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2. Normalizing
Normalizing is one of the heat treatment processes commonly used in the industry. Normalizing can be used as a preliminary heat treatment process to provide a suitable microstructure for the next process heat treatment process, such as providing fine-grained pearlite for eutectic spheroidal annealing, eliminating network carbides, etc. , to provide suitable mechanical properties, such as normalizing treatment of carbon structural steel parts. In addition, normalizing is also commonly used to eliminate certain processing defects. For example, eliminating coarse ferrite blocks, eliminating Wei's organization, and the like. Generally, the normalizing heating temperature is Ac3 + (30 ~ 50) °C, and the temperature is kept for a certain period of time. [General normalizing heat preservation time is based on the burning of the workpiece (that is, the core reaches the required heating temperature)], so that the austenite is homogenized, and then Blow out in the air or in other suitable ways. It must be pointed out that normalizing is to obtain fine pearlite structure. For some alloy steels, air cooling has exceeded its critical cooling rate, resulting in bainite or martensite transformation. At this time, "air cooling" is already quenching. Not just a normal fire.
In actual production, the normalizing heating temperature is often slightly higher than the above temperature. Increasing the heating temperature promotes homogenization of austenite and increases the stability of supercooled austenite. In the normalizing heating temperature of common steel, if the normalizing is used as the pre-heat treatment, the upper limit temperature should be taken, which is beneficial to the homogenization of the structure; if the normalizing is used as the final heat treatment, the lower limit temperature is obtained, and the fine austenite crystal can be obtained. grain.
3. Solution treatment
The solution treatment is to heat the austenitic stainless steel to 1010 ~ 1120 ° C (the specific temperature varies with the steel type). After proper heat preservation, the carbide is dissolved into the austenite matrix as much as possible, and then rapidly cooled to room temperature to prevent the carbide from being precipitated. The supersaturated state is solid-dissolved in the matrix to obtain a heat treatment of the single-phase austenite structure. Secondly, the solution treatment is to obtain a suitable grain size to ensure the high temperature creep resistance of the alloy. The normal delivery state of austenitic stainless steel is a solid solution state.
In a pressure vessel, the solution treatment can play a role in:
(1) For non-ultra-low carbon austenitic stainless steels, solution treatment is an important means to prevent intergranular corrosion.
(2) For thermoformed pressure components, solution treatment can be used to restore the original performance.
(3) When the cold forming or service conditions change the austenite structure, it can be recovered by solution treatment according to the actual situation. For example, cold-formed austenitic stainless steel compression elements should be heat treated after forming at a design temperature in the sensitizing temperature range and when the processing deformation rate exceeds a certain limit (for Type 304 and Type 316 austenitic stainless steels) Pressure element, when the design temperature is 580 ~ 675 ° C, and the processing deformation rate exceeds 20%; or when the design temperature exceeds 675 ° C, and the processing deformation rate exceeds 10%, heat treatment should be performed after cold forming). For example, when the austenitic stainless steel pressure-receiving element is used in a cryogenic condition, it can be subjected to solution treatment after cold forming to restore low temperature toughness.
4. Stabilization
Stabilization treatment: A heat treatment in which the shape and dimensional change of the workpiece can be maintained within a prescribed range under long-term service conditions. According to the "Safety Engineering Dictionary", a heat treatment method for improving the resistance to intergranular corrosion of austenitic stainless steel containing titanium or niobium. When austenitic stainless steel is smelted by adding titanium or bismuth elements several times more than carbon, it is possible to preferentially form carbides of titanium or bismuth prior to the formation of Cr23 C6, and these carbides are hardly dissolved in austenite. When the solder is cooled from a high temperature, even after passing through the sensitization temperature range (850 to 450 ° C) where Cr23 C6 is easily precipitated, Cr23 C6 is not precipitated in a large amount along the grain boundary, thereby greatly improving the ability to resist intergranular corrosion. In order to achieve maximum stability of the steel, stabilization should also be carried out, that is, the member is heated to 900 ° C to fully dissolve Cr23C6 into austenite, and at this time, titanium and niobium are sufficiently formed to form very stable titanium carbide and niobium carbide. Then, it was cooled in the air, and even when passing through the sensitization temperature, Cr 23 C6 was not precipitated at the grain boundary. The stabilized austenitic stainless steel greatly reduces the possibility of intergranular corrosion.
Stabilization is only suitable for applications where a stabilizing element (Ti or Nb) austenitic stainless steel is used in an intergranular corrosive environment. In the production of the pressure vessel, the stabilization treatment can be carried out separately. Considering that the normal delivery state of the austenitic stainless steel is in a solid solution state, the stabilization treatment is an additional heat treatment after the solution treatment.
The stabilization treatment is referred to as stabilization annealing in the related literature, that is, to precipitate or spheroidize the fine microscopic composition in the workpiece. For example, some austenitic stainless steels are subjected to stabilization annealing at around 850 ° C to precipitate TiC, NbC, and TaC to prevent deterioration of intergranular corrosion resistance.
Normalizing is one of the heat treatment processes commonly used in the industry. Normalizing can be used as a preliminary heat treatment process to provide a suitable microstructure for the next process heat treatment process, such as providing fine-grained pearlite for eutectic spheroidal annealing, eliminating network carbides, etc. , to provide suitable mechanical properties, such as normalizing treatment of carbon structural steel parts. In addition, normalizing is also commonly used to eliminate certain processing defects. For example, eliminating coarse ferrite blocks, eliminating Wei's organization, and the like. Generally, the normalizing heating temperature is Ac3 + (30 ~ 50) °C, and the temperature is kept for a certain period of time. [General normalizing heat preservation time is based on the burning of the workpiece (that is, the core reaches the required heating temperature)], so that the austenite is homogenized, and then Blow out in the air or in other suitable ways. It must be pointed out that normalizing is to obtain fine pearlite structure. For some alloy steels, air cooling has exceeded its critical cooling rate, resulting in bainite or martensite transformation. At this time, "air cooling" is already quenching. Not just a normal fire.
In actual production, the normalizing heating temperature is often slightly higher than the above temperature. Increasing the heating temperature promotes homogenization of austenite and increases the stability of supercooled austenite. In the normalizing heating temperature of common steel, if the normalizing is used as the pre-heat treatment, the upper limit temperature should be taken, which is beneficial to the homogenization of the structure; if the normalizing is used as the final heat treatment, the lower limit temperature is obtained, and the fine austenite crystal can be obtained. grain.
3. Solution treatment
The solution treatment is to heat the austenitic stainless steel to 1010 ~ 1120 ° C (the specific temperature varies with the steel type). After proper heat preservation, the carbide is dissolved into the austenite matrix as much as possible, and then rapidly cooled to room temperature to prevent the carbide from being precipitated. The supersaturated state is solid-dissolved in the matrix to obtain a heat treatment of the single-phase austenite structure. Secondly, the solution treatment is to obtain a suitable grain size to ensure the high temperature creep resistance of the alloy. The normal delivery state of austenitic stainless steel is a solid solution state.
In a pressure vessel, the solution treatment can play a role in:
(1) For non-ultra-low carbon austenitic stainless steels, solution treatment is an important means to prevent intergranular corrosion.
(2) For thermoformed pressure components, solution treatment can be used to restore the original performance.
(3) When the cold forming or service conditions change the austenite structure, it can be recovered by solution treatment according to the actual situation. For example, cold-formed austenitic stainless steel compression elements should be heat treated after forming at a design temperature in the sensitizing temperature range and when the processing deformation rate exceeds a certain limit (for Type 304 and Type 316 austenitic stainless steels) Pressure element, when the design temperature is 580 ~ 675 ° C, and the processing deformation rate exceeds 20%; or when the design temperature exceeds 675 ° C, and the processing deformation rate exceeds 10%, heat treatment should be performed after cold forming). For example, when the austenitic stainless steel pressure-receiving element is used in a cryogenic condition, it can be subjected to solution treatment after cold forming to restore low temperature toughness.
4. Stabilization
Stabilization treatment: A heat treatment in which the shape and dimensional change of the workpiece can be maintained within a prescribed range under long-term service conditions. According to the "Safety Engineering Dictionary", a heat treatment method for improving the resistance to intergranular corrosion of austenitic stainless steel containing titanium or niobium. When austenitic stainless steel is smelted by adding titanium or bismuth elements several times more than carbon, it is possible to preferentially form carbides of titanium or bismuth prior to the formation of Cr23 C6, and these carbides are hardly dissolved in austenite. When the solder is cooled from a high temperature, even after passing through the sensitization temperature range (850 to 450 ° C) where Cr23 C6 is easily precipitated, Cr23 C6 is not precipitated in a large amount along the grain boundary, thereby greatly improving the ability to resist intergranular corrosion. In order to achieve maximum stability of the steel, stabilization should also be carried out, that is, the member is heated to 900 ° C to fully dissolve Cr23C6 into austenite, and at this time, titanium and niobium are sufficiently formed to form very stable titanium carbide and niobium carbide. Then, it was cooled in the air, and even when passing through the sensitization temperature, Cr 23 C6 was not precipitated at the grain boundary. The stabilized austenitic stainless steel greatly reduces the possibility of intergranular corrosion.
Stabilization is only suitable for applications where a stabilizing element (Ti or Nb) austenitic stainless steel is used in an intergranular corrosive environment. In the production of the pressure vessel, the stabilization treatment can be carried out separately. Considering that the normal delivery state of the austenitic stainless steel is in a solid solution state, the stabilization treatment is an additional heat treatment after the solution treatment.
The stabilization treatment is referred to as stabilization annealing in the related literature, that is, to precipitate or spheroidize the fine microscopic composition in the workpiece. For example, some austenitic stainless steels are subjected to stabilization annealing at around 850 ° C to precipitate TiC, NbC, and TaC to prevent deterioration of intergranular corrosion resistance.
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