Basic Theory of Heat Treatment on Austenitic Stainless Steel
Austenitic stainless steel is made of austenitic matrix structure that doesn't appear during the phase transition,and it can not be adjusted or changed by heat treatment. The main purpose of heat treatment of austenitic stainless steel is to improve its corrosion resistance, eliminate stress, and harden the material that has been softened.
Because of chromium content in the stainless steel - the role of alloying elements such as nickel, the transformation of austenite to martensite start temperature Ms reduced to below room temperature, high temperature stability of austenite to room temperature or even lower to maintain some of the temperature without change. However, the dissolution of carbon in austenite varies with temperature, solubility at a high temperature are lower than that at a low temperature. Also, σ phase in austenitic stainless steel should be considered. because chromium - nickel austenitic stainless steel in the σ phase is a high chromium containing chromium - iron intermetallic compounds formed in the grain boundary, the same form in their local areas around the chromium depleted zone in the corrosive medium intergranular corrosion caused, especially in strong oxidizing media, so that the material is more sensitive to inter granular corrosion. For the same reason, but also make materials containing Cl-medium increase in the tendency to pitting corrosion. Further more, Austenitic stainless steel in some cases, will lead to δ ferrite in the high temperature ferrite.
Finally, the stress and damage of austenitic stainless steel parts should be payed more attention to. With residual stress of the workpiece and the metal, due to increased energy, atoms are thermodynamically unstable state. when it is heated to a certain temperature, it will quickly return to equilibrium, so that stress can be eliminated. Appropriate treatment is an important method to reduce or eliminate the residual stress in austenitic stainless steel, which is usually called stress relief heat treatment.
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