high-chromium carbides (Cr23C6) precipitate at the grain boundaries, reducing the chromium content in those boundaries. This in turn lowers the corrosion resistance of the grain boundary areas; as a result, austenitic stainless steels generally tend to suffer from intergranular corrosion. To prevent intergranular corrosion in austenitic stainless steels, they should be subjected to solution treatment or stabilization treatment. Solution heat treatment: Austenitic stainless steel is heated to above 1050°C, causing all or almost all of the carbide phases to dissolve within the austenite grains; that is, carbon becomes dissolved in the austenite. The material is then rapidly cooled to room temperature, resulting in a supersaturated state of carbon (the carbon is now stable and no longer has the ability or opportunity to form high-chromium carbides with chromium). This heat treatment method is solution heat treatment. Rapid cooling during solution heat treatment resembles quenching of ordinary steel, but the ‘quenching’ in this case is different from that of ordinary steel; the former is a softening treatment, while the latter is hardening (formation of martensite). The heating temperatures used by the latter to achieve different hardness levels also vary, but they do not reach 1050°C. Stabilization treatment: To prevent carbon from forming high-chromium carbides with chromium, stabilizing elements such as Ti and Nb are added to austenitic steels. When heated to temperatures above 875°C, stable carbides are formed (since Ti and Nb prefer to combine with carbon to form TiC or NbC), which **reduces the concentration of dissolved carbon in the austenite phase; this approach thus serves to preserve chromium at the expense of Ti and Nb. Austenitic stainless steel that has undergone stabilization treatment exhibits better properties than one that has undergone solution heat treatment. Note that the stainless steel grades subjected to stabilization treatment and those subjected to solution treatment are different. In China, 321 is generally used for stabilized stainless steel, while 0Cr18Ni9 is commonly used for stainless steel that has undergone solution treatment.
I wonder what better properties stabilization treatment has compared to solution treatment? Is the main purpose of stabilization to eliminate welding stress? How necessary is it? For example, is it mandatory to perform stabilization treatment on 321?
321 and 304 are two different stainless steel grades; 321 contains titanium, while 304 is 06C19Ni10. Both types of sheets have undergone solution treatment, as specified in GB24511
Heat-resistant stainless steels that are prone to reheat cracking require stabilization heat treatment after welding, in order to eliminate chromium-depleted layers between the grains and prevent stress corrosion in those areas. The main austenitic stainless steels of this type include TP321 and TP347. Conventional austenitic stainless steels (without stabilizing elements such as Ti and Nb) only require solution treatment; if there are special design requirements, stress-relief treatment may also be applied.