The heat treatment of niobium and its alloys includes uniform annealing, stress relief annealing, recrystallization annealing and thermal mechanical treatment. They have their own characteristics and influence each other. Uniform annealing results in non - uniform results. Stress relief annealing affects the grain size and orientation of recrystallization. Thermo-mechanical treatment can stabilize the cold-worked tissue and increase the recrystallization temperature. In the recrystallization annealing process, the precipitation reaction takes place simultaneously. Therefore, when the niobium alloy is used for heat treatment, the special characteristics of some alloy should be analyzed to select the appropriate heat treatment system.
At high temperature, niobium has strong affinity with oxygen, nitrogen, hydrogen and other elements, and is easy to form oxides, nitride, hydrides and other compounds. Contaminate surface, affect mechanical properties and machining properties. Therefore, the heat treatment of niobium and niobium alloys should be carried out in a vacuum or under the protection of purified inert gas.
1. Uniform annealing
During casting, due to the fast cooling speed, the gas in the ingot has no time to be removed, and the alloy contains unstable phase and segregation in the crystal. These defects have great influence on the process performance of uniform and stable tissue. It can be removed effectively by uniform annealing treatment.
The uniform annealing temperature of niobium ingot is 1800 ~ 2000°C. Its void is 10-4 ~ 10-5 ml hg. Insulation time depends on the size of the ingot, generally 5 ~ 10 hours.
Homogenization is achieved by atomic diffusion. In the multielement alloy this process takes a long time, sometimes resulting in grain coarsening, such as Nb - lOTi - lOMo - O.l C alloy at 2040 ° C annealing, organization has yet to reach even when obviously, grain size has grown up, so even after two arc furnace smelting alloy ingot should be corrected by hot working or cold working of excess phase and basic contact condition after homogenizing annealing as well.
2. Stress relief annealing
The deformed metal has higher energy and is in thermodynamic unstable state. Under the skin at lower temperatures, a series of changes occur in tissue and performance, known as recovery. During the recovery period, only a small amount of energy is released, and the properties of the deformed metal tend to recover only partially. It can be seen that the metal can be processed by stress relief annealing in cold working process.
The reactivation energy of niobium and niobium alloy in cold working is less than that of self-diffusion, so the reactivation takes place very quickly at very low temperature. Niobium and most niobium alloys can relieve stress and reduce strain energy when heated for 1 hour at a given temperature. Almost all strain centers producing new grains are eliminated. When reheating to recrystallization temperature, equiaxed grains are formed only in the residual strain center, and in other places, grains continue to grow, but keep the cold working direction. Stress relief annealing is often used in niobium and niobium alloys during cold working.
3. Recrystallization annealing
When the metal is heated to a high enough temperature after deformation, nucleation and growth of the original crystal are reperformed to form a new balanced crystal without distortion. This process is called recrystallization. After recrystallization, the machining structure of the metal is eliminated, the mechanical and physical properties return to the state before cold deformation, and the crystal structure of the new crystal is the same as the original one, but with different orientation. All the potential is released. The recrystallization of niobium and niobium alloy is mainly related to the degree of deformation and the amount of alloying elements. For pure niobium melted by electron beam, the cold deformation is 80-85%, and the recrystallization deformation begins at 1070-1100℃ with 60%, and recrystallization begins at 1200℃.
