Depending on the chemical composition, the purpose of heat treatment, austenitic stainless steel
often used heat treatment methods are solid solution treatment, stabilization annealing, stress removal and sensitization treatment.
1. Solubilization treatment
Austenitic stainless steel solution treatment is to heat the steel to the excess phase dissolved in a solid solution in a certain temperature, to maintain a certain time after the rapid cooling process. The purpose of the austenitic stainless steel solution heat treatment is to re-dissolve the alloy carbides, such as (FeCr) 23C6, and the σ phase, which are produced or precipitated in the previous processing steps, into austenite to obtain a single austenite (Some may have a small amount of δ ferrite), to ensure that the material has good mechanical properties and corrosion resistance, and fully eliminate the stress and cold hardening phenomenon. The solution is suitable for any composition and grade of austenitic stainless steel.
2. Stable annealing
Stabilized annealing is a heat treatment method for austenitic stainless steels containing stabilized titanium or niobium. The purpose of this method is to use titanium, niobium and carbon strong bonding properties, stable carbon, so as not to combine with chromium, and ultimately achieve the purpose of stabilizing chromium to improve the stability of chromium in austenite, To ensure that the material corrosion resistance. The cooling and cooling rates of the austenitic stainless steels have little effect on the stabilization effect. Therefore, in order to prevent deformation of the complex parts or to minimize the stress of the workpiece, a smaller cooling rate such as air cooling or furnace cold.
3. Eliminate stress handling
Determination of austenitic stainless steel to eliminate stress treatment process, should be based on material type, the use of the environment, to eliminate the purpose of stress and workpiece shape size, etc., pay attention to grasp some of the principles. Remove the stress generated during the processing or remove the residual stress after processing. Can be used to solve the heat treatment temperature and fast cooling, Class I, II type austenitic stainless steel can be used to slow the cooling into the type. To ensure the stability of the final size of the workpiece. A low heating temperature and a slow cooling rate can be used. To eliminate the great residual stress. Eliminate the residual stress in the workpiece that may produce new stress in the working environment or to eliminate the welding stress of the large cross-section welding parts, which should be cooled by the melting temperature of the III-type austenitic stainless steel. This situation is best to use Class I or II austenitic stainless steel, slow cooling after heating to eliminate the effect of stress better. In order to eliminate the use of local heating only the residual stress of the workpiece. Should be taken at low temperature and slowly cooled.
The sensitization treatment does not actually belong to the austenitic stainless steel or its products in the manufacturing process should be used in the heat treatment method. But as a procedure used to test the resistance to intergranular corrosion of austenitic stainless steels. The sensitization treatment essentially makes the treatment of austenitic stainless steel more susceptible to intergranular corrosion. For some special use, in order to more rigorously assess the material's resistance to intergranular corrosion, in some standards, the austenite rust sodium sensitization system is more demanding, according to the future use of the workpiece temperature and The carbon content of the material and whether or not with the factors such as the use of different sensitization system. And some of the sensitization of the lift, cooling rate to be controlled. Therefore, in determining the austenitic stainless steel intergranular corrosion tendency of the size, should pay attention to the use of the sensitization system.
5. Austenitic stainless steel cold work to strengthen and stress treatment
Austenitic stainless steel can not be strengthened by heat treatment, but can be strengthened by cold working deformation (cold hardening, deformation strengthening), will increase the strength, plasticity decreased. Austenitic stainless steel or products (springs, bolts, etc.) by the cold deformation after strengthening, there is a large processing stress, the existence of this stress in stress corrosion environment, the use of increased stress corrosion sensitivity, The stability of the. To reduce the stress, can be used to stress treatment. - is heated to 280 ℃ ~ 400 ℃ to maintain 2h ~ 3h after the air cooling or slow cooling. The stress treatment not only reduces the stress of the part, but also increases the hardness and elasticity limits without significant changes in elongation.
First of all, we should pay attention to the austenitic stainless steel solution treatment temperature of the reasonable choice, in the austenitic stainless sodium material standards, the provisions of the solid solution heating temperature range is wide, the actual heat treatment production, can consider the specific composition of steel, Content, the use of the environment, the possible failure of the form and other factors, the rational choice of the best heating temperature. However, care should be taken to prevent the heating temperature from being too high because the melting temperature of the solution treatment is too high, and the grain size of the material which has been refined by the forging rolling may be increased. Coarsening of grain can cause some undesirable consequences.
Second, we should pay attention to the stability of the solid solution on the performance of the impact of the stability of the elements of austenitic stainless steel, solid solution heat treatment and then stabilized, the mechanical properties will decline. Strength and plasticity, toughness have this phenomenon. The reason for the decrease in strength is probably that when the stabilized treatment is carried out, the strong carbide-forming element titanium binds to more carbon to TiC, reducing the degree of carbon in the austenite solid solution, and the TiC will also heat Agglomeration grow up, which will also have an impact on the intensity.
Third, the stability of the heating temperature should not be too high, the general choice between 850 ℃ ~ 930 ℃. Austenitic stainless steel is not suitable for multiple solution treatment, because many times the solution heating, will cause grain growth, knot material properties adversely affected. At the same time, the process should pay attention to pollution, once contaminated, should take measures to eliminate pollution.