Solution treatment refers to a heat treatment process in which the alloy is heated to a high temperature single-phase zone to maintain the temperature, and the excess phase is sufficiently dissolved in the solid solution and then rapidly cooled to obtain a supersaturated solid solution.
Because the operation process is similar to quenching, it is also called “solution hardening”. It is suitable for alloys in which the solid solution is used as a matrix and the solubility changes greatly when the temperature changes. The alloy is first heated to a temperature above the solubility curve and at a suitable temperature below the solidus line for a certain period of time to dissolve the second phase into the solid solution. It is then rapidly cooled in water or other medium to inhibit re-precipitation of the second phase to obtain a supersaturated solid solution at room temperature or a solid solution phase which is usually only present at elevated temperatures. Since it is thermodynamically metastable, desolvation or other transformation will occur under appropriate temperature or stress conditions. It is generally a preliminary heat treatment and its role is to prepare optimum conditions for subsequent heat treatment.
The forming process of the bellows(corrugated expansion joint) is a process in which the substrate undergoes plastic deformation, which causes the structural material (usually austenitic stainless steel) to undergo loading and unloading in the plastic range. Therefore, the performance of the structural material (mainly the yield strength) has undergone a great change, which is called the hardening phenomenon. Generally, after the bellows is formed, the yield strength of the material is doubled.
Although the 18-8 series stainless steel has excellent formability, high fatigue strength, high temperature strength and resistance to multiple corrosions, it has been used in a large number of bellows and has been used successfully. However, it has also been found that stress corrosion or crystal corrosion is still a problem in the destruction of 18-8 series stainless steel. In order to overcome the above two types of corrosion, there are two commonly used methods: selecting a new high nickel bellows material (such as Incolloy 800, Inconel 625, etc.) or solution treatment of the formed bellows. The corrugated pipe material after solution treatment has fine grain and single structure, which can eliminate molding pressure and improve corrosion resistance. After solution treatment, pickling and passivation should be carried out. The bellows after solution treatment has the following characteristics:
The elastic initial stiffness of the bellows after solution treatment does not change much, but the elastic working range of the bellows is significantly reduced compared with the shaped bellows. This difference is beneficial to improve the stress of the pipe support.
The pressure bearing capacity of the bellows after solution treatment is greatly reduced, usually only about half of the shape bellows. Therefore, in the design of the bellows, it is necessary to increase the thickness of the bellows to ensure the column stability and plane stability of the bellows.
The solution treatment has little effect on the fatigue life of the bellows. As long as the reduction of the pressure bearing capacity of the bellows is fully considered, the fatigue life calculation formula of the formed bellows can be used for the life calculation of the solid solution bellows.
The solution treatment can only eliminate the residual stress generated during the welding process of the bellows, and can not eliminate the local stress caused by the deformation when the bellows absorbs the displacement. Therefore, the solution treatment cannot be used to eliminate the stress of the bellows. Corrosion problem.
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