1. Characteristics of Austenitic Stainless Steel
The most basic alloying elements of austenitic stainless steel are chromium and nickel, and the representative grades are chromium-nickel austenitic stainless steels containing about 18% chromium and about 8% nickel. The element ratio of chromium and nickel basically ensures that the structure of the steel is stable austenite. The development of austenitic stainless steel is rapid. In order to meet the needs of different conditions, on the basis of 18-8 steel, changing the content of the quilt or adding other alloying elements gives this type of stainless steel better performance.
Type 18-8 austenitic stainless steel is corrosion-resistant to oxidizing media, such as the atmosphere, dilute nitric acid or medium-concentration nitric acid and concentrated sulfuric acid. It has good corrosion resistance in the range. It is not resistant to corrosion in reducing media, such as hydrochloric acid and sulfurous acid, and is also resistant to corrosion in concentrated nitric acid. In addition, when the austenitic stainless steel is slowly cooled in the range of 850-400 °C after heating, the carbides of chromium will precipitate from the grain boundaries, resulting in local chromium-depleted areas at the grain boundaries, resulting in intergranular corrosion. The intergranular corrosion resistance of austenitic stainless steel is related to the carbon content. The lower the carbon content, the stronger the intergranular corrosion resistance. Austenitic stainless steels are susceptible to stress corrosion cracking. The nickel content in steel plays an important role in improving the resistance to stress corrosion cracking.
2. Heat treatment process of austenitic stainless steel
Depending on the chemical composition and the purpose of heat treatment, austenitic stainless steels are often treated by solution treatment, stabilization annealing treatment, stress relief treatment and sensitization treatment.
2.1. Solid solution treatment
The solid solution treatment of austenitic stainless steel is a process method in which the steel is heated to a certain temperature where the excess phase is fully dissolved into the solid solution, and then rapidly cooled after a certain period of time. The purpose of the solid solution heat treatment of austenitic stainless steel is to remove the alloy carbides, such as (fecr) 23c6, etc., produced or precipitated in the previous processing steps. The phase is redissolved into austenite to obtain a single austenite structure (some may have a small amount of 8-ferrite) to ensure that the material has good mechanical properties and corrosion resistance, fully stress relief and cold work hardening Phenomenon. Solution treatment is suitable for austenitic stainless steels of any composition and grade.
2.2. Stabilization annealing
Stabilization annealing is a heat treatment method for austenitic stainless steels containing stabilizing elements titanium or niobium. The purpose of this method is to use the strong bonding characteristics of titanium, niobium and carbon to stabilize carbon so that it does not combine with chromium as much as possible, and finally achieves the purpose of stabilizing chromium, improving the stability of chromium in austenite, and avoiding crystal formation. boundary precipitation to ensure the corrosion resistance of the material. The cooling method and cooling rate of the stabilization treatment of austenitic stainless steel have little effect on the stabilization effect. Therefore, in order to prevent the deformation of the workpiece with complex shape or to ensure the minimum stress of the workpiece, a smaller cooling rate, such as air cooling or furnace cold can be used.
2.3. Stress Relief Treatment
To determine the stress relief treatment process of austenitic stainless steel, some principles should be mastered according to the material type, use environment, purpose of stress relief and the shape and size of the workpiece.
Remove the stress generated during processing or remove the residual stress after processing. The solution treatment can be used to heat the temperature and cool quickly, and the type I and II austenitic stainless steels can be cooled slowly. In order to ensure the stability of the final size of the workpiece. Low heating temperatures and slow cooling rates can be used. To eliminate large residual stress. To eliminate the residual stress of the workpiece that may generate new stress in the working environment or to eliminate the welding stress of large-section welding parts, the melting heating temperature should be used, and the type III austenitic stainless steel must be cooled quickly. In this case, it is best to use type I or type II austenitic stainless steel, which is cooled slowly after heating, and the effect of stress relief is better. In order to eliminate the residual stress of the workpiece that can only be heated locally. It should be heated at low temperature and cooled slowly.
2.4. Sensitization Treatment
Sensitization treatment is not actually a heat treatment method that should be used in the production process of austenitic stainless steel or its products. Rather, it is a procedure used when testing austenitic stainless steels for resistance to intergranular corrosion.
Sensitizing treatment is essentially a treatment that sensitizes austenitic stainless steel to intergranular corrosion. For some special occasions, in order to more strictly assess the intergranular corrosion resistance of materials, in some standards, the sensitization system for austenite rust sodium is more stringent, according to the temperature and material of the workpiece in the future. Different sensitization systems are used depending on factors such as the carbon content and whether it contains clamp elements. Some also control the heating and cooling rates of the sensitization treatment. Therefore, when judging the intergranular corrosion tendency of austenitic stainless steel, attention should be paid to the sensitization system used.
2.5. Cold Working Strengthening and Stress Relief Treatment of Austenitic Stainless Steel
Austenitic stainless steel cannot be strengthened by heat treatment, but can be strengthened by cold working deformation (cold work hardening, deformation strengthening), which will increase the strength and reduce the plasticity. After austenitic stainless steel or products (springs, bolts, etc.) are strengthened by cold working deformation, there is a large processing stress. The existence of this stress increases the sensitivity of stress corrosion when used in a stress corrosion environment, and affects the small size of the scale. stability. To reduce stress, stress relief treatment can be used. Generally, it is heated to 280 ~ 400 ℃ for 2 ~ 3 hours and then air-cooled or slowly cooled. The stress relief treatment can not only reduce the stress of the part, but also improve the hardness, strength and elastic limit without changing the elongation greatly.
3. Some problems that should be paid attention to in the heat treatment of steel
First of all, attention should be paid to the reasonable selection of the heating temperature for the solid solution treatment of austenitic stainless steel. In the material standard of austenitic stainless sodium, the specified solid solution heating temperature range is wide. In the actual heat treatment production, the specific composition of the steel, The optimal heating temperature should be reasonably selected according to factors such as content, use environment, and possible failure forms. However, care should be taken to prevent the heating temperature from being too high due to the solution treatment, because the heating temperature of the solution treatment is too high, which may cause the grains of the material whose grains have been refined by forging to grow. Grain coarsening can cause some undesirable consequences.
Secondly, attention should be paid to the effect of stabilization treatment on the properties of the solid solution state. Austenitic stainless steel containing stabilizing elements will tend to decrease in mechanical properties when it is stabilized after solution heat treatment. Strength, plasticity, and toughness have this phenomenon. The reason for the decrease in strength may be that during the stabilization treatment, the strong carbide forming element titanium combines with more carbon to form tic, which reduces the strengthening degree of carbon in austenite solid solution, and tic will also be in the process of heating and heat preservation. Agglomerations grow, which also has an effect on strength.
Third, the heating temperature of the stabilization treatment should not be too high, and is generally selected between 850 and 930 °C. Austenitic stainless steel should not be subjected to multiple solution treatment, because multiple solution heating will cause grain growth and adversely affect the performance of the material. At the same time, attention should be paid to pollution during processing. Once contaminated, measures should be taken to eliminate pollution.
