Stainless Steel Solid Solution Treatment

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1. The Principle of Stainless Steel Solution Treatment

The solution treatment is to dissolve the carbides in the matrix and equal γ’to obtain a uniform supersaturated solid solution, which facilitates the re-precipitation of fine particles and uniformly distributed carbides and γ’and other strengthening phases during aging, and at the same time eliminates heat and cold processing. Stress causes the alloy to recrystallize. Secondly, the solution treatment is to obtain a suitable grain size to ensure the high temperature creep resistance of the alloy. The temperature range of solution treatment is about 980~1250℃, which is mainly selected according to the phase precipitation and dissolution law and application requirements of each alloy to ensure the necessary precipitation conditions and certain grain size of the main strengthening phase. For long-term high-temperature alloys, better high-temperature durability and creep properties are required, and a higher solid solution temperature should be selected to obtain a larger grain size; for medium-temperature use, better room temperature hardness, yield strength, For alloys with tensile strength, impact toughness and fatigue strength, a lower solid solution temperature can be used to ensure a smaller grain size. During high-temperature solid solution treatment, various precipitated phases are gradually dissolved and crystal grains grow; during low-temperature solid solution treatment, not only the main strengthening phase is dissolved, but some phases may also be precipitated. For alloys with low supersaturation, a faster cooling rate is usually selected; for alloys with high supersaturation, it is usually cooled in air.

2. The Role of Stainless Steel Solution Treatment

(1) Mainly to improve the plasticity and toughness of steel and alloys, and prepare for precipitation hardening treatment.

(2) Fully dissolve the various phases in the alloy, strengthen the solid solution, improve the toughness and corrosion resistance, eliminate stress and soften, so as to continue processing or forming.

3. Stainless Steel Solid Solution Treatment

(1) The solubility of carbon in austenitic stainless steel has a great influence on temperature. When the austenitic stainless steel is in the temperature range of 400℃～850℃, high chromium carbides will precipitate. When the chromium content falls below the corrosion resistance limit, there is chromium depletion at the grain boundary and intergranular corrosion will occur. , It can turn into powder in severe cases. Therefore, the austenitic stainless steel with the tendency of intergranular corrosion should be subjected to solution heat treatment or stabilization treatment.

(2) Solution heat treatment: heating the austenitic stainless steel to about 1100°C to dissolve the carbide phase completely or substantially, and the carbon is solid-dissolved in the austenite, and then rapidly cooled to room temperature, so that the carbon reaches a supersaturated state. This heat treatment method is solution heat treatment.

(3) The rapid cooling in solution heat treatment seems like the quenching of ordinary steel, but the ‘quenching’ at this time is different from the quenching of ordinary steel. The former is softening treatment and the latter is quenching. The heating temperature adopted by the latter to obtain different hardness is different, but it is not as high as 1100°C.

4. Factors Affecting Intergranular Corrosion

4.1 The influence of ingredients

(1) The influence of C element: The large experiment shows that the carbon content affects the most important factor of the intergranular corrosion of austenitic stainless steel. With the decrease of carbon content, the intergranular corrosion performance of austenitic stainless steel is improved.

(2) The influence of Cr element: In austenitic stainless steel, the increase of Cr content accelerates the intergranular corrosion in the low sensitizing temperature zone, and prolongs the time of intergranular corrosion in the high sensitizing temperature zone. -Generally speaking, the content of Cr in austenitic stainless steel should exceed 13%, if it is lower, it will seriously reduce the ability to resist intergranular corrosion.

(3) The influence of Ni element: Ni is added to the stainless steel to make the steel obtain a completely austenitic structure. In austenitic stainless steel, with the increase of Ni content, the residual ferrite can be completely eliminated. However, the increase of Ni content reduces the solubility of carbon in austenite and promotes the precipitation and growth of carbides, so the content of Ni will increase the susceptibility to intergranular corrosion.

(4) The influence of Ti and Nb elements: the addition of Ti, Nb and other elements in stainless steel that have a stronger binding capacity to C than Cr can combine with C to form stable carbides, which can avoid the formation of lean in austenite Chrome zone. Ti is a strong carbide forming element that can form stable TiC. It can reduce the C content of the matrix, stabilize the Cr content, and can also refine the grains. The main function is to make the C in the steel preferentially form with i. TiC alloy carbides can not form Cr carbides to avoid chromium depletion at grain boundaries and enhance intergranular corrosion resistance. The addition of this stabilizing element can partially inhibit the formation of carbides and reduce the lack of Cr, thereby improving the ability to resist intergranular corrosion.

4.2 Process Factors

(1) The influence of corrosive medium: The type and composition of corrosive substances determine the occurrence of intergranular corrosion and the degree of corrosion. Generally, in acidic media, stainless steel produces more serious intergranular corrosion.

(2) The influence of heat treatment I: In order to ensure that the austenitic stainless steel has the best corrosion resistance, it must have a single-phase austenite structure, so the stainless steel is solutionized. The solution treatment is to heat the austenitic stainless steel to about 1100°C, so that the carbide phase is completely or substantially dissolved, and C is dissolved in the austenite, and then quickly cooled to room temperature, so that C reaches a supersaturated state (C has It is stable, and there is no ability and opportunity to form high chromium carbides with Cr). The sensitization temperature is 450°C ~ 850°C. Among the sensitization temperature, 650°C is the most dangerous. Therefore, in the cooling stage of solution treatment, the speed should be fast and try to avoid the sensitization temperature zone. In the heating or cooling process, the shorter the residence time in the sensitization temperature zone, the smaller the chance of intergranular corrosion. Therefore, the solution treatment is in the two stages of heating and cooling. Increasing the heating and cooling speed is to increase the austenitic stainless steel U Shape tube resistant to intercrystalline corrosion.

Effective measures to eclipse. Austenitic stainless steel should be heated slowly to 400°C during solution treatment to reduce the internal force, and then quickly heated from 400°C to about 900°C to avoid more carbide precipitation, and then heated to 1050° Above C, quickly cool down after heat preservation. Because the carbides of austenitic stainless steel are precipitated at a temperature of 450 °C ~ 850 °C after solid solution, especially at 600 °C ~ 700 °C, the precipitation is more, and the precipitation is the most at around 650 °C, so it must be stable after heat preservation. Rapid cooling to below 300°C, so as to keep the austenite supersaturated with carbides at room temperature.

In order to improve the resistance of austenitic stainless steel U-shaped tube to intergranular corrosion, the faster the cooling rate is, the better without causing deformation of the workpiece. The effect of water cooling is better than forced air cooling. There are more forced air cooling in the industry, and it is not as convenient as forced air cooling to operate. Cooling to below 300°C within 3 minutes, with great practical experience, the workpiece can meet the requirements of use (metallographic analysis) within this cooling rate range.

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